Modern vacuum tubes, mostly miniature style
In electronics, a vacuum tube, an electron tube, or just a tube (North America), or valve (Britain and some other regions), is a device that controls electric current between electrodes in an evacuated container. Vacuum tubes mostly rely on thermionic emission of electrons from a hot filament or a cathode heated by the filament. This type is called a thermionic tube or thermionic valve. A phototube, however, achieves electron emission through the photoelectric effect. Not all electronic circuit valves/electron tubes are vacuum tubes (evacuated); gas-filled tubes are similar devices containing a gas, typically at low pressure, which exploit phenomena related to electric discharge in gases, usually without a heater.
The simplest vacuum tube, the diode, contains only a heater, a heated electron-emitting cathode (the filament itself acts as the cathode in some diodes), and a plate (anode). Current can only flow in one direction through the device between the two electrodes, as electrons emitted by the cathode travel through the tube and are collected by the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grid or grids. Tubes with grids can be used for many purposes, including amplification, rectification, switching, oscillation, and display.
Invented in 1904 by John Ambrose Fleming, vacuum tubes were a basic component for electronics throughout the first half of the twentieth century, which saw the diffusion of radio, television, radar, sound reinforcement, sound recording and reproduction, large telephone networks, analog and digital computers, and industrial process control. Although some applications had counterparts using earlier technologies such as the spark gap transmitter or mechanical computers, it was the invention of the vacuum tube that made these technologies widespread and practical. In the 1940s the invention of semiconductor devices made it possible to produce solid-state devices, which are smaller, more efficient, more reliable, more durable, and cheaper than tubes. Hence, from the mid-1950s solid-state devices such as transistors gradually replaced tubes. The cathode-ray tube (CRT) remained the basis for televisions and video monitors until superseded in the 21st century. However, there are still a few applications for which tubes are preferred to semiconductors; for example, the magnetron used in microwave ovens, and certain high-frequency amplifiers.
Thermionic valve: Classifications
One classification of vacuum tubes is by the number of active electrodes, (neglecting the filament or heater). A device with two active elements is a diode, usually used for rectification. Devices with three elements are triodes used for amplification and switching. Additional electrodes create tetrodes, pentodes, and so forth, which have multiple additional functions made possible by the additional controllable electrodes.
Other classifications are:
- by frequency range (audio, radio, VHF, UHF, microwave)
- by power rating (small-signal, audio power, high-power radio transmitting)
- by cathode/filament type (indirectly heated, directly heated) and Warm-up time (including "bright-emitter" or "dull-emitter")
- by characteristic curves design (e.g., sharp- versus remote-cutoff in some pentodes)
- by application (receiving tubes, transmitting tubes, amplifying or switching, rectification, mixing)
- specialized parameters (long life, very low microphonic sensitivity and low-noise audio amplification, rugged/military versions
- specialized functions (light or radiation detectors, video imaging tubes)
- tubes used to display information (Nixie tubes, "magic eye" tubes, Vacuum fluorescent displays, CRTs)
Multiple classifications may apply to a device; for example similar dual triodes can be used for audio preamplification and as flip-flops in computers, although linearity is important in the former case and long life in the latter.
Tubes have different functions, such as cathode ray tubes which create a beam of electrons for display purposes (such as the television picture tube) in addition to more specialized functions such as electron microscopy and electron beam lithography. X-ray tubes are also vacuum tubes. Phototubes and photomultipliers rely on electron flow through a vacuum, though in those cases electron emission from the cathode depends on energy from photons rather than thermionic emission. Since these sorts of "vacuum tubes" have functions other than electronic amplification and rectification they are described in their own articles.
Thermionic valve: Description
Diode: electrons from the hot cathode flow towards the positive anode, but not vice versa
Triode: voltage applied to the grid controls plate (anode) current.
A vacuum tube consists of two or more electrodes in a vacuum inside an airtight enclosure. Most tubes have glass envelopes, though ceramic and metal envelopes (atop insulating bases) have been used. The electrodes are attached to leads which pass through the envelope via an airtight seal. Most vacuum tubes have a limited lifetime, due to the filament or heater burning out or other failure modes, so they are made as replaceable units; the electrode leads connect to pins on the tube's base which plug into a tube socket. Tubes were a frequent cause of failure in electronic equipment, and consumers were expected to be able to replace tubes themselves. In addition to the base terminals, some tubes had an electrode terminating at a top cap. The principal reason for doing this was to avoid leakage resistance through the tube base, particularly for the high impedance grid input. The bases were commonly made with phenolic insulation which performs poorly as an insulator in humid conditions. Other reasons for using a top cap include improving stability by reducing grid-to-anode capacitance, improved high-frequency performance, keeping a very high plate voltage away from lower voltages, and accommodating one more electrode than allowed by the base. There was even an occasional design that had two top cap connections.
The earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases electrons into the vacuum, a process called thermionic emission, originally known as the "Edison Effect". A second electrode, the anode or plate, will attract those electrons if it is at a more positive voltage. The result is a net flow of electrons from the filament to plate. However, electrons cannot flow in the reverse direction because the plate is not heated and does not emit electrons. The filament (cathode) has a dual function: it emits electrons when heated; and, together with the plate, it creates an electric field due to the potential difference between them. Such a tube with only two electrodes is termed a diode, and is used for rectification. Since current can only pass in one direction, such a diode (or rectifier) will convert alternating current (AC) to pulsating DC. Diodes can therefore be used in a DC power supply, as a demodulator of amplitude modulated (AM) radio signals and for similar functions.
Early tubes used the filament as the cathode, this is called a "directly heated" tube. Most modern tubes are "indirectly heated" by a "heater" element inside a metal tube that is the cathode. The heater is electrically isolated from the surrounding cathode and simply serves to heat the cathode sufficiently for thermionic emission of electrons. The electrical isolation allows all the tubes' heaters to be supplied from a common circuit (which can be AC without inducing hum) while allowing the cathodes in different tubes to operate at different voltages. H. J. Round invented the indirectly heated tube around 1913.
The filaments require constant and often considerable power, even when amplifying signals at the microwatt level. Power is also dissipated when the electrons from the cathode slam into the anode (plate) and heat it; this can occur even in an idle amplifier due to quiescent currents necessary to ensure linearity and low distortion. In a power amplifier, this heating can be considerable and can destroy the tube if driven beyond its safe limits. Since the tube contains a vacuum, the anodes in most small and medium power tubes are cooled by radiation through the glass envelope. In some special high power applications, the anode forms part of the vacuum envelope to conduct heat to an external heat sink, usually cooled by a blower, or water-jacket.
Klystrons and magnetrons often operate their anodes (called collectors in klystrons) at ground potential to facilitate cooling, particularly with water, without high-voltage insulation. These tubes instead operate with high negative voltages on the filament and cathode.
Except for diodes, additional electrodes are positioned between the cathode and the plate (anode). These electrodes are referred to as grids as they are not solid electrodes but sparse elements through which electrons can pass on their way to the plate. The vacuum tube is then known as a triode, tetrode, pentode, etc., depending on the number of grids. A triode has three electrodes: the anode, cathode, and one grid, and so on. The first grid, known as the control grid, (and sometimes other grids) transforms the diode into a voltage-controlled device: the voltage applied to the control grid affects the current between the cathode and the plate. When held negative with respect to the cathode, the control grid creates an electric field which repels electrons emitted by the cathode, thus reducing or even stopping the current between cathode and anode. As long as the control grid is negative relative to the cathode, essentially no current flows into it, yet a change of several volts on the control grid is sufficient to make a large difference in the plate current, possibly changing the output by hundreds of volts (depending on the circuit). The solid-state device which operates most like the pentode tube is the junction field-effect transistor (JFET), although vacuum tubes typically operate at over a hundred volts, unlike most semiconductors in most applications.
Thermionic valve: History and development
One of Edison's experimental bulbs
The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes tubes. The many scientists and inventors who experimented with such tubes include Thomas Edison, Eugen Goldstein, Nikola Tesla, and Johann Wilhelm Hittorf. With the exception of early light bulbs, such tubes were only used in scientific research or as novelties. The groundwork laid by these scientists and inventors, however, was critical to the development of subsequent vacuum tube technology.
Although thermionic emission was originally reported in 1873 by Frederick Guthrie, it was Thomas Edison's apparently independent discovery of the phenomenon in 1883 that became well known. Although Edison was aware of the unidirectional property of current flow between the filament and the anode, his interest (and patent) concentrated on the sensitivity of the anode current to the current through the filament (and thus filament temperature). Little practical use was ever made of this property (however early radios often implemented volume controls through varying the filament current of amplifying tubes). It was only years later that John Ambrose Fleming utilized the rectifying property of the diode tube to detect (demodulate) radio signals, a substantial improvement on the early cat's-whisker detector already used for rectification.
However actual amplification by a vacuum tube only became practical with Lee De Forest's 1907 invention of the three-terminal "audion" tube, a crude form of what was to become the triode. Being essentially the first electronic amplifier, such tubes were instrumental in long-distance telephony (such as the first coast-to-coast telephone line in the US) and public address systems, and introduced a far superior and versatile technology for use in radio transmitters and receivers. The electronics revolution of the 20th century arguably began with the invention of the triode vacuum tube.
Thermionic valve: Diodes
The English physicist John Ambrose Fleming worked as an engineering consultant for firms including Edison Telephone and the Marconi Company. In 1904, as a result of experiments conducted on Edison effect bulbs imported from the USA, he developed a device he called an "oscillation valve" (because it passes current in only one direction). The heated filament, or cathode, was capable of thermionic emission of electrons that would flow to the plate (or anode) when it was at a positive voltage with respect to the cathode. Electrons, however, could not pass in the reverse direction because the plate was not heated and thus not capable of thermionic emission of electrons.
Later known as the Fleming valve, it could be used as a rectifier of alternating current and as a radio wave detector. This greatly improved the crystal set which rectified the radio signal using an early solid-state diode based on a crystal and a so-called cat's whisker, an adjustable point contact. Unlike modern semiconductors, such a diode required painstaking adjustment of the contact to the crystal in order for it to rectify.
The tube was relatively immune to vibration, and thus vastly superior on shipboard duty, particularly for navy ships with the shock of weapon fire commonly knocking the sensitive but delicate galena off its sensitive point (the tube was in general no more sensitive as a radio detector, but was adjustment free). The diode tube was a reliable alternative for detecting radio signals.
As electronic engineering advanced, notably during World War II, this function of a diode came to be considered as one type of demodulation. While firmly established by history, the term "detector" is not of itself descriptive, and should be considered outdated.
Higher power diode tubes or power rectifiers found their way into power supply applications until they were eventually replaced first by selenium, and later, by silicon rectifiers in the 1960s.
Thermionic valve: Triodes
The first triode, the De Forest Audion
, invented in 1906
Triodes as they evolved over 40 years of tube manufacture, from the RE16 in 1918 to a 1960s era miniature tube.
Triode symbol. From top to bottom: plate (anode), control grid, cathode, heater (filament)
Originally, the only use for tubes in radio circuits was for rectification, not amplification. In 1906, Robert von Lieben filed for a patent for a cathode ray tube which included magnetic deflection. This could be used for amplifying audio signals and was intended for use in telephony equipment. He would later go on to help refine the triode vacuum tube.
However, it was Lee De Forest who is credited with inventing the triode tube in 1907 while continuing experiments to improve his original Audion tube, a crude forerunner of the triode. By placing an additional electrode between the filament (cathode) and plate (anode), he discovered the ability of the resulting device to amplify signals of all frequencies. As the voltage applied to the so-called control grid (or simply "grid") was lowered from the cathode's voltage to somewhat more negative voltages, the amount of current from the filament to the plate would be reduced.
The negative electrostatic field created by the grid in the vicinity of the cathode would inhibit passage of emitted electrons and reduce the current to the plate. Thus, a few volts' difference at the grid would make a large change in the plate current and could lead to a much larger voltage change at the plate; the result was voltage and power amplification. In 1908, De Forest was granted a patent (U.S. Patent 879,532) for such a three-electrode version of his original Audion tube for use as an electronic amplifier in radio communications. This eventually became known as the triode.
General Electric Company Pliotron, Chemical Heritage Foundation
De Forest's device was not a hard vacuum tube, as he erroneously believed that it depended on the presence of residual gas remaining after evacuation. In its Audion leaflets, the De Forest company even warned against any operation which might lead to too high a vacuum. In 1912 De Forest brought the audion to Harold Arnold in AT&T's engineering department. Arnold recommended that AT&T purchase the patent. He developed high-vacuum tubes which were tested in the summer of 1913 on AT&T's long distance network.
The Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his sound-on-film process in Berlin, Germany. Tigerstedt's innovation was to make the electrodes concentric cylinders with the cathode at the centre, thus greatly increasing the collection of emitted electrons at the anode. The first true vacuum triodes in production were the Pliotrons developed by Irving Langmuir at the General Electric research laboratory (Schenectady, New York) in 1915. Langmuir was one of the first scientists to realize that a harder vacuum would improve the amplifying behaviour of the triode, having improved Gaede's diffusion vacuum pump.
Pliotrons were closely followed by the French type 'TM' and later the English type 'R' which were in widespread use by the allied military by 1916. These types were the first true hard vacuum tubes; early diodes and triodes performed as such despite a rather high residual gas pressure. Techniques to produce and maintain better vacua in tubes were then developed. Historically, vacuum levels in production vacuum tubes typically ranged from 10 µPa down to 10 nPa.
The triode and its derivatives (tetrodes and pentodes) are transconductance devices, in which the controlling signal applied to the grid is a voltage, and the resulting amplified signal appearing at the anode is a current. Compare this to the behavior of the bipolar junction transistor, in which the controlling signal is a current and the output is also a current.
For vacuum tubes, transconductance or mutual conductance (gm) is defined as the change in the plate(anode)/cathode current divided by the corresponding change in the grid to cathode voltage, with a constant plate(anode) to cathode voltage. Typical values of gm for a small-signal vacuum tube are 1 to 10 millisiemens. It is one of the three 'constants' of a vacuum tube, the other two being its gain μ and plate resistance Rp or Ra. The Van der Bijl equation defines their relationship as follows:
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortion at low volumes. Plotting plate current as a function of applied grid voltage, it was seen that there was a range of grid voltages for which the transfer characteristics were approximately linear.
To use this range, a negative bias voltage had to be applied to the grid to position the DC operating point in the linear region. This was called the idle condition, and the plate current at this point the "idle current". The controlling voltage was superimposed onto the bias voltage, resulting in a linear variation of plate current in response to both positive and negative variation of the input voltage around that point.
This concept is called grid bias. Many early radio sets had a third battery called the "C battery" (unrelated to the present-day C cell, for which the letter denotes its size and shape). The C cell's positive terminal was connected to the cathode of the tubes (or "ground" in most circuits) and whose negative terminal supplied this bias voltage to the grids of the tubes.
Later circuits, after tubes were made with heaters isolated from their cathodes, used cathode biasing, avoiding the need for a separate negative power supply. For cathode biasing, a relatively low-value resistor is connected between the cathode and ground. This makes the cathode positive with respect to the grid, which is at ground potential for DC.
However C batteries continued to be included in some equipment even when the "A" and "B" batteries had been replaced by power from the AC mains. That was possible because there was essentially no current draw on these batteries; they could thus last for many years (often longer than all the tubes) without requiring replacement.
When triodes were first used in radio transmitters and receivers, it was found that tuned amplification stages had a tendency to oscillate unless their gain was very limited. This was due to the parasitic capacitance between the plate (the amplifier's output) and the control grid (the amplifier's input), known as the Miller capacitance.
Eventually the technique of neutralization was developed whereby the RF transformer connected to the plate (anode) would include an additional winding in the opposite phase. This winding would be connected back to the grid through a small capacitor, and when properly adjusted would cancel the Miller capacitance. This technique was employed and led to the success of the Neutrodyne radio during the 1920s. However, neutralization required careful adjustment and proved unsatisfactory when used over a wide range of frequencies.
Thermionic valve: Tetrodes and pentodes
Tetrode symbol. From top to bottom: plate (anode), screen grid, control grid, cathode, heater (filament)
To combat the stability problems and limited voltage gain due to the Miller effect, the physicist Walter H. Schottky invented the tetrode tube in 1919. He showed that the addition of a second grid, located between the control grid and the plate (anode), known as the screen grid, could solve these problems. ("Screen" in this case refers to electrical "screening" or shielding, not physical construction: all "grid" electrodes in between the cathode and plate are "screens" of some sort rather than solid electrodes since they must allow for the passage of electrons directly from the cathode to the plate). A positive voltage slightly lower than the plate (anode) voltage was applied to it, and was bypassed (for high frequencies) to ground with a capacitor. This arrangement decoupled the anode and the control grid, essentially eliminating the Miller capacitance and its associated problems. Consequently, higher voltage gains from a single tube became possible, reducing the number of tubes required in many circuits. This two-grid tube is called a tetrode, meaning four active electrodes, and was common by 1926.
At certain values of plate voltage and current, the tetrode characteristic curves are kinked due to secondary emission.
However, the tetrode had one new problem. In any tube, electrons strike the anode with sufficient energy to cause the emission of electrons from its surface. In a triode this so-called secondary emission of electrons is not important since they are simply re-captured by the more positive anode (plate). But in a tetrode they can be captured by the screen grid (thus also acting as an anode) since it is also at a high voltage, thus robbing them from the plate current and reducing the amplification of the device. Since secondary electrons can outnumber the primary electrons, in the worst case, particularly as the plate voltage dips below the screen voltage, the plate current can decrease with increasing plate voltage. This is the so-called "tetrode kink" and is an example of negative resistance which can itself cause instability. The otherwise undesirable negative resistance was exploited to produce an extremely simple oscillator circuit only requiring connection of the plate to a resonant LC circuit to oscillate; this was effective over a wide frequency range. The so-called dynatron oscillator thus operated on the same principle of negative resistance as the tunnel diode oscillator many years later. Another undesirable consequence of secondary emission is that in extreme cases enough charge can flow to the screen grid to overheat and destroy it. Later tetrodes had anodes treated to reduce secondary emission; earlier ones such as the type 77 sharp-cutoff pentode connected as a tetrode made better dynatrons.
The solution was to add another grid between the screen grid and the main anode, called the suppressor grid (since it suppressed secondary emission current toward the screen grid). This grid was held at the cathode (or "ground") voltage and its negative voltage (relative to the anode) electrostatically repelled secondary electrons so that they would be collected by the anode after all. This three-grid tube is called a pentode, meaning five electrodes. The pentode was invented in 1926 by Bernard D. H. Tellegen and became generally favored over the simple tetrode. Pentodes are made in two classes: those with the suppressor grid wired internally to the cathode (e.g. EL84/6BQ5) and those with the suppressor grid wired to a separate pin for user access (e.g. 803, 837). An alternative solution for power applications is the beam tetrode or "beam power tube", discussed below.
Thermionic valve: Multifunction and multisection tubes
The pentagrid converter contained five grids between the cathode and the plate (anode).
Superheterodyne receivers require a local oscillator and mixer, combined in the function of a single pentagrid converter tube. Various alternatives such as using a combination of a triode with a hexode and even an octode have been used for this purpose. The additional grids include both control grids (at a low potential) and screen grids (at a high voltage). Many designs used such a screen grid as an additional anode to provide feedback for the oscillator function, whose current was added to that of the incoming radio frequency signal. The pentagrid converter thus became widely used in AM receivers including the miniature tube version of the "All American Five". Octodes such as the 7A8 were rarely used in the US, but much more common in Europe, particularly in battery operated radios where the lower power consumption was an advantage.
To further reduce the cost and complexity of radio equipment, two separate structures (triode and pentode for instance) could be combined in the bulb of a single multisection tube. An early example was the Loewe 3NF. This 1920s device had three triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver. As the Loewe set had only one tube socket, it was able to substantially undercut the competition since, in Germany, state tax was levied by the number of sockets. However, reliability was compromised, and production costs for the tube were much greater. In a sense, these were akin to integrated circuits. In the US, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also had a single tube socket, but because it used a four-pin base, the additional element connections were made on a "mezzanine" platform at the top of the tube base.
By 1940 multisection tubes had become commonplace. There were constraints, however, due to patents and other licensing considerations (see British Valve Association). Constraints due to the number of external pins (leads) often forced the functions to share some of those external connections such as their cathode connections (in addition to the heater connection). The RCA Type 55 was a double diode triode used as a detector, automatic gain control rectifier and audio preamplifier in early AC powered radios. These sets often included the 53 Dual Triode Audio Output. Another early type of multi-section tube, the 6SN7, is a "dual triode" which performs the functions of two triode tubes, while taking up half as much space and costing less. The 12AX7 is a dual "high mu" (high voltage gain) triode in a miniature enclosure, and became widely used in audio signal amplifiers, instruments, and guitar amplifiers.
The introduction of the miniature tube base (see below) which could have 9 pins, more than previously available, allowed other multi-section tubes to be introduced, such as the 6GH8/ECF82 triode-pentode, quite popular in television receivers. The desire to include even more functions in one envelope resulted in the General Electric Compactron which had 12 pins. A typical example, the 6AG11, contained two triodes and two diodes.
Some otherwise conventional tubes do not fall into standard categories; the 6AR8, 6JH8 and 6ME8 had several common grids, followed by a pair of beam deflection electrodes which deflected the current towards either of two anodes. It was sometimes known as the 'sheet beam' tube, and was used in some color TV sets for color demodulation. The similar 7360 was popular as a balanced SSB (de)modulator.
Thermionic valve: Beam power tubes
6L6 tubes in glass envelopes
The beam power tube is usually a tetrode with the addition of beam-forming electrodes, which take the place of the suppressor grid. These angled plates (not to be confused with the anode) focus the electron stream onto certain spots on the anode which can withstand the heat generated by the impact of massive numbers of electrons, while also providing pentode behavior. The positioning of the elements in a beam power tube uses a design called "critical-distance geometry", which minimizes the "tetrode kink", plate to control grid capacitance, screen grid current, and secondary emission from the anode, thus increasing power conversion efficiency. The control grid and screen grid are also wound with the same pitch, or number of wires per inch. The two grids are positioned so that the control grid creates "sheets" of electrons which pass between the screen-grid wires. They're aligned to be equidistant from, say, the bottom of the tube.
Aligning the grid wires also helps to reduce screen current, which represents wasted energy. This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes. EMI engineers Cabot Bull and Sidney Rodda developed the design which became the 6L6, the first popular beam power tube, introduced by RCA in 1936 and later corresponding tubes in Europe the KT66, KT77 and KT88 made by the Marconi-Osram Valve subsidiary of GEC (the KT standing for "Kinkless Tetrode").
"Pentode operation" of beam power tubes is often described in manufacturers' handbooks and data sheets, resulting in some confusion in terminology. They are not pentodes, of course.
Variations of the 6L6 design are still widely used in tube guitar amplifiers, making it one of the longest-lived electronic device families in history. Similar design strategies are used in the construction of large ceramic power tetrodes used in radio transmitters.
Beam power tubes can be connected as triodes for improved audio tonal quality but in triode mode deliver significantly reduced power output.
Thermionic valve: Gas-filled tubes
Gas-filled tubes such as discharge tubes and cold cathode tubes are not hard vacuum tubes, though are always filled with gas at less than sea-level atmospheric pressure. Types such as the voltage-regulator tube and thyratron resemble hard vacuum tubes and fit in sockets designed for vacuum tubes. Their distinctive orange, red, or purple glow during operation indicates the presence of gas; electrons flowing in a vacuum do not produce light within that region. These types may still be referred to as "electron tubes" as they do perform electronic functions. High-power rectifiers use mercury vapor to achieve a lower forward voltage drop than high-vacuum tubes.
Subminiature CV4501 tube (SQ version of EF72), 35 mm long
x 10 mm diameter
Thermionic valve: Miniature tubes
Early tubes used a metal or glass envelope atop an insulating bakelite base. In 1938 a technique was developed to use an all-glass construction with the pins fused in the glass base of the envelope. This was used in the design of a much smaller tube outline, known as the miniature tube, having 7 or 9 pins. Making tubes smaller reduced the voltage where they could safely operate, and also reduced the power dissipation of the filament. Miniature tubes became predominant in consumer applications such as radio receivers and hi-fi amplifiers. However the larger older styles continued to be used especially as higher power rectifiers, in higher power audio output stages and as transmitting tubes.
RCA 6DS4 "Nuvistor" triode, circa 20 mm high
by 11 mm diameter
Subminiature tubes with a size roughly that of half a cigarette were used in hearing-aid amplifiers. These tubes did not have pins plugging into a socket but were soldered in place. The "acorn tube" (named due to its shape) was also very small, as was the metal-cased RCA nuvistor from 1959, about the size of a thimble. The nuvistor was developed to compete with the early transistors and operated at higher frequencies than those early transistors could. The small size supported especially high-frequency operation; nuvistors were used in UHF television tuners and some HiFi FM radio tuners (Sansui 500A) until replaced by high-frequency capable transistors.
Thermionic valve: Improvements in construction and performance
Commercial packaging for vacuum tubes used in the latter half of the 20th century including boxes for individual tubes (bottom right), sleeves for rows of the boxes (left), and bags that smaller tubes would be put in by a store upon purchase (top right)
The earliest vacuum tubes strongly resembled incandescent light bulbs and were made by lamp manufacturers, who had the equipment needed to manufacture glass envelopes and the vacuum pumps required to evacuate the enclosures. De Forest used Heinrich Geissler's mercury displacement pump, which left behind a partial vacuum. The development of the diffusion pump in 1915 and improvement by Irving Langmuir led to the development of high-vacuum tubes. After World War I, specialized manufacturers using more economical construction methods were set up to fill the growing demand for broadcast receivers. Bare tungsten filaments operated at a temperature of around 2200 °C. The development of oxide-coated filaments in the mid-1920s reduced filament operating temperature to a dull red heat (around 700 °C), which in turn reduced thermal distortion of the tube structure and allowed closer spacing of tube elements. This in turn improved tube gain, since the gain of a triode is inversely proportional to the spacing between grid and cathode. Bare tungsten filaments remain in use in small transmitting tubes but are brittle and tend to fracture if handled roughly – e.g. in the postal services. These tubes are best suited to stationary equipment where impact and vibration is not present.
Thermionic valve: Indirectly heated cathodes
The desire to power electronic equipment using AC mains power faced a difficulty with respect to the powering of the tubes' filaments, as these were also the cathode of each tube. Powering the filaments directly from a power transformer introduced mains-frequency (50 or 60 Hz) hum into audio stages. The invention of the "equipotential cathode" reduced this problem, with the filaments being powered by a balanced AC power transformer winding having a grounded center tap.
A superior solution, and one which allowed each cathode to "float" at a different voltage, was that of the indirectly heated cathode: a cylinder of oxide-coated nickel acted as electron-emitting cathode, and was electrically isolated from the filament inside it. Indirectly heated cathodes enable the cathode circuit to be separated from the heater circuit. The filament, no longer electrically connected to the tube's electrodes, became simply known as a "heater", and could as well be powered by AC without any introduction of hum. In the 1930s indirectly heated cathode tubes became widespread in equipment using AC power. Directly heated cathode tubes continued to be widely used in battery-powered equipment as their filaments required considerably less power than the heaters required with indirectly heated cathodes.
Tubes designed for high gain audio applications may have twisted heater wires to cancel out stray electric fields, fields that could induce objectionable hum into the program material.
Heaters may be energized with either alternating current (AC) or direct current (DC). DC is often used where low hum is required.
Thermionic valve: Use in electronic computers
The 1946 ENIAC
computer used 17,468 vacuum tubes and consumed 150 kW
Vacuum tubes used as switches made electronic computing possible for the first time, but the cost and relatively short mean time to failure of tubes were limiting factors. "The common wisdom was that valves-which, like light bulbs, contained a hot glowing filament-could never be used satisfactorily in large numbers, for they were unreliable, and in a large installation too many would fail in too short a time". Tommy Flowers, who later designed Colossus, "discovered that, so long as valves were switched on and left on, they could operate reliably for very long periods, especially if their 'heaters' were run on a reduced current". In 1934 Flowers built a successful experimental installation using over 3,000 tubes in small independent modules; when a tube failed, it was possible to switch off one module and keep the others going, thereby reducing the risk of another tube failure being caused; this installation was accepted by the Post Office (who operated telephone exchanges). Flowers was also a pioneer of using tubes as very fast (compared to electromechanical devices) electronic switches. Later work confirmed that tube unreliability was not as serious an issue as generally believed; the 1946 ENIAC, with over 17,000 tubes, had a tube failure (which took 15 minutes to locate) on average every two days. The quality of the tubes was a factor, and the diversion of skilled people during the Second World War lowered the general quality of tubes. During the war Colossus was instrumental in breaking German codes. After the war, development continued with tube-based computers including, military computers ENIAC and Whirlwind, the Ferranti Mark 1 (the first commercially available electronic computer), and UNIVAC I, also available commercially.
Thermionic valve: Colossus
Flowers's Colossus and its successor Colossus Mk2 were built by the British during World War II to substantially speed up the task of breaking the German high level Lorenz encryption. Using about 1,500 vacuum tubes (2,400 for Mk2), Colossus replaced an earlier machine based on relay and switch logic (the Heath Robinson). Colossus was able to break in a matter of hours messages that had previously taken several weeks; it was also much more reliable. Colossus was the first use of vacuum tubes working in concert on such a large scale for a single machine.
Once Colossus was built and installed, it ran continuously, powered by dual redundant diesel generators, the wartime mains supply being considered too unreliable. The only time it was switched off was for conversion to Mk2, with the addition of more tubes. Another nine Colossus Mk2s were built, and all ten machines were surprisingly reliable. The ten machines drew 15 kilowatts of power each continuously, largely for the tube heaters.
A Colossus reconstruction was switched on in 1996; it was upgraded to Mk2 configuration in 2004; it found the key for a wartime German ciphertext in 2007.
Thermionic valve: Whirlwind and "special-quality" tubes
To meet the reliability requirements of the 1951 US digital computer Whirlwind, "special-quality" tubes with extended life, and a long-lasting cathode in particular, were produced. The problem of short lifetime was traced to evaporation of silicon, used in the tungsten alloy to make the heater wire easier to draw. Elimination of silicon from the heater wire alloy (and more frequent replacement of the wire drawing dies) allowed production of tubes that were reliable enough for the Whirlwind project. The tubes developed for Whirlwind were later used in the giant SAGE air-defense computer system. SAGE computers were dual installations, with one operating, and the other in standby. To locate potential tube failures in the standby computer, heater voltages were reduced, which caused failures of tubes which would otherwise fail in service. These computers continued in service years after other tube computers had been superseded.
High-purity nickel tubing and cathode coatings free of materials that can poison emission (such as silicates and aluminum) also contribute to long cathode life. The first such "computer tube" was Sylvania's 7AK7 of 1948. Computers were the first tube devices to run tubes at cutoff (enough negative grid voltage to make them cease conduction) for quite-extended periods of time. When their grids became less negative, they failed to conduct. While hot but non-conductive, an insulating layer ("cathode interface") developed between the nickel sleeve and the oxide coating. What was described above cured this problem.
By the late 1950s it was routine for special-quality small-signal tubes to last for hundreds of thousands of hours, if operated conservatively. This increased reliability also made mid-cable amplifiers in submarine cables possible.
Thermionic valve: Heat generation and cooling
The anode (plate) of this transmitting triode has been designed to dissipate up to 500 W
A considerable amount of heat is produced when tubes operate, both from the filament (heater) but also from the stream of electrons bombarding the plate. In power amplifiers this source of heat will exceed the power due to cathode heating. A few types of tube permit operation with the anodes at a dull red heat; in other types, red heat indicates severe overload.
The requirements for heat removal can significantly change the appearance of high-power vacuum tubes. High power audio amplifiers and rectifiers required larger envelopes to dissipate heat. Transmitting tubes could be much larger still.
Heat escapes the device by black body radiation from the anode (plate) as infrared radiation, and by convection of air over the tube envelope. Convection is not possible in most tubes since the anode is surrounded by vacuum.
Tubes which generate relatively little heat, such as the 1.4-volt filament directly heated tubes designed for use in battery-powered equipment, often have shiny metal anodes. 1T4, 1R5 and 1A7 are examples. Gas-filled tubes such as thyratrons may also use a shiny metal anode, since the gas present inside the tube allows for heat convection from the anode to the glass enclosure.
The anode is often treated to make its surface emit more infrared energy. High-power amplifier tubes are designed with external anodes which can be cooled by convection, forced air or circulating water. The water-cooled 80 kg, 1.25 MW 8974 is among the largest commercial tubes available today.
In a water-cooled tube, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator to prevent high voltage leakage through the cooling water to the radiator system. Water as usually supplied has ions which conduct electricity; deionized water, a good insulator, is required. Such systems usually have a built-in water-conductance monitor which will shut down the high-tension supply if the conductance becomes too high.
The screen grid may also generate considerable heat. Limits to screen grid dissipation, in addition to plate dissipation, are listed for power devices. If these are exceeded then tube failure is likely.
Thermionic valve: Tube packages
Metal-cased tubes with octal bases
High power GS-9B triode transmitting tube with heat sink at bottom.
Most modern tubes have glass envelopes, but metal, fused quartz (silica) and ceramic have also been used. A first version of the 6L6 used a metal envelope sealed with glass beads, while a glass disk fused to the metal was used in later versions. Metal and ceramic are used almost exclusively for power tubes above 2 kW dissipation. The nuvistor was a modern receiving tube using a very small metal and ceramic package.
The internal elements of tubes have always been connected to external circuitry via pins at their base which plug into a socket. Subminiature tubes were produced using wire leads rather than sockets, however these were restricted to rather specialized applications. In addition to the connections at the base of the tube, many early triodes connected the grid using a metal cap at the top of the tube; this reduces stray capacitance between the grid and the plate leads. Tube caps were also used for the plate (anode) connection, particularly in transmitting tubes and tubes using a very high plate voltage.
High-power tubes such as transmitting tubes have packages designed more to enhance heat transfer. In some tubes, the metal envelope is also the anode. The 4CX1000A is an external anode tube of this sort. Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation. Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac 4CM2500KG, a forced water-cooled power tetrode capable of dissipating 2.5 megawatts. By comparison, the largest power transistor can only dissipate about 1 kilowatt.
Thermionic valve: Names
The generic name "[thermionic] valve" used in the UK derives from the unidirectional current flow allowed by the earliest device, the thermionic diode emitting electrons from a heated filament, by analogy with a non-return valve in a water pipe. The US names "vacuum tube", "electron tube", and "thermionic tube" all simply describe a tubular envelope which has been evacuated ("vacuum"), has a heater, and controls electron flow.
In many cases manufacturers and the military gave tubes designations which said nothing about their purpose (e.g., 1614). In the early days some manufacturers used proprietary names which might convey some information, but only about their products; the KT66 and KT88 were "Kinkless Tetrodes". Later, consumer tubes were given names which conveyed some information, with the same name often used generically by several manufacturers. In the US, Radio Electronics Television Manufacturers' Association (RETMA) designations comprise a number, followed by one or two letters, and a number. The first number is the (rounded) heater voltage; the letters designate a particular tube but say nothing about its structure; and the final number is the total number of electrodes (without distinguishing between, say, a tube with many electrodes, or two sets of electrodes in a single envelope-a double triode, for example). For example, the 12AX7 is a double triode (two sets of three electrodes plus heater) with a 12.6V heater (which, as it happens, can also be connected to run from 6.3V). The "AX" has no meaning other than to designate this particular tube according to its characteristics. Similar, but not identical, tubes are the 12AD7, 12AE7...12AT7, 12AU7, 12AV7, 12AW7 (rare!), 12AY7, and the 12AZ7.
A system widely used in Europe known as the Mullard-Philips tube designation, also extended to transistors, uses a letter, followed by one or more further letters, and a number. The type designator specifies the heater voltage or current (one letter), the functions of all sections of the tube (one letter per section), the socket type (first digit), and the particular tube (remaining digits). For example, the ECC83 (equivalent to the 12AX7) is a 6.3V (E) double triode (CC) with a miniature base (8). In this system special-quality tubes (e.g., for long-life computer use) are indicated by moving the number immediately after the first letter: the E83CC is a special-quality equivalent of the ECC83, the E55L a power pentode with no consumer equivalent.
Thermionic valve: Special-purpose tubes
Voltage-regulator tube in operation. Low pressure gas within tube glows due to current flow.
Some special-purpose tubes are constructed with particular gases in the envelope. For instance, voltage-regulator tubes contain various inert gases such as argon, helium or neon, which will ionize at predictable voltages. The thyratron is a special-purpose tube filled with low-pressure gas or mercury vapor. Like vacuum tubes, it contains a hot cathode and an anode, but also a control electrode which behaves somewhat like the grid of a triode. When the control electrode starts conduction, the gas ionizes, after which the control electrode can no longer stop the current; the tube "latches" into conduction. Removing anode (plate) voltage lets the gas de-ionize, restoring its non-conductive state.
Some thyratrons can carry large currents for their physical size. One example is the miniature type 2D21, often seen in 1950s jukeboxes as control switches for relays. A cold-cathode version of the thyratron, which uses a pool of mercury for its cathode, is called an ignitron; some can switch thousands of amperes. Thyratrons containing hydrogen have a very consistent time delay between their turn-on pulse and full conduction; they behave much like modern silicon-controlled rectifiers, also called thyristors due to their functional similarity to thyratrons. Hydrogen thyratrons have long been used in radar transmitters.
An extremely specialized tube is the krytron, which is used for extremely precise and rapid high-voltage switching. Krytrons with certain specifications are suitable to initiate the precise sequence of detonations used to set off a nuclear weapon, and are heavily controlled at an international level.
X-ray tubes are used in medical imaging among other uses. X-ray tubes used for continuous-duty operation in fluoroscopy and CT imaging equipment may use a focused cathode and a rotating anode to dissipate the large amounts of heat thereby generated. These are housed in an oil-filled aluminium housing to provide cooling.
The photomultiplier tube is an extremely sensitive detector of light, which uses the photoelectric effect and secondary emission, rather than thermionic emission, to generate and amplify electrical signals. Nuclear medicine imaging equipment and liquid scintillation counters use photomultiplier tube arrays to detect low-intensity scintillation due to ionizing radiation.
Thermionic valve: Powering the tube
Thermionic valve: Batteries
Batteries provided the voltages required by tubes in early radio sets. Three different voltages were generally required, using three different batteries designated as the A, B, and C battery. The "A" battery or LT (low-tension) battery provided the filament voltage. Tube heaters were designed for single, double or triple-cell lead-acid batteries, giving nominal heater voltages of 2 V, 4 V or 6 V. In portable radios, dry batteries were sometimes used with 1.5 or 1 V heaters. Reducing filament consumption improved the life span of batteries. By 1955 towards the end of the tube era, tubes using only 50 mA down to as little as 10 mA for the heaters had been developed.
The high voltage applied to the anode (plate) was provided by the "B" battery or the HT (high-tension) supply or battery. These were generally of dry cell construction and typically came in 22.5-, 45-, 67.5-, 90-, 120- or 135-volt versions.
Batteries for a vacuum tube circuit. The C battery is highlighted.
Early sets used a grid bias battery or "C" battery which was connected to provide a negative voltage. Since virtually no current flows through a tube's grid connection, these batteries had very low drain and lasted the longest. Even after AC power supplies became commonplace, some radio sets continued to be built with C batteries, as they would almost never need replacing. However more modern circuits were designed using cathode biasing, eliminating the need for a third power supply voltage; this became practical with tubes using indirect heating of the cathode.
The "C battery" for bias is a designation having no relation to the "C cell" battery size.
Thermionic valve: AC power
Battery replacement was a major operating cost for early radio receiver users. The development of the battery eliminator, and, in 1925, batteryless receivers operated by household power, reduced operating costs and contributed to the growing popularity of radio. A power supply using a transformer with several windings, one or more rectifiers (which may themselves be vacuum tubes), and large filter capacitors provided the required direct current voltages from the alternating current source.
As a cost reduction measure, especially in high-volume consumer receivers, all the tube heaters could be connected in series across the AC supply using heaters requiring the same current and with a similar warm-up time. In one such design, a tap on the tube heater string supplied the 6 volts needed for the dial light. By deriving the high voltage from a half-wave rectifier directly connected to the AC mains, the heavy and costly power transformer was eliminated. This also allowed such receivers to operate on direct current, a so-called AC/DC receiver design. Many different US consumer AM radio manufacturers of the era used a virtually identical circuit, given the nickname All American Five.
Where the mains voltage was in the 100-120V range, this limited voltage proved suitable only for low-power receivers. Television receivers either required a transformer or could use a voltage doubling circuit. Where 230 V nominal mains voltage was used, television receivers as well could dispense with a power transformer.
Transformer-less power supplies required safety precautions in their design to limit the shock hazard to users, such as electrically insulated cabinets and an interlock tying the power cord to the cabinet back, so the line cord was necessarily disconnected if the user or service person opened the cabinet. A cheater cord was a power cord ending in the special socket used by the safety interlock; servicers could then power the device with the hazardous voltages exposed.
To avoid the warm-up delay, "instant on" television receivers passed a small heating current through their tubes even when the set was nominally off. At switch on, full heating current was provided and the set would play almost immediately.
Thermionic valve: Reliability
Tube tester manufactured in 1930
One reliability problem of tubes with oxide cathodes is the possibility that the cathode may slowly become "poisoned" by gas molecules from other elements in the tube, which reduce its ability to emit electrons. Trapped gases or slow gas leaks can also damage the cathode or cause plate (anode) current runaway due to ionization of free gas molecules. Vacuum hardness and proper selection of construction materials are the major influences on tube lifetime. Depending on the material, temperature and construction, the surface material of the cathode may also diffuse onto other elements. The resistive heaters that heat the cathodes may break in a manner similar to incandescent lamp filaments, but rarely do, since they operate at much lower temperatures than lamps.
The heater's failure mode is typically a stress-related fracture of the tungsten wire or at a weld point and generally occurs after accruing many thermal (power on-off) cycles. Tungsten wire has a very low resistance when at room temperature. A negative temperature coefficient device, such as a thermistor, may be incorporated in the equipment's heater supply or a ramp-up circuit may be employed to allow the heater or filaments to reach operating temperature more gradually than if powered-up in a step-function. Low-cost radios had tubes with heaters connected in series, with a total voltage equal to that of the line (mains). Some receivers made before World War II had series-string heaters with total voltage less than that of the mains. Some had a resistance wire running the length of the power cord to drop the voltage to the tubes. Others had series resistors made like regular tubes; they were called ballast tubes.
Following World War II, tubes intended to be used in series heater strings were redesigned to all have the same ("controlled") warm-up time. Earlier designs had quite-different thermal time constants. The audio output stage, for instance, had a larger cathode, and warmed up more slowly than lower-powered tubes. The result was that heaters that warmed up faster also temporarily had higher resistance, because of their positive temperature coefficient. This disproportionate resistance caused them to temporarily operate with heater voltages well above their ratings, and shortened their life.
Another important reliability problem is caused by air leakage into the tube. Usually oxygen in the air reacts chemically with the hot filament or cathode, quickly ruining it. Designers developed tube designs that sealed reliably. This was why most tubes were constructed of glass. Metal alloys (such as Cunife and Fernico) and glasses had been developed for light bulbs that expanded and contracted in similar amounts, as temperature changed. These made it easy to construct an insulating envelope of glass, while passing connection wires through the glass to the electrodes.
When a vacuum tube is overloaded or operated past its design dissipation, its anode (plate) may glow red. In consumer equipment, a glowing plate is universally a sign of an overloaded tube. However, some large transmitting tubes are designed to operate with their anodes at red, orange, or in rare cases, white heat.
"Special quality" versions of standard tubes were often made, designed for improved performance in some respect, such as a longer life cathode, low noise construction, mechanical ruggedness via ruggedized filaments, low microphony, for applications where the tube will spend much of its time cut off, etc. The only way to know the particular features of a special quality part is by reading the data sheet. Names may reflect the standard name (12AU7==>12AU7A, its equivalent ECC82==>E82CC, etc.), or be absolutely anything (standard and special-quality equivalents of the same tube include 12AU7, ECC82, B329, CV491, E2163, E812CC, M8136, CV4003, 6067, VX7058, 5814A and 12AU7A).
The longest recorded valve life was earned by a Mazda AC/P pentode valve (serial No. 4418) in operation at the BBC's main Northern Ireland transmitter at Lisnagarvey. The valve was in service from 1935 until 1961 and had a recorded life of 232,592 hours. The BBC maintained meticulous records of their valves' lives with periodic returns to their central valve stores.
Thermionic valve: Vacuum
Getter in opened tube; silvery deposit from getter
Dead vacuum fluorescent display (air has leaked in and the getter spot has become white)
The highest possible vacuum is desired in a tube. Remaining gas atoms will ionize and conduct electricity between the elements in an undesired manner. In a defective tube residual air pressure will lead to ionization, becoming visible as a pink-purple glow discharge between the tube elements.
To prevent gases from compromising the tube's vacuum, modern tubes are constructed with "getters", which are usually small, circular troughs filled with metals that oxidize quickly, barium being the most common. While the tube envelope is being evacuated, the internal parts except the getter are heated by RF induction heating to evolve any remaining gas from the metal parts. The tube is then sealed and the getter is heated to a high temperature, again by radio frequency induction heating, which causes the getter material to vaporize and react with any residual gas. The vapor is deposited on the inside of the glass envelope, leaving a silver-colored metallic patch which continues to absorb small amounts of gas that may leak into the tube during its working life. Great care is taken with the valve design to ensure this material is not deposited on any of the working electrodes. If a tube develops a serious leak in the envelope, this deposit turns a white color as it reacts with atmospheric oxygen. Large transmitting and specialized tubes often use more exotic getter materials, such as zirconium. Early gettered tubes used phosphorus-based getters, and these tubes are easily identifiable, as the phosphorus leaves a characteristic orange or rainbow deposit on the glass. The use of phosphorus was short-lived and was quickly replaced by the superior barium getters. Unlike the barium getters, the phosphorus did not absorb any further gases once it had fired.
Getters act by chemically combining with residual or infiltrating gases, but are unable to counteract (non-reactive) inert gases. A known problem, mostly affecting valves with large envelopes such as cathode ray tubes and camera tubes such as iconoscopes, orthicons, and image orthicons, comes from helium infiltration. The effect appears as impaired or absent functioning, and as a diffuse glow along the electron stream inside the tube. This effect cannot be rectified (short of re-evacuation and resealing), and is responsible for working examples of such tubes becoming rarer and rarer. Unused ("New Old Stock") tubes can also exhibit inert gas infiltration, so there is no long-term guarantee of these tube types surviving into the future.
Thermionic valve: Transmitting tubes
Large transmitting tubes have carbonized tungsten filaments containing a small trace (1% to 2%) of thorium. An extremely thin (molecular) layer of thorium atoms forms on the outside of the wire's carbonized layer and, when heated, serve as an efficient source of electrons. The thorium slowly evaporates from the wire surface, while new thorium atoms diffuse to the surface to replace them. Such thoriated tungsten cathodes usually deliver lifetimes in the tens of thousands of hours. The end-of-life scenario for a thoriated-tungsten filament is when the carbonized layer has mostly been converted back into another form of tungsten carbide and emission begins to drop off rapidly; a complete loss of thorium has never been found to be a factor in the end-of-life in a tube with this type of emitter. WAAY-TV in Huntsville, Alabama achieved 163,000 hours of service from an Eimac external cavity klystron in the visual circuit of its transmitter; this is the highest documented service life for this type of tube. It has been said that transmitters with vacuum tubes are better able to survive lightning strikes than transistor transmitters do. While it was commonly believed that at RF power levels above approx. 20 kilowatts, vacuum tubes were more efficient than solid state circuits, this is no longer the case, especially in medium wave (AM broadcast) service where solid state transmitters at nearly all power levels have measurably higher efficiency. FM broadcast transmitters with solid state power amplifiers up to approx. 15 kW also show better overall mains-power efficiency than tube-based power amplifiers.
Thermionic valve: Receiving tubes
Cathodes in small "receiving" tubes are coated with a mixture of barium oxide and strontium oxide, sometimes with addition of calcium oxide or aluminium oxide. An electric heater is inserted into the cathode sleeve, and insulated from it electrically by a coating of aluminium oxide. This complex construction causes barium and strontium atoms to diffuse to the surface of the cathode and emit electrons when heated to about 780 degrees Celsius.
Thermionic valve: Failure modes
Thermionic valve: Catastrophic failures
A catastrophic failure is one which suddenly makes the vacuum tube unusable. A crack in the glass envelope will allow air into the tube and destroy it. Cracks may result from stress in the glass, bent pins or impacts; tube sockets must allow for thermal expansion, to prevent stress in the glass at the pins. Stress may accumulate if a metal shield or other object presses on the tube envelope and causes differential heating of the glass. Glass may also be damaged by high-voltage arcing.
Tube heaters may also fail without warning, especially if exposed to over voltage or as a result of manufacturing defects. Tube heaters do not normally fail by evaporation like lamp filaments, since they operate at much lower temperature. The surge of inrush current when the heater is first energized causes stress in the heater, and can be avoided by slowly warming the heaters, gradually increasing current with a NTC thermistor included in the circuit. Tubes intended for series-string operation of the heaters across the supply have a specified controlled warm-up time to avoid excess voltage on some heaters as others warm up. Directly heated filament-type cathodes as used in battery-operated tubes or some rectifiers may fail if the filament sags, causing internal arcing. Excess heater-to-cathode voltage in indirectly heated cathodes can break down the insulation between elements and destroy the heater.
Arcing between tube elements can destroy the tube. An arc can be caused by applying voltage to the anode (plate) before the cathode has come up to operating temperature, or by drawing excess current through a rectifier, which damages the emission coating. Arcs can also be initiated by any loose material inside the tube, or by excess screen voltage. An arc inside the tube allows gas to evolve from the tube materials, and may deposit conductive material on internal insulating spacers.
Tube rectifiers have limited current capability and exceeding ratings will eventually destroy a tube.
Thermionic valve: Degenerative failures
Degenerative failures are those caused by the slow deterioration of performance over time.
Overheating of internal parts, such as control grids or mica spacer insulators, can result in trapped gas escaping into the tube; this can reduce performance. A getter is used to absorb gases evolved during tube operation, but has only a limited ability to combine with gas. Control of the envelope temperature prevents some types of gassing. A tube with an unusually high level of internal gas may exhibit a visible blue glow when plate voltage is applied. The getter (being a highly reactive metal) is effective against many atmospheric gases, but has no (or very limited) chemical reactivity to inert gases such as helium. One progressive type of failure, especially with physically large envelopes such as those used by camera tubes and cathode-ray tubes, comes from helium infiltration. The exact mechanism is not clear: the metal-to-glass lead-in seals are one possible infiltration site.
Gas and ions within the tube contribute to grid current which can disturb operation of a vacuum tube circuit. Another effect of overheating is the slow deposit of metallic vapors on internal spacers, resulting in inter-element leakage.
Tubes on standby for long periods, with heater voltage applied, may develop high cathode interface resistance and display poor emission characteristics. This effect occurred especially in pulse and digital circuits, where tubes had no plate current flowing for extended times. Tubes designed specifically for this mode of operation were made.
Cathode depletion is the loss of emission after thousands of hours of normal use. Sometimes emission can be restored for a time by raising heater voltage, either for a short time or a permanent increase of a few percent. Cathode depletion was uncommon in signal tubes but was a frequent cause of failure of monochrome television cathode-ray tubes. Usable life of this expensive component was sometimes extended by fitting a boost transformer to increase heater voltage.
Thermionic valve: Other failures
Vacuum tubes may have or develop defects in operation that make an individual tube unsuitable in a given device, although it may perform satisfactorily in another application. Microphonics refers to internal vibrations of tube elements which modulate the tube's signal in an undesirable way; sound or vibration pick-up may affect the signals, or even cause uncontrolled howling if a feedback path develops between a microphonic tube and, for example, a loudspeaker. Leakage current between AC heaters and the cathode may couple into the circuit, or electrons emitted directly from the ends of the heater may also inject hum into the signal. Leakage current due to internal contamination may also inject noise. Some of these effects make tubes unsuitable for small-signal audio use, although unobjectionable for many purposes. Selecting the best of a batch of nominally identical tubes for critical applications can produce better results.
Tube pins are designed to facilitate installation and removal from the socket but, due to the high operating temperatures of these devices and/or ingress of dirt and dust over time, pins can develop non-conducting or high resistance surface films. Pins can be easily cleaned to restore conductance to normal standards.
Thermionic valve: Testing
Universal vacuum tube tester
Vacuum tubes can be tested outside of their circuitry using a vacuum tube tester.
Thermionic valve: Other vacuum tube devices
Most small signal vacuum tube devices have been superseded by semiconductors, but some vacuum tube electronic devices are still in common use. The magnetron is the type of tube used in all microwave ovens. In spite of the advancing state of the art in power semiconductor technology, the vacuum tube still has reliability and cost advantages for high-frequency RF power generation.
Some tubes, such as magnetrons, traveling-wave tubes, carcinotrons, and klystrons, combine magnetic and electrostatic effects. These are efficient (usually narrow-band) RF generators and still find use in radar, microwave ovens and industrial heating. Traveling-wave tubes (TWTs) are very good amplifiers and are even used in some communications satellites. High-powered klystron amplifier tubes can provide hundreds of kilowatts in the UHF range.
Thermionic valve: Cathode ray tubes
The cathode ray tube (CRT) is a vacuum tube used particularly for display purposes. Although there are still many televisions and computer monitors using cathode ray tubes, they are rapidly being replaced by flat panel displays whose quality has greatly improved even as their prices drop. This is also true of digital oscilloscopes (based on internal computers and analog to digital converters), although traditional analog scopes (dependent upon CRTs) continue to be produced, are economical, and preferred by many technicians. At one time many radios used "magic eye tubes", a specialized sort of CRT used in place of a meter movement to indicate signal strength, or input level in a tape recorder. A modern indicator device, the vacuum fluorescent display (VFD) is also a sort of cathode ray tube.
X-ray tube is also a special type of Cathode ray tube, whilst x-ray emits when high voltage electrons hit the anode.
Gyrotrons or vacuum masers, used to generate high-power millimeter band waves, are magnetic vacuum tubes in which a small relativistic effect, due to the high voltage, is used for bunching the electrons. Gyrotrons can generate very high powers (hundreds of kilowatts). Free electron lasers, used to generate high-power coherent light and even X-rays, are highly relativistic vacuum tubes driven by high-energy particle accelerators. Thus these are sorts of cathode ray tubes.
Thermionic valve: Electron multipliers
A photomultiplier is a phototube whose sensitivity is greatly increased through the use of electron multiplication. This works on the principle of secondary emission, whereby a single electron emitted by the photocathode strikes a special sort of anode known as a dynode causing more electrons to be released from that dynode. Those electrons are accelerated toward another dynode at a higher voltage, releasing more secondary electrons; as many as 15 such stages provide a huge amplification. Despite great advances in solid state photodetectors, the single-photon detection capability of photomultiplier tubes makes this vacuum tube device excel in certain applications. Such a tube can also be used for detection of ionizing radiation as an alternative to the Geiger–Müller tube (itself not an actual vacuum tube). Historically, the image orthicon TV camera tube widely used in television studios prior to the development of modern CCD arrays also used multistage electron multiplication.
For decades, electron-tube designers tried to augment amplifying tubes with electron multipliers in order to increase gain, but these suffered from short life because the material used for the dynodes "poisoned" the tube's hot cathode. (For instance, the interesting RCA 1630 secondary-emission tube was marketed, but did not last.) However, eventually, Philips of the Netherlands developed the EFP60 tube that had a satisfactory lifetime, and was used in at least one product, a laboratory pulse generator. By that time, however, transistors were rapidly improving, making such developments superfluous.
One variant called a "channel electron multiplier" does not use individual dynodes but consists of a curved tube, such as a helix, coated on the inside with material with good secondary emission. One type had a funnel of sorts to capture the secondary electrons. The continuous dynode was resistive, and its ends were connected to enough voltage to create repeated cascades of electrons. The microchannel plate consists of an array of single stage electron multipliers over an image plane; several of these can then be stacked. This can be used, for instance, as an image intensifier in which the discrete channels substitute for focussing.
Tektronix made a high-performance wideband oscilloscope CRT with a channel electron multiplier plate behind the phosphor layer. This plate was a bundled array of a huge number of short individual c.e.m. tubes that accepted a low-current beam and intensified it to provide a display of practical brightness. (The electron optics of the wideband electron gun could not provide enough current to directly excite the phosphor.)
Thermionic valve: Vacuum tubes in the 21st century
Thermionic valve: Niche applications
Although vacuum tubes have been largely replaced by solid-state devices in most amplifying, switching, and rectifying applications, there are certain exceptions. In addition to the special functions noted above, tubes still have some niche applications.
In general, vacuum tubes are much less susceptible than corresponding solid-state components to transient overvoltages, such as mains voltage surges or lightning, the electromagnetic pulse effect of nuclear explosions or geomagnetic storms produced by giant solar flares. This property kept them in use for certain military applications long after more practical and less expensive solid-state technology was available for the same applications, as for example with the MiG-25. In that plane, output power of the radar is about one kilowatt and it can burn through a channel under interference.
Vacuum tubes are still practical alternatives to solid state in generating high power at radio frequencies in applications such as industrial radio frequency heating, particle accelerators, and broadcast transmitters. This is particularly true at microwave frequencies where such devices as the klystron and traveling-wave tube provide amplification at power levels unattainable using current semiconductor devices. The household microwave oven uses a magnetron tube to efficiently generate hundreds of watts of microwave power.
In military applications, a high-power vacuum tube can generate a 10–100 megawatt signal that can burn out an unprotected receiver's frontend. Such devices are considered non-nuclear electromagnetic weapons; they were introduced in the late 1990s by US and Russia.
Thermionic valve: Audiophiles
70-watt tube-hybrid audio amplifier selling for US$2,680 in 2011, about 10 times the price of a comparable model using transistors.
Enough people prefer tube sound to make tube amplifiers commercially viable in three areas: musical instrument (guitar) amplifiers, devices used in recording studios, and audiophile equipment.
Many guitarists prefer using valve amplifiers to solid-state models, often due to the way they tend to distort when overdriven. (Any amplifier can only accurately amplify a signal to a certain volume; past this limit, the amplifier will begin to distort the signal. Different circuits will distort the signal in different ways; some guitarists prefer the distortion characteristics of vacuum tubes.) Most popular vintage models use vacuum tubes.
Thermionic valve: Vacuum fluorescent display
Typical VFD used in a videocassette recorder
A modern display technology using a variation of cathode ray tube is often used in videocassette recorders, DVD players and recorders, microwave oven control panels, and automotive dashboards. Rather than raster scanning, these vacuum fluorescent displays (VFD) switch control grids and anode voltages on and off, for instance, to display discrete characters. The VFD uses phosphor-coated anodes as in other display cathode ray tubes. Because the filaments are in view, they must be operated at temperatures where the filament does not glow visibly. This is possible using more recent cathode technology, and these tubes also operate with quite low anode voltages (often less than 50 volts) unlike cathode ray tubes. Their high brightness allows reading the display in bright daylight. VFD tubes are flat and rectangular, as well as relatively thin. Typical VFD phosphors emit a broad spectrum of greenish-white light, permitting use of color filters, though different phosphors can give other colors even within the same display. The design of these tubes provides a bright glow despite the low energy of the incident electrons. This is because the distance between the cathode and anode is relatively small. (This technology is distinct from fluorescent lighting, which uses a discharge tube.)
Thermionic valve: Vacuum tubes using field electron emitters
In the early years of the 21st century there has been renewed interest in vacuum tubes, this time with the electron emitter formed on a flat silicon substrate, as in integrated circuit technology. This subject is now called vacuum nanoelectronics. The most common design uses a cold cathode in the form of a large-area field electron source (for example a field emitter array). With these devices, electrons are field-emitted from a large number of closely spaced individual emission sites.
Such integrated microtubes may find application in microwave devices including mobile phones, for Bluetooth and Wi-Fi transmission, in radar and for satellite communication. As of 2012 they were being studied for possible applications in field emission display technology, but there were significant production problems.
As of 2014, NASA's Ames Research Center was reported on working on vacuum-channel transistors produced using CMOS techniques.
Thermionic valve: See also
- Bogey device, close to manufacturer's stated parameter values
- List of vacuum tubes, a list of type numbers.
- List of vacuum tube computers
- Mullard-Philips tube designation
- Nixie tube, a gas-filled display device sometimes misidentified as a vacuum tube
- Fetron, a solid-state plug-compatible replace for vacuum tubes
- RETMA tube designation
- RMA tube designation
- Russian tube designations
- Tube caddy
- Tube tester
- Valve amplifier
Thermionic valve: Patents
- U.S. Patent 803,684 – Instrument for converting alternating electric currents into continuous currents (Fleming valve patent)
- U.S. Patent 841,387 – Device for amplifying feeble electrical currents
- U.S. Patent 879,532 – De Forest's Audion
Thermionic valve: References
- Reich, Herbert J. (April 13, 2013). Principles of Electron Tubes. Literary Licensing, LLC. ISBN 978-1258664060.
- Fundamental Amplifier Techniques with Electron Tubes: Theory and Practice with Design Methods for Self Construction. Elektor Electronics. January 1, 2011. ISBN 978-0905705934.
- "RCA Electron Tube 6BN6/6KS6". Retrieved 2015-04-13.
- Hoddeson, L. "The Vacuum Tube". PBS. Retrieved 6 May 2012.
- Jones, Morgan (2012). Valve Amplifiers. Elsevier. p. 580. ISBN 0080966403.
- Olsen, George Henry (2013). Electronics: A General Introduction for the Non-Specialist. Springer. p. 391. ISBN 1489965351.
- Rogers, D. C. "Triode amplifiers in the frequency range 100 Mc/s to 420 Mc/s". Journal of the British Institution of Radio Engineers. 11 (12): 569–575., p.571
- Bray, John (2002). Innovation and the Communications Revolution: From the Victorian Pioneers to Broadband Internet. IET.
- Guthrie, Frederick (1876). Magnetism and Electricity. London and Glasgow: William Collins, Sons, & Company.
- Thomas A. Edison U.S. Patent 307,031 "Electrical Indicator", Issue date: 1884
- Guarnieri, M. (2012). "The age of vacuum tubes: Early devices and the rise of radio communications". IEEE Ind. Electron. M. 6 (1): 41–43. doi:10.1109/MIE.2012.2182822.
- White, Thomas, United States Early Radio History
- "Robert von Lieben - Patent Nr 179807 Dated November 19, 1906" (PDF). Kaiserliches Patentamt. 19 November 1906. Retrieved 30 March 2008.
- Räisänen, Antti V.; Lehto, Arto (2003). Radio Engineering for Wireless Communication and Sensor Applications. Artech House. p. 7. ISBN 1580536697.
- J.Jenkins and W.H.Jarvis, "Basic Principles of Electronics, Volume 1 Thermionics", Pergamon Press (1966), Ch.1.10 p.9
- Guarnieri, M. (2012). "The age of vacuum tubes: the conquest of analog communications". IEEE Ind. Electron. M. 6 (2): 52–54. doi:10.1109/MIE.2012.2193274.
- Introduction to Thermionic Valves (Vacuum Tubes), Colin J. Seymour
- "Philips Historical Products: Philips Vacuum Tubes". Retrieved 3 November 2013.
- Baker, Bonnie (2008). Analog circuits. Newnes. p. 391. ISBN 0-7506-8627-8.
- Modjeski, Roger A. "Mu, Gm and Rp and how Tubes are matched". Välljud AB. Retrieved 22 April 2011.
- ISBN 0-672-21983-2.
Amplification factor or voltage gain is the amount the signal at the control grid is increased in amplitude after passing through the tube, which is also referred to as the Greek letter μ (mu) or voltage gain (Vg) of the tube.
- C H Gardner (1965) The Story of the Valve, Radio Constructor (See particularly the section "Glass Base Construction")
- L.W. Turner (ed.) Electronics Engineer's Reference Book, 4th ed. Newnes-Butterworth, London 1976 Buy book ISBN 0-408-00168-2 pages 7–2 through 7-6
- Guarnieri, M. (2012). "The age of Vacuum Tubes: Merging with Digital Computing". IEEE Ind. Electron. M. 6 (3): 52–55. doi:10.1109/MIE.2012.2207830.
- From part of Copeland's "Colossus" available online
- Randall, Alexander 5th (14 February 2006). "A lost interview with ENIAC co-inventor J. Presper Eckert". Computer World. Retrieved 2011-04-25.
- The National Museum of Computing – Rebuilding Colossus
- The National Museum of Computing – The Colossus Gallery
- RCA "Transmitting Tubes Manual" TT-5 1962, p. 10
- (PDF) http://www.cpii.com/docs/datasheets/78/4CM2500KG%20June%202011.pdf.
The maximum anode dissipation rating is 2500 kilowatts.
- The Oxford Companion to the History of Modern Science, J. L. Heilbron , Oxford University Press 2003, 9780195112290, "valve, thermionic"
- Okamura, Sōgo (1994). History of electron tubes. IOS Press. pp. 133–. ISBN 978-90-5199-145-1. Retrieved 9 May 2011.
- National Valve Museum: audio double triodes ECC81, 2, and 3
- Certified by BBC central valve stores, Motspur Park
- Mazda Data Booklet 1968 Page 112.
- 31 Alumni. "The Klystron & Cactus". Retrieved 29 December 2013.
- Tomer, Robert B. (1960), Getting the most out of vacuum tubes, Indianapolis, IN, USA: Howard W. Sams, LCCN 60-13843. available on the Internet Archive. Chapter 1
- , 60, chapter 2
- , 60, chapter 3
- Broad, William J. "Nuclear Pulse (I): Awakening to the Chaos Factor", Science. 29 May 1981 212: 1009–1012
- Y Butt, The Space Review, 2011 "... geomagnetic storms, on occasion, can induce more powerful pulses than the E3 pulse from even megaton type nuclear weapons."
- Price of $4,680 for the "super enhanced version." Includes 90-day warranty on tubes "under normal operation conditions." See Model no: SE-300B-70W
- Rolls RA200 100 W RMS/Channel @ 4 Ohms Power Amplifier. Full Compass. Retrieved on 2011-05-09.
- Barbour, E. (1998). "The cool sound of tubes – vacuum tube musical applications". Spectrum, IEEE. 35 (8). IEEE. pp. 24–35.
- Ackerman, Evan. "Vacuum tubes could be the future of computing". Dvice. Dvice. Retrieved 8 February 2013.
- Anthony, Sebastian. "The vacuum tube strikes back: NASA’s tiny 460GHz vacuum transistor that could one day replace silicon FETs". ExtremeTech.
Thermionic valve: Further reading
- Basic Electronics : Volumes 1-5; Van Valkenburgh, Nooger, Neville; John F. Rider Publisher; 1955.
- Spangenberg, Karl R. (1948). Vacuum Tubes. McGraw-Hill. OCLC 567981. LCC TK7872.V3.
- Millman, J. & Seely, S. Electronics, 2nd ed. McGraw-Hill, 1951.
- Shiers, George, "The First Electron Tube", Scientific American, March 1969, p. 104.
- Tyne, Gerald, Saga of the Vacuum Tube, Ziff Publishing, 1943, (reprint 1994 Prompt Publications), pp. 30–83.
- Stokes, John, 70 Years of Radio Tubes and Valves, Vestal Press, NY, 1982, pp. 3–9.
- Thrower, Keith, History of the British Radio Valve to 1940, MMA International, 1982, pp 9–13.
- Eastman, Austin V., Fundamentals of Vacuum Tubes, McGraw-Hill, 1949
- Philips Technical Library. Books published in the UK in the 1940s and 1950s by Cleaver Hume Press on design and application of vacuum tubes.
- RCA Radiotron Designer's Handbook, 1953 (4th Edition). Contains chapters on the design and application of receiving tubes.
- Wireless World. "Radio Designer's Handbook". UK reprint of the above.
- RCA "Receiving Tube Manual" RC15, RC26 (1947, 1968) Issued every two years, contains details of the technical specs of the tubes that RCA sold.
Thermionic valve: External links
||Wikimedia Commons has media related to Vacuum tubes.
- http://www.pentalabs.com/Limited-Warranty/Tube-Maintenance-Education/How-a-Vacuum-Tube-Works – The history of vacuum tubes
- http://www.cfp-radio.com/realisations/rea03/rea03.html – (FR) How to build a vacuum tube tester.
- The Thermionic Detector – HJ van der Bijl (October 1919)
- How vacuum tubes really work – Thermionic emission and vacuum tube theory, using introductory college-level mathematics.
- The Vacuum Tube FAQ – FAQ from rec.audio
- The invention of the thermionic valve. Fleming discovers the thermionic (or oscillation) valve, or 'diode'.
- Tubes Vs. Transistors : Is There an Audible Difference? – 1972 AES paper on audible differences in sound quality between vacuum tubes and transistors.
- The Virtual Valve Museum
- The cathode ray tube site
- O'Neill's Electronic museum – vacuum tube museum
- Vacuum tubes for beginners – Japanese Version
- NJ7P Tube Database – Data manual for tubes used in North America.
- Vacuum tube data sheet locator
- Characteristics and datasheets
- Video of amateur radio operator making his own vacuum tube triodes
- Tuning eye tubes.
- Archive film of Mullard factory Blackburn
- Western Electric specifications sheets for 1940s and 1950s electron and vacuum tubes
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And today the goods by request "Thermionic valve" in Minnesota can be shipped to Minneapolis, Saint Paul, Rochester, Bloomington, Duluth, Brooklyn Park, Plymouth, Maple Grove, Woodbury, St. Cloud, Eagan, Eden Prairie, Coon Rapids, Blaine, Burnsville, Lakeville, Minnetonka, Apple Valley, Edina, St. Louis Park, Moorhead, Mankato, Maplewood, Shakopee, Richfield, Cottage Grove, Roseville, Inver Grove Heights, Andover, Brooklyn Center, Savage, Oakdale, Fridley, Winona, Shoreview, Ramsey, Owatonna, Chanhassen, Prior Lake, White Bear Lake, Chaska, Austin, Elk River, Champlin, Faribault, Rosemount, Crystal, Farmington, Hastings, New Brighton, etc.
And of course, any products related with "Thermionic valve" in Mississippi can be sent to Jackson, Gulfport, Southaven, Hattiesburg, Biloxi, Meridian, Tupelo, Greenville, Olive Branch, Horn Lake, Clinton, Pearl, Ridgeland, Starkville, Columbus, Vicksburg, Pascagoula, Clarksdale, Oxford, Laurel, Gautier, Ocean Springs, Madison, Brandon, Greenwood, Cleveland, Natchez, Long Beach, Corinth, Hernando, Moss Point, McComb, Canton, Carriere, Grenada, Brookhaven, Indianola, Yazoo City, West Point, Picayune, Petal.
No doubt, the products by request "Thermionic valve" in Missouri can be shipped to such cities as Kansas City, St. Louis, Springfield, Independence, Columbia, Lee’s Summit, O’Fallon, St. Joseph, St. Charles, Blue Springs, St. Peters, Florissant, Joplin, Chesterfield, Jefferson City, Cape Girardeau, Oakville, Wildwood, University City, Ballwin, Raytown, Liberty, Wentzville, Mehlville, Kirkwood, Maryland Heights, Hazelwood, Gladstone, Grandview, Belton, Webster Groves, Sedalia, Ferguson, Arnold, Affton and smaller towns.
And today the products by request "Thermionic valve" in Montana can be shipped to Billings, Missoula, Great Falls, Bozeman, Butte, Helena, Kalispell, Havre, Anaconda, Miles City, Belgrade, Livingston, Laurel, Whitefish, Lewistown, Sidney and smaller towns.
And any things related with "Thermionic valve" in Nebraska can be sent to Omaha, Lincoln, Bellevue, Grand Island, Kearney, Fremont, Hastings, Norfolk, North Platte, Papillion, Columbus, La Vista, Scottsbluff, South Sioux City, Beatrice, Lexington and smaller towns.
Undoubtedly, the products by request "Thermionic valve" in Nevada can be shipped to such cities as Las Vegas, Henderson, Reno, North Las Vegas, Sparks, Carson City, Fernley, Elko, Mesquite, Boulder City, Fallon, Winnemucca, West Wendover, Ely, Yerington, Carlin, Lovelock, Wells, Caliente and smaller towns.
As always, the goods by request "Thermionic valve" in New Hampshire can be shipped to Manchester, Nashua, Concord, Derry, Dover, Rochester, Salem, Merrimack, Hudson, Londonderry, Keene, Bedford, Portsmouth, Goffstown, Laconia, Hampton, Milford, Durham, Exeter, Windham, Hooksett, Claremont, Lebanon, Pelham, Somersworth, Hanover, Amherst, Raymond, Conway, Berlin, and other cities and towns.
Usually, the goods by your query "Thermionic valve" in New Jersey can be shipped to Newark, Jersey City, Paterson, Elizabeth, Edison, Woodbridge, Lakewood, Toms River, Hamilton, Trenton, Clifton, Camden, Brick, Cherry Hill, Passaic, Middletown, Union City, Old Bridge, Gloucester Township, East Orange, Bayonne, Franklin, North Bergen, Vineland, Union, Piscataway, New Brunswick, Jackson, Wayne, Irvington, Parsippany-Troy Hills, Howell, Perth Amboy, Hoboken, Plainfield, West New York, Washington Township, East Brunswick, Bloomfield, West Orange, Evesham, Bridgewater, South Brunswick, Egg Harbor, Manchester, Hackensack, Sayreville, Mount Laurel, Berkeley, North Brunswick, and so on.
Of course, the goods named "Thermionic valve" in New Mexico can be shipped to Albuquerque, Las Cruces, Rio Rancho, Santa Fe, Roswell, Farmington, South Valley, Clovis, Hobbs, Alamogordo, Carlsbad, Gallup, Deming, Los Lunas, Chaparral, Sunland Park, Las Vegas, Portales, Los Alamos, North Valley, Artesia, Lovington, Silver City, Española.
Naturally, the products related to the term "Thermionic valve" in New York can be delivered to the following cities: New York, Buffalo, Rochester, Yonkers, Syracuse, Albany, New Rochelle, Mount Vernon, Schenectady, Utica, White Plains, Troy, Niagara Falls, Binghamton, Rome, Long Beach, Poughkeepsie, North Tonawanda, Jamestown, Ithaca, Elmira, Newburgh, Middletown, Auburn, Watertown, Glen Cove, Saratoga Springs, Kingston, Peekskill, Lockport, Plattsburgh, Cortland, Amsterdam, Oswego, Lackawanna, Cohoes, Rye, Gloversville, Beacon, Batavia, Tonawanda, Glens Falls, Olean, Oneonta, Geneva, Dunkirk, Fulton, Oneida, Corning, Ogdensburg, Canandaigua, Watervliet...
Usually, the goods related with "Thermionic valve" in North Carolina can be purchased if you live in Charlotte, Raleigh, Greensboro, Durham, Winston-Salem, Fayetteville, Cary, Wilmington, High Point, Greenville, Asheville, Concord, Gastonia, Jacksonville, Chapel Hill, Rocky Mount, Huntersville, Burlington, Wilson, Kannapolis, Apex, Hickory, Wake Forest, Indian Trail, Mooresville, Goldsboro, Monroe, Salisbury, Holly Springs, Matthews, New Bern, Sanford, Cornelius, Garner, Thomasville, Statesville, Asheboro, Mint Hill, Fuquay-Varina, Morrisville, Kernersville, Lumberton, Kinston, Carrboro, Havelock, Shelby, Clemmons, Lexington, Clayton, Boone, and other cities and towns.
It goes without saying that the goods related with "Thermionic valve" in North Dakota can be received in such cities as Fargo, Bismarck, Grand Forks, Minot, West Fargo, Williston, Dickinson, Mandan, Jamestown, Wahpeton, Devils Lake, Watford City, Valley City, Grafton, Lincoln, Beulah, Rugby, Stanley, Horace, Casselton, New Town, Hazen, Bottineau, Lisbon, Carrington, etc.
And the goods by request "Thermionic valve" in Ohio can be received in Columbus, Cleveland, Cincinnati, Toledo, Akron, Dayton, Parma, Canton, Youngstown, Lorain, Hamilton, Springfield, Kettering, Elyria, Lakewood, Cuyahoga Falls, Euclid, Middletown, Mansfield, Newark, Mentor, Cleveland Heights, Beavercreek, Strongsville, Fairfield, Dublin, Warren, Findlay, Lancaster, Lima, Huber Heights, Marion, Westerville, Reynoldsburg, Grove City, Stow, Delaware, Brunswick, Upper Arlington, Gahanna, Westlake, North Olmsted, Fairborn, Massillon, Mason, North Royalton, Bowling Green, North Ridgeville, Kent, Garfield Heights, etc.
As you know, the goods related with "Thermionic valve" in Oklahoma can be received in Oklahoma City, Tulsa, Norman, Broken Arrow, Lawton, Edmond, Moore, Midwest City, Enid, Stillwater, Muskogee, Bartlesville, Owasso, Shawnee, Yukon, Ardmore, Ponca City, Bixby, Duncan, Del City, Jenks, Sapulpa, Mustang, Sand Springs, Bethany, Altus, Claremore, El Reno, McAlester, Ada, Durant, Tahlequah, Chickasha, Miami, Glenpool, Elk City, Woodward, Okmulgee, Choctaw, Weatherford, Guymon, Guthrie, Warr Acres and smaller towns.
As always, the goods related with "Thermionic valve" in Oregon can be delivered to Portland, Salem, Eugene, Gresham, Hillsboro, Beaverton, Bend, Medford, Springfield, Corvallis, Albany, Tigard, Lake Oswego, Keizer, Grants Pass, Oregon City, McMinnville, Redmond, Tualatin, West Linn, Woodburn, Forest Grove, Newberg, Wilsonville, Roseburg, Klamath Falls, Ashland, Milwaukie, Sherwood, Happy Valley, Central Point, Canby, Hermiston, Pendleton, Troutdale, Lebanon, Coos Bay, The Dalles, Dallas, St. Helens, La Grande, Cornelius, Gladstone, Ontario, Sandy, Newport, Monmouth.
No need to say, any things related with "Thermionic valve" in Pennsylvania can be sent to Philadelphia, Pittsburgh, Allentown, Erie, Reading, Scranton, Bethlehem, Lancaster, Harrisburg, Altoona, York, Wilkes-Barre, Chester, Williamsport, Easton, Lebanon, Hazleton, New Castle, Johnstown, McKeesport, Hermitage, Greensburg, Pottsville, Sharon, Butler, Washington, Meadville, New Kensington, Coatesville, St. Marys, Lower Burrell, Oil City, Nanticoke, Uniontown, and other cities and towns.
Naturally, the goods named "Thermionic valve" in Rhode Island can be bought in Providence, Warwick, Cranston, Pawtucket, East Providence, Woonsocket, Coventry, Cumberland, North Providence, South Kingstown, West Warwick, Johnston, North Kingstown, Newport, Bristol, Westerly, Smithfield, Lincoln, Central Falls, Portsmouth, Barrington, Middletown, Burrillville, Narragansett, Tiverton, East Greenwich, North Smithfield, Warren, Scituate, and other cities and towns.
No need to say, any products related with "Thermionic valve" in South Carolina can be received in such cities as Columbia, Charleston, North Charleston, Mount Pleasant, Rock Hill, Greenville, Summerville, Sumter, Hilton Head Island, Spartanburg, Florence, Goose Creek, Aiken, Myrtle Beach, Anderson, Greer, Mauldin, Greenwood, North Augusta, Easley, Simpsonville, Hanahan, Lexington, Conway, West Columbia, North Myrtle Beach, Clemson, Orangeburg, Cayce, Bluffton, Beaufort, Gaffney, Irmo, Fort Mill, Port Royal, Forest Acres, Newberry, and other cities and towns.
And today any products related with "Thermionic valve" in South Dakota can be delivered to the following cities: Sioux Falls, Rapid City, Aberdeen, Brookings, Watertown, Mitchell, Yankton, Pierre, Huron, Spearfish, Vermillion, etc.
As you know, any products related with "Thermionic valve" in Tennessee can be shipped to such cities as Memphis, Nashville, Knoxville, Chattanooga, Clarksville, Murfreesboro, Franklin, Jackson, Johnson City, Bartlett, Hendersonville, Kingsport, Collierville, Smyrna, Cleveland, Brentwood, Germantown, Columbia, Spring Hill, La Vergne, Gallatin, Cookeville, Mount Juliet, Lebanon, Morristown, Oak Ridge, Maryville, Bristol, Farragut, Shelbyville, East Ridge, Tullahoma, etc.
As you know, the products by request "Thermionic valve" in Texas can be bought in Houston, San Antonio, Dallas, Austin, Fort Worth, El Paso, Arlington, Corpus Christi, Plano, Laredo, Lubbock, Garland, Irving, Amarillo, Grand Prairie, Brownsville, McKinney, Frisco, Pasadena, Mesquite, Killeen, McAllen, Carrollton, Midland, Waco, Denton, Abilene, Odessa, Beaumont, Round Rock, The Woodlands, Richardson, Pearland, College Station, Wichita Falls, Lewisville, Tyler, San Angelo, League City, Allen, Sugar Land, Edinburg, Mission, Longview, Bryan, Pharr, Baytown, Missouri City, Temple, Flower Mound, New Braunfels, North Richland Hills, Conroe, Victoria, Cedar Park, Harlingen, Atascocita, Mansfield, Georgetown, San Marcos, Rowlett, Pflugerville, Port Arthur, Spring, Euless, DeSoto, Grapevine, Galveston, and other cities.
It goes without saying that the products by request "Thermionic valve" in Utah can be shipped to Salt Lake City, West Valley City, Provo, West Jordan, Orem, Sandy, Ogden, St. George, Layton, Taylorsville, South Jordan, Logan, Lehi, Murray, Bountiful, Draper, Riverton, Roy, Spanish Fork, Pleasant Grove, Cottonwood Heights, Tooele, Springville, Cedar City, Midvale. And, of course, Kaysville, Holladay, American Fork, Clearfield, Syracuse, South Salt Lake, Herriman, Eagle Mountain, Clinton, Washington, Payson, Farmington, Brigham City, Saratoga Springs, North Ogden, South Ogden, North Salt Lake, Highland, Centerville, Hurricane, Heber City, West Haven, Lindon...
As always, the goods by your query "Thermionic valve" in Vermont can be delivered to the following cities: Burlington, South Burlington, Rutland, Barre, Montpelier, Winooski, St. Albans, Newport, Vergennes, etc.
It goes without saying that the products by request "Thermionic valve" in Virginia can be purchased if you live in Virginia Beach, Norfolk, Chesapeake, Richmond, Newport News, Alexandria, Hampton, Roanoke, Portsmouth, Suffolk, Lynchburg, Harrisonburg, Charlottesville, Danville, Manassas, Petersburg, Fredericksburg, Winchester, Salem, Staunton, Fairfax, Hopewell, Waynesboro, Colonial Heights, Radford, Bristol, Manassas Park, Williamsburg, Falls Church, Martinsville, Poquoson, and other cities and towns.
Of course, the products related to the term "Thermionic valve" in Washington can be delivered to the following cities: Seattle, Spokane, Tacoma, Vancouver, Bellevue, Kent, Everett, Renton, Federal Way, Yakima, Spokane Valley, Kirkland, Bellingham, Kennewick, Auburn, Pasco, Marysville, Lakewood, Redmond, Shoreline, Richland, Sammamish, Burien, Olympia, Lacey. It is also available for the people living in Edmonds, Puyallup, Bremerton, Lynnwood, Bothell, Longview, Issaquah, Wenatchee, Mount Vernon, University Place, Walla Walla, Pullman, Des Moines, Lake Stevens, SeaTac, Maple Valley, Mercer Island, Bainbridge Island, Oak Harbor, Kenmore, Moses Lake, Camas, Mukilteo, Mountlake Terrace, Tukwila, and other cities.
And the products related to the term "Thermionic valve" in West Virginia can be bought in Charleston, Huntington, Morgantown, Parkersburg, Wheeling, Weirton, Fairmont, Martinsburg, Beckley, Clarksburg, South Charleston, St. Albans, Vienna, Bluefield, etc.
No need to say, the goods by request "Thermionic valve" in Wisconsin can be delivered to the following cities: Milwaukee, Madison, Green Bay, Kenosha, Racine, Appleton, Waukesha, Oshkosh, Eau Claire, Janesville, West Allis, La Crosse, Sheboygan, Wauwatosa, Fond du Lac, New Berlin, Wausau. It's also available for those who live in Brookfield, Beloit, Greenfield, Franklin, Oak Creek, Manitowoc, West Bend, Sun Prairie, Superior, Stevens Point, Neenah, Fitchburg, Muskego, Watertown, De Pere, Mequon, South Milwaukee, Marshfield, and other cities.
As always, the products related to the term "Thermionic valve" in Wyoming can be delivered to the following cities: Cheyenne, Casper, Laramie, Gillette, Rock Springs, Sheridan, Green River, Evanston, Riverton, Jackson, Cody, Rawlins, Lander, Torrington, Powell, Douglas, Worland, etc.
Canada Delivery, Shipping to Canada
And of course, the goods related with "Thermionic valve" in Canada can be purchased if you live in Toronto, Montreal, Calgary, Ottawa, Edmonton, Mississauga, Winnipeg, Vancouver, Brampton, Hamilton, Quebec City, Surrey, Laval, Halifax, London, Markham, Vaughan, Gatineau, Longueuil, Burnaby, Saskatoon, Kitchener, Windsor, Regina, Richmond, Richmond Hill.
And other cities and towns, such as Oakville, Burlington, Greater Sudbury, Sherbrooke, Oshawa, Saguenay, Lévis, Barrie, Abbotsford, St. Catharines, Trois-Rivières, Cambridge, Coquitlam, Kingston, Whitby, Guelph, Kelowna, Saanich, Ajax, Thunder Bay, Terrebonne, St. John's, Langley, Chatham-Kent, Delta.
And also in Waterloo, Cape Breton, Brantford, Strathcona County, Saint-Jean-sur-Richelieu, Red Deer, Pickering, Kamloops, Clarington, North Vancouver, Milton, Nanaimo, Lethbridge, Niagara Falls, Repentigny, Victoria, Newmarket, Brossard, Peterborough, Chilliwack, Maple Ridge, Sault Ste. Marie, Kawartha Lakes, Sarnia, Prince George.
As well as in Drummondville, Saint John, Moncton, Saint-Jérôme, New Westminster, Wood Buffalo, Granby, Norfolk County, St. Albert, Medicine Hat, Caledon, Halton Hills, Port Coquitlam, Fredericton, Grande Prairie, North Bay, Blainville, Saint-Hyacinthe, Aurora, Welland, Shawinigan, Dollard-des-Ormeaux, Belleville, North Vancouver, and other cities.
Actually, any things related with "Thermionic valve" can be shipped to any place in Canada, including Ontario, Quebec, British Columbia, Alberta, Manitoba, Saskatchewan, Nova Scotia, New Brunswick, Newfoundland and Labrador, and Prince Edward Island.
UK Delivery, Shipping to the United Kingdom
As always, any things related with "Thermionic valve" in the United Kingdom can be sent to London, Birmingham, Leeds, Glasgow, Sheffield, Bradford, Edinburgh, Liverpool, Manchester, Bristol, Wakefield, Cardiff, Coventry, Nottingham, Leicester, Sunderland, Belfast, Newcastle upon Tyne, Brighton, Hull, Plymouth, Stoke-on-Trent.
The delivery is also available in Wolverhampton, Derby, Swansea, Southampton, Salford, Aberdeen, Westminster, Portsmouth, York, Peterborough, Dundee, Lancaster, Oxford, Newport, Preston, St Albans, Norwich, Chester, Cambridge, Salisbury, Exeter, Gloucester. The delivery is also available in Lisburn, Chichester, Winchester, Londonderry, Carlisle, Worcester, Bath, Durham, Lincoln, Hereford, Armagh, Inverness, Stirling, Canterbury, Lichfield, Newry, Ripon, Bangor, Truro, Ely, Wells, St. Davids...
Basically, the goods by your query "Thermionic valve" can be shipped to any place in the UK, including England, Scotland, Wales, and Northern Ireland.
Ireland Delivery, Shipping to Ireland
Undoubtedly, the goods by your query "Thermionic valve" in Ireland can be sent to Dublin, Cork, Limerick, Galway, Waterford, Drogheda, Dundalk, Swords, Bray, Navan, Ennis, Kilkenny, Tralee, Carlow, Newbridge, Naas, Athlone, Portlaoise, Mullingar, Wexford, Balbriggan, Letterkenny, Celbridge, Sligo. It is also available for the people living in Clonmel, Greystones, Malahide, Leixlip, Carrigaline, Tullamore, Killarney, Arklow, Maynooth, Cobh, Castlebar, Midleton, Mallow, Ashbourne, Ballina, Laytown-Bettystown-Mornington, Enniscorthy, Wicklow, Tramore, Cavan, etc.
Generally, the goods by request "Thermionic valve" can be shipped to any place in Ireland, including Leinster, Ulster, Munster, and Connacht.
Australia Delivery, Shipping to Australia
And of course, the goods named "Thermionic valve" in Australia can be shipped to Sydney, Melbourne, Brisbane, Perth, Adelaide, Gold Coast, Tweed Heads, Newcastle, Maitland, Canberra, Queanbeyan, Sunshine Coast, Wollongong, Hobart, Geelong, Townsville, Cairns, Darwin, Toowoomba, Ballarat, Bendigo, Albury, Wodonga, Launceston, Mackay.
The delivery is also available in Rockhampton, Bunbury, Bundaberg, Coffs Harbour, Wagga Wagga, Hervey Bay, Mildura, Wentworth, Shepparton, Mooroopna, Gladstone, Tannum Sands, Port Macquarie, Tamworth, Traralgon, Morwell, Orange, Geraldton, Bowral, Mittagong, Dubbo, Busselton, Bathurst, Nowra, Bomaderry, Warrnambool, Albany, Warragul, Drouin, Kalgoorlie, Boulder, Devonport.
Actually, the products by request "Thermionic valve" can be shipped to any place in Australia, including New South Wales, Victoria, Queensland, Western Australia, South Australia, Tasmania, Australian Capital Territory, and Northern Territory.
New Zealand Delivery, Shipping to New Zealand
And the products by request "Thermionic valve" in New Zealand can be purchased if you live in Auckland, Wellington, Christchurch, Hamilton, Tauranga, Napier-Hastings, Dunedin, Lower Hutt, Palmerston North, Nelson, Rotorua, New Plymouth, Whangarei, Invercargill, Whanganui, Gisborne, Porirua, Invercargill, Nelson, Upper Hutt, Gisborne, Blenheim, Pukekohe, Timaru, Taupo...
In other words, the goods related with "Thermionic valve" can be shipped to any place in New Zealand, including North Island, South Island, Waiheke Island, and smaller islands.
It goes without saying thatany products related withcan be received inAnd, of course,.
Abkhazia: Gagra, Gudauta, Lake Ritsa, New Athos, Ochamchire, Pitsunda, Sukhumi, Tsandryphsh, etc.
Afghanistan: Herat, Jalalabad, Kabul, Kandahar, Kunduz, Mazar-i-Sharif, Taloqan, etc.
Albania: Berat, Butrint, Dhërmi, Durrës, Gjirokastër, Himarë, Korçë, Pogradec, Qeparo, Sarandë, Shkodër, Tirana, Velipojë, Vlorë, etc.
Algeria: Algiers, Oran, etc.
American Virgin Islands: Charlotte Amalie, etc.
Andorra: Andorra la Vella, Arinsal, El Pas de la Casa, Encamp, Grandvalira, Ordino, Pal, Soldeu, Vallnord, etc.
Angola: Benguela, Luanda, etc.
Anguilla: The Valley, West End, etc.
Antigua and Barbuda: Saint John’s, etc.
Argentina: Buenos Aires, Colón, Córdoba, El Calafate, La Plata, Los Glaciares, Mar del Plata, Mendoza, Pinamar, Puerto Iguazú, Puerto Madryn, Rosario, Salta, San Carlos de Bariloche, San Martín de los Andes, San Miguel de Tucumán, San Rafael, Tandil, Tierra del Fuego, Ushuaia, Villa Carlos Paz, Villa Gesell, Villa La Angostura, Villa de Merlo, etc.
Armenia: Dilijan, Etchmiadzin, Goris, Gyumri, Jermuk, Sevan, Stepanavan, Tsaghkadzor, Vagharshapat, Vanadzor, Yeghegnadzor, Yerevan, etc.
Aruba: Oranjestad, etc.
Australia: Adelaide, Brisbane, Byron Bay, Cairns, Canberra, Darwin, Gold Coast, Great Barrier Reef, Hobart, Melbourne, Perth, Sydney, Tasmania, etc.
Austria: Abtenau, Alpbach, Austrian Alps, Bad Gastein, Bad Hofgastein, Bad Kleinkirchheim, Dürnstein, Flachau, Fugen, Graz, Innsbruck, Ischgl, Kaprun, Kitzbühel, Klagenfurt, Kufstein, Lech, Leogang, Lienz, Linz, Maria Alm, Mayrhofen, Neustift im Stubaital, Obergurgl, Saalbach-Hinterglemm, Saalfelden, Salzburg, Schladming, Seefeld, Serfaus, St. Anton, St. Johann im Pongau, Sölden, Tux, Tyrol, Vienna, Villach, Wachau, Wagrain, Zell am See, etc.
Azerbaijan: Baku, Ganja, Lankaran, Quba, Qusar, Shahdag, Sheki, Stepanakert, etc.
Bahamas: Andros, Eleuthera, Exuma, Freeport, Grand Bahama, Nassau, New Providence, Paradise Island, etc.
Bahrain: Manama, etc.
Bangladesh: Chittagong, Cox's Bazar, Dhaka, Khulna, Narayanganj, Rajshahi, Sylhet, etc.
Barbados: Bridgetown, etc.
Belarus: Babruysk, Białowieża Forest, Brest Belarus, Gomel, Grodno, Lahoysk, Maladzyechna, Minsk, Mogilev, Nesvizh, Pinsk, Silichi, Vitebsk, etc.
Belgium: Antwerp, Ardennes, Blankenberge, Bouillon, Bruges, Brussels, Charleroi, De Haan, De Panne, Durbuy, Flanders, Ghent, Hasselt, Kortrijk, Leuven, Liège, Namur, Nieuwpoort, Ostend, Spa, Ypres, Zeebrugge, etc.
Belize: Ambergris Caye, Belize City, Caye Caulker, Placencia, San Pedro, etc.
Benin: Cotonou, etc.
Bermuda: Hamilton, etc.
Bhutan: Paro, Thimphu, etc.
Bolivia: Cochabamba, El Alto, La Paz, Oruro, Quillacollo, Santa Cruz de la Sierra, Sucre, Uyuni, etc.
Bosnia and Herzegovina: Banja Luka, Bihać, Jahorina, Medjugorje, Mostar, Neum, Sarajevo, Travnik, Trebinje, etc.
Botswana: Gaborone, Maun, etc.
Brazil: Amazon River, Amazonia, Angra dos Reis, Arraial do Cabo, Atlantic Forest, Balneário Camboriú, Belo Horizonte, Belém, Bombinhas, Brasília, Búzios, Cabo Frio, Camaçari, Campinas, Campos do Jordão, Caraguatatuba, Copacabana, Costa do Sauípe, Curitiba, Duque de Caxias, Fernando de Noronha, Florianópolis, Fortaleza, Foz do Iguaçu, Goiânia, Gramado, Guarujá, Guarulhos, Iguazu Falls, Ilha Grande, Ilhabela, Ilhéus, Ipanema, Itacaré, Maceió, Manaus, Morro de São Paulo, Natal, Niterói, Osasco, Ouro Preto, Paraty, Petrópolis, Porto Alegre, Porto Seguro, Praia do Forte, Recife, Ribeirão Preto, Rio de Janeiro, Salvador, Santos, São Gonçalo, São José dos Campos, São Luís, São Paulo, São Sebastião, Trancoso, Ubatuba, Vila do Abraão, etc.
British Virgin Islands: Tortola, etc.
Brunei: Bandar Seri Begawan, etc.
Bulgaria: Albena, Balchik, Bansko, Blagoevgrad, Borovets, Burgas, Chernomorets, Dobrinishte, Golden Sands, Kiten, Koprivshtitsa, Lozenets, Nesebar, Obzor, Pamporovo, Pirin, Pleven, Plovdiv, Pomorie, Primorsko, Ravda, Razlog, Rila, Ruse, Samokov, Sandanski, Shumen, Sofia, Sozopol, Stara Zagora, Sunny Beach, Sveti Vlas, Tsarevo, Varna, Veliko Tarnovo, etc.
Burkina Faso: Bobo-Dioulasso, Ouagadougou, etc.
Burundi: Bujumbura, etc.
Cambodia: Angkor, Battambang, Kampot, Kep, Phnom Penh, Siem Reap, Sihanoukville, etc.
Cameroon: Bafoussam, Bamenda, Douala, Garoua, Kribi, Limbe, Maroua, Yaoundé, etc.
Canada: Alberta, Banff, Brampton, British Columbia, Burnaby, Calgary, Charlottetown, Edmonton, Fort McMurray, Gatineau, Halifax, Hamilton, Jasper, Kamloops, Kelowna, Kingston, Kitchener, Laval, London, Longueuil, Manitoba, Markham, Mississauga, Moncton, Mont-Tremblant, Montreal, Nanaimo, New Brunswick, Niagara Falls, Niagara on the Lake, Nova Scotia, Ontario, Ottawa, Prince Edward Island, Quebec, Regina, Richmond, Saskatchewan, Saskatoon, Surrey, Toronto, Vancouver, Vaughan, Victoria, Whistler, Whitehorse, Windsor, Winnipeg, Yukon, etc.
Cape Verde: Boa Vista Cape Verde, Sal, etc.
Caribbean Netherlands:, etc.
Cayman Islands: George Town, Grand Cayman, West Bay, etc.
Chad: N'Djamena, etc.
Chile: Antofagasta, Arica, Atacama, Coquimbo, Easter Island, Hanga Roa, Iquique, La Serena, Patagonia, Pucón, Puerto Montt, Puerto Natales, Puerto Varas, Punta Arenas, San Pedro de Atacama, Santiago, Torres del Paine, Valdivia, Valparaíso, Villarrica, Viña del Mar, etc.
China: Anshun, Baishan, Baoding, Baoshan, Baotou, Beijing, Binzhou, Changchun, Changsha, Changzhi, Chengdu, Chongqing, Dali, Dalian, Datong, Dengfeng, Diqing, Dongguan, Emeishan, Foshan, Great Wall of China, Guangdong, Guangzhou, Guilin, Guiyang, Hainan, Hangzhou, Harbin, Honghe, Huashan, Huizhou, Jiangmen, Jiangxi, Jiaxing, Jilin, Jinan, Jincheng, Jingdezhen, Jinzhong, Jiujiang, Jiuzhaigou, Kunming, Langfang, Lanzhou, Laoshan, Leshan, Lhasa, Lianyungang, Lijiang, Linfen, Linyi, Luoyang, Lushan, Lüliang, Mianyang, Nanchang, Nanchong, Nanjing, Nantong, Ngawa, Ningbo, Qiandongnan, Qingdao, Qingyuan, Qinhuangdao, Qufu, Qujing, Rizhao, Sanya, Shanghai, Shangri-La, Shantou, Shanxi, Shaoguan, Shaolin, Shaoxing, Shenyang, Shenzhen, Shigatse, Shijiazhuang, Sichuan, Suzhou, Tai'an, Taiyuan, Taizhou Jiangsu, Tangshan, Tianjin, Tibet, Weifang, Weihai, Wuhan, Wulingyuan, Wutai, Wuxi, Xi'an, Xiamen, Xinzhou, Xishuangbanna, Ya'an, Yanbian, Yangtze, Yangzhou, Yantai, Yellow River, Yibin, Yinchuan, Yiwu, Yuncheng, Yunnan, Zhangjiajie, Zhanjiang, Zhejiang, Zhengzhou, Zhongshan, Zhongwei, Zhoushan, Zhuhai, Zunyi, etc.
Colombia: Barranquilla, Bogotá, Bucaramanga, Cali, Cartagena, Medellín, Pereira, San Andrés, Santa Marta, Villa de Leyva, Villavicencio, etc.
Comoros: Moroni, etc.
Costa Rica: Alajuela, Jacó, La Fortuna, Manuel Antonio, Monteverde, Puerto Viejo de Talamanca, Puntarenas, Quepos, San José, Santa Teresa, Tamarindo, Tortuguero, etc.
Croatia: Baška Voda, Baška, Bibinje, Biograd na Moru, Bol, Brač, Brela, Cavtat, Cres, Dalmatia, Fažana, Hvar, Istria, Ičići, Korčula, Krk, Lopud, Lovran, Lošinj, Makarska, Mali Lošinj, Malinska, Medulin, Mlini, Nin, Novi Vinodolski, Novigrad, Omiš, Opatija, Orebić, Pag, Podstrana, Poreč, Pula, Rab, Rabac, Rijeka, Rovinj, Split, Stari Grad, Sukošan, Supetar, Trogir, Tučepi, Umag, Vrsar, Zadar, Zagreb, Čiovo, Šibenik, etc.
Cuba: Baracoa, Camagüey, Cayo Coco, Cayo Largo, Cayo Santa María, Cienfuegos, Guantánamo, Havana, Holguín, Pinar del Río, Remedios Cuba, Sancti Spíritus, Santa Clara Cuba, Santiago de Cuba, Trinidad, Varadero, Viñales, etc.
Curaçao: Sint Michiel, Westpunt, Willemstad, etc.
Cyprus: Ayia Napa, Coral Bay Cyprus, Famagusta, Kouklia, Kyrenia, Larnaca, Limassol, Nicosia, Paphos, Paralimni, Peyia, Pissouri, Polis, Protaras, etc.
Czech Republic: Bohemia, Brno, Děčín, Frymburk, Frýdek-Místek, Harrachov, Hradec Králové, Jihlava, Karlovy Vary, Kladno, Krkonoše, Kutná Hora, Liberec, Marienbad, Mikulov, Mladá Boleslav, Mělník, Olomouc, Ostrava, Pardubice, Plzeň, Poděbrady, Prague, Teplice, Třeboň, Zlín, Znojmo, Ústí nad Labem, České Budějovice, Český Krumlov, Špindlerův Mlýn, etc.
Democratic Republic of the Congo: Kinshasa, etc.
Denmark: Aalborg, Aarhus, Billund, Copenhagen, Ebeltoft, Esbjerg, Frederikshavn, Greenland, Helsingør, Herning, Hirtshals, Hjørring, Holstebro, Jutland, Odense, Silkeborg, Skagen, Skive, Sønderborg, Vejle, Viborg, etc.
Djibouti: Djibouti City, etc.
Dominican Republic: Boca Chica, Bávaro, Cabarete, La Romana, Las Terrenas, Puerto Plata, Punta Cana, Santiago de los Caballeros, Santo Domingo, Sosúa, etc.
East Timor: Dili, etc.
Ecuador: Baños, Cuenca, Galápagos Islands, Guayaquil, Manta, Otavalo, Puerto Ayora, Puerto López, Quito, Salinas, etc.
Egypt: Abu Simbel, Al Qusair, Alexandria, Aswan, Cairo, Dahab, El Alamein, El Gouna, El Hadaba, Faiyum, Giza, Hurghada, Luxor, Marsa Alam, Mersa Matruh, Naama Bay, Nabq Bay, Nile, Nuweiba, Port Said, Red Sea, Safaga, Sahl Hasheesh, Scharm asch-Schaich, Sharks Bay, Sinai, Suez, Taba, Valley of the Kings, etc.
El Salvador: La Libertad, San Salvador, etc.
Equatorial Guinea: Malabo, etc.
Eritrea: Asmara, etc.
Estonia: Haapsalu, Kuressaare, Narva, Pärnu, Saaremaa, Tallinn, Tartu, etc.
Ethiopia: Addis Ababa, Bahir Dar, Gondar, etc.
Falkland Islands: Stanley, etc.
Faroe Islands: Sørvágur, Tórshavn, etc.
Fiji: Nadi, Suva, Viti Levu Island, etc.
Finland: Espoo, Helsinki, Imatra, Joensuu, Jyväskylä, Jämsä, Kotka, Kuopio, Kuusamo, Lahti, Lapland, Lappeenranta, Levi, Mariehamn, Mikkeli, Moomin World, Naantali, Nilsiä, Oulu, Pori, Porvoo, Pyhätunturi, Rovaniemi, Rukatunturi, Saariselkä, Saimaa, Tampere, Turku, Vaasa, Vantaa, Vuokatti, Åland Islands, etc.
France: Aix-en-Provence, Ajaccio, Alsace, Annecy, Antibes, Aquitaine, Arles, Avignon, Avoriaz, Bayonne, Beaune, Besançon, Biarritz, Bonifacio, Bordeaux, Briançon, Brittany, Burgundy, Cabourg, Cagnes-sur-Mer, Calais, Calvi, Canet-en-Roussillon, Cannes, Carcassonne, Cassis, Chambéry, Chamonix, Colmar, Corsica, Courchevel, Deauville, Dijon, Dunkirk, French Alps, French Riviera, Fréjus, Grenoble, Honfleur, La Ciotat, La Plagne, La Rochelle, Le Grau-du-Roi, Le Havre, Les Arcs, Les Gets, Les Menuires, Lille, Limoges, Lourdes, Lyon, Mandelieu-la-Napoule, Marseille, Megève, Menton, Montpellier, Morzine, Méribel, Nantes, Narbonne, Nice, Nord-Pas-de-Calais, Normandy, Nîmes, Paradiski, Paris, Pas-de-Calais, Perpignan, Portes du Soleil, Porto-Vecchio, Provence, Périgueux, Reims, Rhône-Alpes, Rouen, Saint-Gervais-les-Bains, Saint-Malo, Saint-Martin-de-Belleville, Saint-Rémy-de-Provence, Saint-Tropez, Saintes-Maries-de-la-Mer, Strasbourg, The Three Valleys, Tignes, Toulouse, Trouville-sur-Mer, Val Thorens, Val-d'Isère, Versailles, Étretat, Île-de-France, etc.
French Guiana: Cayenne, Kourou, etc.
French Polynesia: Bora Bora, Mo'orea, Papeete, Tahiti, etc.
Gabon: Libreville, etc.
Gambia: Banjul, Serekunda, etc.
Georgia: Bakuriani, Batumi, Borjomi, Gori, Gudauri, Kobuleti, Kutaisi, Mestia, Mtskheta, Poti, Sighnaghi, Stepantsminda, Tbilisi, Telavi, Zugdidi, etc.
Germany: Aachen, Augsburg, Bad Birnbach, Bad Driburg, Bad Ems, Bad Füssing, Bad Godesberg, Bad Harzburg, Bad Homburg, Bad Kissingen, Bad Kreuznach, Bad Mergentheim, Bad Neuenahr-Ahrweiler, Bad Reichenhall, Bad Salzuflen, Bad Schandau, Baden-Baden, Baden-Württemberg, Bamberg, Bavaria, Berchtesgaden, Bergen auf Rügen, Berlin, Bernkastel-Kues, Bielefeld, Binz, Bochum, Bonn, Bottrop, Brandenburg, Braunlage, Braunschweig, Bremen, Bremerhaven, Brilon, Chemnitz, Cochem, Cologne, Cuxhaven, Dortmund, Dresden, Duisburg, Düsseldorf, Eisenach, Erfurt, Erlangen, Essen, Europa-Park, Flensburg, Frankfurt, Freiburg, Friedrichshafen, Fürth, Füssen, Garmisch-Partenkirchen, Gelsenkirchen, Glowe, Goslar, Görlitz, Göttingen, Hamburg, Hanover, Heidelberg, Heiligendamm, Heligoland, Hesse, Ingolstadt, Inzell, Karlsruhe, Kiel, Koblenz, Krefeld, Lake Constance, Leipzig, Lindau, Lower Saxony, Lübeck, Magdeburg, Mainz, Mannheim, Marburg, Mecklenburg-Vorpommern, Medebach, Monschau, Munich, Mönchengladbach, Mülheim an der Ruhr, Münster, Neuschwanstein Castle, Neuss, Norddeich, Norden, Norderney, North Rhine-Westphalia, Nuremberg, Oberhausen, Oberstdorf, Oldenburg, Olsberg, Osnabrück, Paderborn, Potsdam, Putbus, Quedlinburg, Rathen, Regensburg, Rhineland-Palatinate, Rostock, Rothenburg ob der Tauber, Ruhpolding, Rust, Rügen, Saarbrücken, Saarland, Sassnitz, Saxony, Saxony-Anhalt, Schleswig-Holstein, Schmallenberg, Schwerin, Schönau am Königsee, Sindelfingen, Solingen, Speyer, Stralsund, Stuttgart, Sylt, Thuringia, Travemünde, Trier, Ulm, Warnemünde, Weimar, Wernigerode, Westerland, Wiesbaden, Winterberg, Wolfsburg, Wuppertal, Würzburg, Xanten, Zingst, etc.
Ghana: Accra, Kumasi, etc.
Greece: Acharavi, Aegina, Afantou, Afytos, Agios Gordios, Andros, Arkadia, Athens, Cephalonia, Chania, Chaniotis, Chios, Corfu, Corinth, Crete, Cyclades, Dassia, Delphi, Dodecanese, Faliraki, Halkidiki, Heraklion, Hersonissos, Hydra, Ialysos, Ionian Islands, Kalamata, Kalavryta, Kalymnos, Kardamaina, Karpathos, Kassandra, Kastoria, Katerini, Kavos, Kefalos, Kokkari, Kos, Kriopigi, Laganas, Lefkada, Lemnos, Lesbos, Lindos, Loutraki, Marathokampos, Meteora, Mithymna, Monemvasia, Mount Athos, Mykonos, Mytilene, Nafplio, Naxos, Neos Marmaras, Paleokastritsa, Parga, Patmos, Patras, Pefkochori, Pefkos, Peloponnese, Polychrono, Poros, Pythagoreio, Rethymno, Rhodes, Samos, Samothrace, Santorini, Sidari, Sithonia, Sparta, Spetses, Sporades, Syros, Thasos, Thessaloniki, Tingaki, Zakynthos, etc.
Guadeloupe: Saint-François, etc.
Guam: Tamuning, Tumon, etc.
Guatemala: Antigua Guatemala, etc.
Guinea: Conakry, etc.
Guinea-Bissau: Bissau, etc.
Guyana: Georgetown, etc.
Haiti: Cap-Haitien, Port-au-Prince, etc.
Honduras: Roatán, Tegucigalpa, etc.
Hong Kong: Causeway Bay, Hong Kong Island, Kowloon, Mong Kok, New Territories, Repulse Bay, Tsim Sha Tsui, Wan Chai, etc.
Hungary: Budapest, Eger, Gyula, Hajdúszoboszló, Hévíz, Lake Balaton, Pécs, Siófok, Szeged, Zalakaros, etc.
Iceland: Akureyri, Blue Lagoon, Borgarnes, Egilsstaðir, Garðabær, Hafnarfjörður, Hveragerði, Höfn, Keflavík, Kópavogur, Reykjavik, Selfoss, Vík í Mýrdal, Ísafjörður, etc.
India: Agra, Ahmedabad, Ajmer, Allahabad, Amritsar, Andhra Pradesh, Assam, Aurangabad, Bangalore, Bhopal, Bikaner, Chandigarh, Chennai, Chhattisgarh, Darjeeling, Dehradun, Delhi, Dharamshala, Fatehpur Sikri, Gangtok, Goa, Gujarat, Gurgaon, Guwahati, Gwalior, Haridwar, Himachal Pradesh, Hyderabad, Indore, Jabalpur, Jaipur, Jaisalmer, Jalandhar, Jodhpur, Kanpur, Karnataka, Kerala, Khajuraho, Kochi, Kolhapur, Kolkata, Ladakh, Leh, Lucknow, Ludhiana, Madhya Pradesh, Madikeri, Madurai, Maharashtra, Manali, Mangalore, Mathura, Mount Abu, Mumbai, Munnar, Mussoorie, Mysore, Nagpur, Nainital, Nashik, Navi Mumbai, New Delhi, Noida, Ooty, Pachmarhi, Palakkad, Pune, Punjab, Pushkar, Raipur, Rajasthan, Ramnagar, Rishikesh, Sawai Madhopur, Shimla, Sikkim, Srinagar, Tamil Nadu, Thane, Thiruvananthapuram, Tirupati, Udaipur, Ujjain, Uttar Pradesh, Uttarakhand, Varanasi, Varkala, Vijayawada, Visakhapatnam, etc.
Indonesia: Bali, Balikpapan, Bandung, Batu, Bintan, Bogor, Borobudur, Denpasar, Jakarta, Java, Jimbaran, Kalimantan, Kuta, Lombok, Makassar, Malang, Mataram, Medan, Nusa Dua, Padang, Palembang, Pekanbaru, Sanur, Semarang, Seminyak, Sumatra, Surabaya, Surakarta, Ubud, Yogyakarta, etc.
Iran: Isfahan, Mashhad, Shiraz, Tehran, etc.
Iraq: Baghdad, Basra, Duhok, Erbil, Karbala, Sulaymaniyah, etc.
Ireland: Achill Island, Bray, Bundoran, Carlow, Clifden, Connemara, Cork, Dingle, Donegal, Doolin, Drogheda, Dublin, Dundalk, Ennis, Galway, Glendalough, Kenmare, Kilkenny, Killarney, Letterkenny, Limerick, Navan, Shannon, Swords, Tralee, Waterford, Westport, etc.
Isle of Man: Douglas, etc.
Israel: Acre, Amirim, Arad, Ashdod, Ashkelon, Bat Yam, Beersheba, Caesarea, Dead Sea, Eilat, Ein Bokek, Galilee, Golan Heights, Gush Dan, Haifa, Hermon, Herzliya, Jaffa, Jerusalem, Katzrin, Metula, Mitzpe Ramon, Nahariya, Nazareth, Netanya, Petah Tikva, Ramat Gan, Ramot, Rishon LeZion, Rosh Pinna, Safed, Sea of Galilee, Tel Aviv, Tiberias, Zikhron Ya'akov, etc.
Italy: Abano Terme, Abruzzo, Agrigento, Alassio, Alberobello, Alghero, Amalfi Coast, Aosta Valley, Apulia, Arezzo, Arona, Arzachena, Asciano, Ascoli Piceno, Assisi, Asti, Bardolino, Bari, Basilicata, Baveno, Bellagio, Bellaria-Igea Marina, Benevento, Bergamo, Bologna, Bolzano, Bordighera, Bormio, Bracciano, Brescia, Breuil-Cervinia, Brindisi, Cagliari, Calabria, Campania, Canazei, Caorle, Capri, Carrara, Castelnuovo Berardenga, Castiglion Fiorentino, Castiglione d'Orcia, Castiglione del Lago, Castiglione della Pescaia, Catania, Cefalù, Cervia, Cesenatico, Chianciano Terme, Chieti, Chioggia, Cinque Terre, Città della Pieve, Civitavecchia, Cortina d'Ampezzo, Cortona, Costa Smeralda, Courmayeur, Desenzano del Garda, Dolomites, Elba, Emilia-Romagna, Ercolano, Fasano, Fassa Valley, Ferrara, Finale Ligure, Fiumicino, Florence, Forte dei Marmi, Gaeta, Gallipoli, Genoa, Golfo Aranci, Greve in Chianti, Grosseto, Gubbio, Herculaneum, Imperia, Ischia, Italian Alps, Jesolo, L'Aquila, La Spezia, Lake Como, Lake Garda, Lake Maggiore, Lampedusa, Lazio, Lazise, Lecco, Lerici, Lido di Jesolo, Lignano Sabbiadoro, Liguria, Livigno, Livorno, Lombardy, Lucca, Madonna di Campiglio, Malcesine, Manarola, Mantua, Maratea, Massa, Matera, Menaggio, Merano, Messina, Mestre, Milan, Milazzo, Monopoli, Montalcino, Montecatini Terme, Montepulciano, Monterosso al Mare, Monza, Naples, Nardò, Novara, Olbia, Ortisei, Ostuni, Otranto, Padua, Palermo, Parma, Perugia, Pescara, Peschici, Peschiera del Garda, Piacenza, Piedmont, Pienza, Pisa, Pistoia, Pitigliano, Polignano a Mare, Pompeii, Porto Cervo, Porto Cesareo, Portoferraio, Portofino, Positano, Prato, Ragusa, Rapallo, Rapolano Terme, Ravenna, Riccione, Rimini, Riomaggiore, Riva del Garda, Rome, Salerno, San Casciano dei Bagni, San Gimignano, Sanremo, Sardinia, Savona, Sestriere, Sicily, Siena, Sinalunga, Siracusa, Sirmione, Sorrento, Sottomarina, Sperlonga, Stresa, Sëlva, Taormina, Taranto, Terracina, Tivoli, Torrita di Siena, Trani, Trapani, Trentino-Alto Adige, Trento, Treviso, Trieste, Tropea, Turin, Tuscany, Umbria, Urbino, Val Gardena, Veneto, Venice, Ventimiglia, Verbania, Vernazza, Verona, Vesuvius, Viareggio, Vicenza, Vieste, Viterbo, etc.
Ivory Coast: Abidjan, Assinie-Mafia, Bouaké, San-Pédro, Yamoussoukro, etc.
Jamaica: Kingston, Montego Bay, Negril, Ocho Rios, Port Antonio, Runaway Bay, etc.
Japan: Atami, Fujisawa, Fukuoka, Furano, Hakodate, Hakone, Hakuba, Hamamatsu, Hiroshima, Hokkaido, Ishigaki, Itō, Kagoshima, Kanagawa, Kanazawa, Karuizawa, Kawasaki, Kobe, Kutchan, Kyoto, Lake Suwa, Matsumoto, Miyakojima, Nagasaki, Nagoya, Naha, Nanjō, Nikkō, Okinawa, Onna, Osaka, Sapporo, Sendai, Shizuoka, Takayama, Tokyo, Yokohama, etc.
Jordan: Amman, Aqaba, Irbid, Jerash, Madaba, Petra, Sweimeh, Wadi Musa, Wadi Rum, Zarqa, etc.
Kazakhstan: Aktau, Aktobe, Almaty, Astana, Atyrau, Burabay, Karagandy, Kokshetau, Kostanay, Lake Balkhash, Oskemen, Pavlodar, Semey, Shymbulak, Shymkent, Taraz, etc.
Kenya: Kisumu, Lake Victoria, Masai Mara, Mombasa, Nairobi, Ukunda, etc.
Kiribati: South Tarawa, etc.
Kongo: Brazzaville, Pointe-Noire, etc.
Kosovo: Pristina, Prizren, etc.
Kuwait: Hawally, Kuwait City, Salmiya, etc.
Kyrgyzstan: Bishkek, Bosteri, Cholpon-Ata, Issyk Kul, Karakol, Osh, etc.
Laos: Luang Prabang, Vang Vieng, Vientiane, etc.
Latvia: Cēsis, Daugavpils, Jelgava, Jūrmala, Liepāja, Riga, Rēzekne, Sigulda, Ventspils, etc.
Lebanon: Baalbeck, Beirut, Byblos, Faraya, Jounieh, Mzaar Kfardebian, Tripoli, etc.
Lesotho: Maseru, etc.
Liberia: Monrovia, etc.
Libya: Benghazi, Tripoli, etc.
Liechtenstein: Schaan, Vaduz, etc.
Lithuania: Druskininkai, Kaunas, Klaipėda, Nida, Palanga, Panevėžys, Trakai, Vilnius, Šiauliai, Šventoji, etc.
Luxembourg: Differdange, Dudelange, Echternach, Esch-sur-Alzette, Luxembourg City, Vianden, etc.
Macedonia: Bitola, Mavrovo, Ohrid, Skopje, etc.
Madagascar: Antananarivo, etc.
Malawi: Blantyre, Lilongwe, etc.
Malaysia: Borneo, George Town, Ipoh, Johor Bahru, Johor, Kedah, Kota Bharu, Kota Kinabalu, Kuah, Kuala Lumpur, Kuala Terengganu, Kuantan, Kuching, Langkawi, Malacca, Penang, Putrajaya, Sabah, Sarawak, Selangor, Shah Alam, etc.
Maldives: Kaafu Atoll, Malé, etc.
Mali: Bamako, etc.
Malta: Birżebbuġa, Buġibba, Gozo, Gżira, Mellieħa, Paceville, Pembroke, Qawra, Sliema, St. Julian's, St. Paul's Bay, Valletta, etc.
Martinique: Fort-de-France, Les Trois-Îlets, Sainte-Luce, etc.
Mauritania: Mérida, Nouakchott, Puerto Escondido, Puerto Peñasco, etc.
Mauritius: Port Louis, etc.
Mexico: Acapulco, Akumal, Cabo San Lucas, Cancún, Chetumal, Chichen Itza, Chihuahua, Ciudad Juárez, Cozumel, Cuernavaca, Guadalajara, Guanajuato, Isla Mujeres, Los Cabos, Manzanillo, Mazatlán, Monterrey, Oaxaca, Playa del Carmen, Puebla, Puerto Aventuras, Puerto Morelos, Puerto Vallarta, Querétaro, Riviera Maya, San Cristóbal de las Casas, San Miguel de Allende, San Miguel de Cozumel, Tijuana, Tulum, etc.
Moldova: Bălți, Chișinău, Tiraspol, etc.
Monaco: Monte Carlo, etc.
Mongolia: Darkhan, Erdenet, Ulaanbaatar, etc.
Montenegro: Bar, Bečići, Bijela, Budva, Cetinje, Dobra Voda, Dobrota, Herceg Novi, Igalo, Kolašin, Kotor, Miločer, Nikšić, Perast, Petrovac, Podgorica, Prčanj, Sutomore, Sveti Stefan, Tivat, Ulcinj, Žabljak, etc.
Montserrat: Plymouth, etc.
Morocco: Agadir, Asilah, Casablanca, Chefchaouen, El Jadida, Essaouira, Fez, Marrakesh, Meknes, Merzouga, Mohammedia, Nador, Ouarzazate, Rabat, Tangier, Taroudant, Tinghir, Tétouan, etc.
Mozambique: Maputo, etc.
Myanmar: Mandalay, Naypyidaw, Nyaung Shwe, Yangon, etc.
Namibia: Rundu, Swakopmund, Walvis Bay, Windhoek, etc.
Nepal: Chitwan, Himalayas, Kathmandu, Lukla, Lumbini, Mount Everest, Nagarkot, Namche Bazaar, Patan, Pokhara, Tengboche, etc.
Netherlands: 's-Hertogenbosch, Alkmaar, Amersfoort, Amsterdam, Arnhem, Breda, Delft, Domburg, Dordrecht, Eindhoven, Groningen, Haarlem, Leiden, Maastricht, Nijmegen, Noordwijk, Rotterdam, Texel, The Hague, Utrecht, Valkenburg aan de Geul, Wijk aan Zee, Zandvoort, etc.
New Zealand: Auckland, Christchurch, Dunedin, Gisborne, Hamilton, Hastings, Invercargill, Kaikoura, Lower Hutt, Napier, Nelson, New Plymouth, North Island, Palmerston North, Porirua, Queenstown, Rotorua, South Island, Taupo, Tauranga, Waiheke Island, Wanaka, Wellington, Whangarei, etc.
Nicaragua: Granada, Managua, etc.
Nigeria: Abuja, Benin City, Calabar, Enugu, Ibadan, Ilorin, Jos, Kaduna, Lagos, Owerri, Port Harcourt, Uyo, etc.
North Korea: Pyongyang, etc.
Northern Mariana Islands: Saipan, etc.
Norway: Beitostølen, Bergen, Bodø, Gardermoen, Geilo, Geirangerfjord, Hardangerfjord, Hemsedal, Kirkenes, Kristiansand, Larvik, Lillehammer, Lofoten, Narvik, Nordland, Oslo, Sognefjord, Stavanger, Stryn, Svalbard, Tromsø, Trondheim, Ålesund, etc.
Oman: Muscat, Nizwa, Salalah, Seeb, etc.
Pakistan: Bhurban, Faisalabad, Islamabad, Karachi, Lahore, Peshawar, Rawalpindi, etc.
Palau: Koror, Peleliu, etc.
Palestine: Beit Sahour, Bethlehem, Hebron, Jenin, Jericho, Nablus, Ramallah, etc.
Panama: Bocas del Toro, etc.
Papua New Guinea: Port Moresby, etc.
Paraguay: Asunción, Ciudad Del Este, Encarnación, Panama City, etc.
Peru: Arequipa, Ayacucho, Cajamarca, Chiclayo, Cusco, Huancayo, Huanchaco, Huaraz, Ica, Iquitos, Lima, Machu Picchu, Máncora, Nazca, Ollantaytambo, Paracas, Pisco, Piura, Puerto Maldonado, Puno, Tacna, Tarapoto, Trujillo, Urubamba, etc.
Philippines: Angeles City, Antipolo, Bacolod, Bacoor, Baguio, Batangas, Bohol, Boracay, Cagayan de Oro, Calamba, Caloocan, Cebu, Coron, Dasmariñas, Davao, Dumaguete, El Nido, General Santos, Iloilo City, Kalibo, Lapu-Lapu City, Las Piñas, Luzon, Mactan, Makati, Mandaue, Manila, Marikina, Mindanao, Muntinlupa, Olongapo, Palawan, Panglao, Parañaque, Pasay, Pasig, Puerto Galera, Puerto Princesa, Quezon City, Tagaytay, Tagbilaran, Taguig, Valenzuela, Visayas, Zamboanga, etc.
Poland: Białka Tatrzańska, Białowieża Forest, Białystok, Bielsko-Biała, Bukowina Tatrzańska, Bydgoszcz, Elbląg, Gdańsk, Gdynia, Giżycko, Gorzów Wielkopolski, Katowice, Kielce, Kołobrzeg, Kraków, Krynica Morska, Krynica-Zdrój, Lublin, Malbork, Mikołajki, Mrągowo, Olsztyn, Opole, Oświęcim, Poznań, Rzeszów, Sopot, Szczecin, Słubice, Tarnów, Toruń, Tricity, Warsaw, Wrocław, Zakopane, Zielona Góra, Łódź, Świnoujście, etc.
Portugal: Albufeira, Algarve, Aljezur, Almancil, Armação de Pêra, Azores, Braga, Cabanas de Tavira, Carvoeiro, Cascais, Castro Marim, Coimbra, Estoril, Faro, Funchal, Fátima, Guimarães, Lagoa, Lagos, Lisbon, Loulé, Madeira, Monte Gordo, Nazaré, Olhão, Ponta Delgada, Portimão, Porto, Praia da Luz, Quarteira, Sesimbra, Silves, Sintra, Tavira, Vila Real de Santo António, Vila do Bispo, Vilamoura, Évora, etc.
Puerto Rico: Bayamón, Caguas, Carolina, Ponce, San Juan, Vieques, etc.
Qatar: Doha, etc.
Romania: Bran, Brașov, Bucharest, Cluj-Napoca, Constanța, Poiana Brașov, Sibiu, Sighișoara, Timișoara, Transylvania, etc.
Russia: Abakan, Abrau-Dyurso, Abzakovo, Adler, Altai Republic, Alupka, Alushta, Anadyr, Anapa, Angarsk, Arkhangelsk, Arkhipo Osipovka, Arkhyz, Armavir, Astrakhan, Bakhchysarai, Balaklava, Balakovo, Balashikha, Baltic Sea, Barnaul, Belgorod, Belokurikha, Biysk, Black Sea, Blagoveshchensk, Bolshoy Utrish, Bratsk, Bryansk, Caucasian Mineral Waters, Cheboksary, Chelyabinsk, Cherepovets, Cherkessk, Chita, Chornomorske, Crimea, Curonian Spit, Dagomys, Divnomorskoye, Dombay, Domodedovo, Dzerzhinsk, Dzhankhot, Dzhemete, Dzhubga, Elektrostal, Elista, Engels, Estosadok, Feodosia, Foros, Gaspra, Gatchina, Gelendzhik, Golden Ring, Golubitskaya, Gorky Gorod, Gornaya Karusel, Gorno-Altaysk, Goryachy Klyuch, Grozny, Gurzuf, Irkutsk, Ivanovo, Izhevsk, Kabardinka, Kaliningrad, Kaluga, Kamchatka, Kamensk-Uralsky, Karelia, Kazan, Kemerovo, Kerch, Khabarovsk, Khanty-Mansiysk, Khibiny, Khimki, Khosta, Kirov, Kirovsk, Kislovodsk, Kizhi, Koktebel, Kolomna, Komsomolsk on Amur, Konakovo, Koreiz, Korolev, Kostroma, Krasnaya Polyana, Krasnodar Krai, Krasnodar, Krasnogorsk, Krasnoyarsk, Kudepsta, Kurgan, Kursk, Kyzyl, Lake Baikal, Lake Seliger, Lazarevskoye, Lipetsk, Listvyanka, Loo, Lyubertsy, Magadan, Magnitogorsk, Makhachkala, Massandra, Matsesta, Maykop, Miass, Mineralnye Vody, Moscow, Mount Elbrus, Murmansk, Murom, Mytishchi, Naberezhnye Chelny, Nakhodka, Nalchik, Naryan-Mar, Nebug, Nizhnekamsk, Nizhnevartovsk, Nizhny Novgorod, Nizhny Tagil, Norilsk, Novokuznetsk, Novorossiysk, Novosibirsk, Novyi Svit, Novyy Urengoy, Obninsk, Odintsovo, Olginka, Omsk, Orenburg, Orsk, Oryol, Partenit, Penza, Pereslavl Zalessky, Perm, Pervouralsk, Petergof, Petropavlovsk-Kamchatsky, Petrozavodsk, Plyos, Podolsk, Popovka, Primorsko-Akhtarsk, Pskov, Pulkovo, Pushkin, Pushkino, Pyatigorsk, Repino, Rosa Khutor, Rostov-on-Don, Ryazan, Rybachye, Rybinsk, Saint Petersburg, Sakhalin, Saky, Salekhard, Samara, Saransk, Saratov, Sea of Azov, Sergiyev Posad, Serpukhov, Sestroretsk, Sevastopol, Shakhty, Sheregesh, Sheremetyevo, Siberia, Simeiz, Simferopol, Smolensk, Sochi, Solovetsky Islands, Sortavala, Stary Oskol, Stavropol, Sterlitamak, Sudak, Sukko, Surgut, Suzdal, Svetlogorsk, Syktyvkar, Syzran, Taganrog, Taman, Tambov, Tarusa, Temryuk, Terskol, Tobolsk, Tolyatti, Tomsk, Torzhok, Tuapse, Tula, Tver, Tyumen, Ufa, Uglich, Ukhta, Ulan-Ude, Ulyanovsk, Usinsk, Ussuriysk, Utes, Valaam, Valday, Vardane, Velikiye Luki, Veliky Novgorod, Veliky Ustyug, Vityazevo, Vladikavkaz, Vladimir, Vladivostok, Vnukovo International Airport, Volga, Volgograd, Vologda, Volzhskiy, Vorkuta, Voronezh, Vyborg, Yakhroma, Yakornaya Shchel, Yakutsk, Yalta, Yaroslavl, Yekaterinburg, Yelets, Yenisei, Yessentuki, Yevpatoria, Yeysk, Yoshkar-Ola, Yuzhno-Sakhalinsk, Zavidovo, Zelenogradsk, Zheleznovodsk, Zhukovsky, Zvenigorod, etc.
Rwanda: Butare, Gisenyi, Kibuye, Kigali, etc.
Réunion: Saint-Denis, etc.
Saint Barthélemy: Gustavia, etc.
Saint Kitts and Nevis: Basseterre, etc.
Saint Lucia: Anse La Raye, Castries, Gros Islet, Soufrière, etc.
Saint Martin:, etc.
Saint Vincent and the Grenadines: Kingstown, etc.
Samoa: Apia, etc.
San Marino: City of San Marino, etc.
Saudi Arabia: Abha, Al Khobar, Buraydah, Dammam, Jeddah, Jizan, Jubail, Mecca, Medina, Riyadh, Ta'if, Tabuk, Yanbu, etc.
Senegal: Dakar, etc.
Serbia: Belgrade, Kopaonik, Niš, Novi Sad, Palić, Stara Planina, Subotica, Zlatibor, etc.
Seychelles: La Digue, Mahé, Praslin, etc.
Sierra Leone: Freetown, etc.
Singapore: Changi, Sentosa, etc.
Sint Maarten:, etc.
Slovakia: Bratislava, Jasná, Liptov, Tatranská Lomnica, Vysoké Tatry, Štrbské Pleso, etc.
Slovenia: Bled, Bohinj, Bovec, Kranjska Gora, Ljubljana, Maribor, Piran, Portorož, Rogaška Slatina, etc.
Solomon Islands: Honiara, etc.
Somalia: Mogadishu, etc.
Somaliland: Hargeisa, etc.
South Africa: Ballito, Benoni, Bloemfontein, Boksburg, Cape Town, Drakensberg, Durban, East London, George, Johannesburg, Kempton Park, Kimberley, Knysna, Kruger National Park, Marloth Park, Mossel Bay, Nelspruit, Pietermaritzburg, Plettenberg Bay, Polokwane, Port Elizabeth, Potchefstroom, Pretoria, Rustenburg, Sandton, Stellenbosch, Umhlanga, etc.
South Korea: Busan, Daegu, Daejeon, Gangneung, Gapyeong, Gwangju, Gwangyang, Gyeongju, Incheon, Jejudo, Jeonju, Pyeongchang, Seogwipo, Seoul, Sokcho, Suwon, Ulsan, Yangyang, Yeosu, etc.
Spain: A Coruña, Alcúdia, Algeciras, Alicante, Almería, Altea, Andalusia, Antequera, Aragon, Asturias, Ayamonte, Baiona, Balearic Islands, Barbate, Barcelona, Basque Country, Benalmádena, Benidorm, Benissa, Besalú, Bilbao, Blanes, Buñol, Cadaqués, Cala d'Or, Calella, Calonge, Calp, Calvià, Cambados, Cambrils, Canary Islands, Cangas de Onís, Cantabria, Cartagena, Castilla-La Mancha, Catalonia, Chiclana de la Frontera, Costa Blanca, Costa Brava, Costa Dorada, Costa del Maresme, Costa del Sol, Cádiz, Córdoba, Dénia, El Puerto de Santa María, Empuriabrava, Estepona, Figueres, Formentera, Fuerteventura, Galicia, Gijón, Girona, Gran Canaria, Granada, Ibiza, Jerez de la Frontera, L'Escala, L'Estartit, L'Hospitalet de Llobregat, La Pineda, Lanzarote, Llançà, Lleida, Lloret de Mar, Madrid, Magaluf, Malgrat de Mar, Mallorca, Marbella, Maspalomas, Menorca, Mijas, Mojácar, Moraira, Murcia, Málaga, Navarre, Nerja, O Grove, Ourense, Oviedo, Palma Nova, Palma, Pals, Poio, Pollença, Pontevedra, PortAventura, Portonovo, Ronda, Roquetas de Mar, Roses, Salamanca, Salou, San Sebastian, Sant Antoni de Portmany, Santander, Santiago de Compostela, Santillana del Mar, Sanxenxo, Seville, Sidges, Sierra Nevada, Tarifa, Tarragona, Tenerife, Toledo, Torremolinos, Torrevieja, Torroella de Montgrí, Tossa de Mar, Valencia, Vigo, Vélez-Málaga, Xàbia, Zaragoza, etc.
Sri Lanka: Anuradhapura, Bentota, Beruwala, Colombo, Dambulla, Galle, Hikkaduwa, Jaffna, Kandy, Mirissa, Negombo, Nuwara Eliya, Sigiriya, Tangalle, Trincomalee, Unawatuna, Weligama, etc.
Sudan: Khartoum, Port Sudan, etc.
Suriname: Lelydorp, Nieuw Nickerie, Paramaribo, etc.
Swaziland: Lobamba, Mbabane, etc.
Sweden: Bohuslän, Borgholm, Borlänge, Dalarna, Falkenberg, Falun, Gothenburg, Gotland, Halmstad, Helsingborg, Jönköping, Kalmar, Karlshamn, Karlskrona, Karlstad, Kiruna, Kristianstad, Linköping, Lund, Malmö, Norrköping, Solna, Stockholm, Umeå, Uppsala, Vimmerby, Visby, Västerås, Växjö, Ystad, Ängelholm, Åre, Öland, Örebro, Östersund, etc.
Switzerland: Adelboden, Andermatt, Anzère, Arosa, Ascona, Basel, Bellinzona, Bern, Crans-Montana, Davos, Engelberg, Fribourg, Geneva, Grindelwald, Grächen, Gstaad, Haute-Nendaz, Interlaken, Jungfrau, Klosters, Lake Maggiore, Lausanne, Lauterbrunnen, Leukerbad, Locarno, Lucerne, Lugano, Matterhorn, Montreux, Nendaz, Neuchâtel, Pontresina, Portes du Soleil, Saanen, Saas-Fee, Sierre, Silvaplana, Sion, St. Gallen, St. Moritz, Swiss Alps, Ticino, Valais, Verbier, Vevey, Veysonnaz, Wengen, Zermatt, Zug, Zürich, etc.
Syria: Aleppo, Damascus, Deir ez-Zor, Latakia, Palmyra, Tartus, etc.
Taiwan: Hsinchu, Kaohsiung, Taichung, Tainan, Taipei, etc.
Tajikistan: Dushanbe, Isfara, Khujand, etc.
Tanzania: Dar es Salaam, Mount Kilimanjaro, Serengeti, Zanzibar, etc.
Thailand: Ayutthaya, Bangkok, Chiang Mai, Chiang Rai, Chonburi, Hua Hin, Kanchanaburi, Karon, Khao Sok, Ko Chang, Ko Lanta, Ko Phangan, Ko Samui, Krabi, Pai, Patong, Pattaya, Phi Phi Islands, Phuket, Prachuap Khiri Khan, Ranong, River Kwai, Udon Thani, etc.
Togo: Lomé, etc.
Tonga: Nukuʻalofa, Tunis, etc.
Trinidad and Tobago: Port of Spain, etc.
Tunisia: Djerba, Hammamet, Midoun, Monastir, Port El Kantaoui, Sousse, etc.
Turkey: Adana, Alacati, Alanya, Ankara, Antakya, Antalya, Ayvalık, Beldibi, Belek, Bodrum, Bozcaada, Bursa, Büyükada, Cappadocia, Dalyan, Datça, Denizli, Didim, Edirne, Ephesus, Erzincan, Erzurum, Eskişehir, Fethiye, Gaziantep, Göreme, Göynük, Istanbul, Kalkan, Kayseri, Kaş, Kemer, Konakli, Konya, Kuşadası, Lara, Mahmutlar, Manavgat, Marmaris, Mersin, Olympos, Palandöken, Pamukkale, Prince Islands, Samsun, Sapanca, Sarıkamış, Selçuk, Side, Tarsus, Tekirova, Trabzon, Troy, Turkish Riviera, Uludağ, Van, Çamyuva, Çanakkale, Çeşme, Çıralı, Ölüdeniz, Ürgüp, İskenderun, İzmir, İçmeler, Şanlıurfa, etc.
Turkmenistan: Ashgabat, Avaza, etc.
Turks and Caicos Islands: Cockburn Town, North Caicos, Pine Cay, Providenciales, etc.
Uganda: Kampala, etc.
Ukraine: Berdiansk, Bila Tserkva, Boryspil, Bukovel, Cherkasy, Chernihiv, Chernivtsi, Dnipropetrovsk, Donetsk, Ivano-Frankivsk, Kamianets-Podilskyi, Kharkiv, Kherson, Kiev, Koblevo, Kremenchuk, Kryvyi Rih, Luhansk, Lviv, Mariupol, Melitopol, Mykolaiv, Odessa, Poltava, Slavske, Sumy, Truskavets, Uzhgorod, Vinnytsia, Yaremche, Yasinya, Zaporizhia, Zatoka, Zhytomyr, etc.
United Arab Emirates: Abu Dhabi, Ajman, Dubai, Persian Gulf, Ras Al Khaimah, Sharjah, etc.
United Kingdom: Aberdeen, Bath, Belfast, Blackpool, Bournemouth, Bradford, Brighton, Bristol, Cambridge, Canterbury, Cardiff, Channel Tunnel, Cheltenham, Chester, Cornwall, Coventry, Cumbria, Derry, Devon, Dorset, Dover, Eastbourne, Edinburgh, England, English Channel, Exeter, Folkestone, Fort William, Glasgow, Hampshire, Harrogate, Inverness, Isle of Wight, Kent, Lancashire, Leeds, Leicester, Liverpool, Llandudno, London, Manchester, Mansfield, Milton Keynes, Newcastle, Newquay, Northern Ireland, Norwich, Nottingham, Oban, Oxford, Paignton, Plymouth, Portmeirion, Portsmouth, Reading, Sandown, Scarborough, Scotland, Shanklin, Sheffield, Somerset, Southampton, St Albans, Stonehenge, Sussex, Swansea, Torquay, Wales, Whitby, Windsor, York, etc.
United States: Akron, Alabama, Alaska, Albuquerque, Amarillo, Anaheim, Anchorage, Ann Arbor, Arizona, Arkansas, Arlington, Aspen, Atlanta, Aurora, Austin, Bakersfield, Baltimore, Baton Rouge, Beaver Creek, Big Bear Lake, Billings, Biloxi, Birmingham, Boca Raton, Boise, Boston, Breckenridge, Brooklyn, Buffalo, California, Carlsbad, Carmel-by-the-Sea, Chandler, Charlotte, Chesapeake, Cheyenne, Chicago, Chula Vista, Cincinnati, Clearwater, Cleveland, Colorado Springs, Colorado, Columbus Georgia, Columbus, Connecticut, Corpus Christi, Costa Mesa, Cupertino, Dallas, Dana Point, Daytona Beach, Death Valley, Delaware, Delray Beach, Denver, Des Moines, Destin, Detroit, Durham, El Paso, Estes Park, Fargo, Fayetteville, Florida, Fontana, Fort Lauderdale, Fort Myers, Fort Walton Beach, Fort Wayne, Fort Worth, Fremont, Fresno, Galveston, Garland, Georgia, Gilbert, Glendale, Grand Canyon, Grand Rapids, Grand Teton, Great Smoky Mountains, Greensboro, Gulfport, Hawaii, Henderson, Hialeah, Hollywood, Honolulu, Hot Springs, Houston, Huntington Beach, Idaho, Illinois, Indiana, Indianapolis, Iowa, Irving, Jackson Mississippi, Jackson Wyoming, Jacksonville, Jersey City, Juneau, Kansas City, Kansas, Kentucky, Key Largo, Key West, La Jolla, Laguna Beach, Lahaina, Lake Tahoe, Laredo, Las Vegas, Lexington, Lincoln, Little Rock, Long Beach, Los Angeles, Louisiana, Louisville, Lubbock, Madison, Maine, Malibu, Mammoth Lakes, Manhattan, Marathon, Maryland, Massachusetts, Memphis, Mesa, Mexico City, Miami Beach, Miami, Michigan, Milwaukee, Minneapolis, Minnesota, Mississippi, Missouri, Moab, Modesto, Montana, Monterey, Montgomery, Moreno Valley, Mountain View, Myrtle Beach, Napa, Naples, Nashville, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New Orleans, New York City, New York, Newark, Newport Beach, Newport, Norfolk, North Carolina, North Dakota, North Las Vegas, Oakland, Ocean City, Oceanside, Ohio, Oklahoma City, Oklahoma, Omaha, Oregon, Orlando, Oxnard, Palm Coast, Palm Desert, Palm Springs, Palo Alto, Panama City Beach, Park City, Pasadena, Pennsylvania, Pensacola, Philadelphia, Phoenix, Pittsburgh, Plano, Pompano Beach, Portland, Portland, Providence, Raleigh, Reno, Rhode Island, Richmond, Riverside, Rochester, Rocky Mountains, Sacramento, Saint Paul, Salt Lake City, San Antonio, San Bernardino, San Diego, San Francisco, San Jose, Sanibel, Santa Ana, Santa Barbara, Santa Cruz, Santa Fe, Santa Monica, Sarasota, Savannah, Scottsdale, Seattle, Shreveport, Silicon Valley, South Carolina, South Dakota, South Lake Tahoe, Spokane, Springfield, Squaw Valley, St. Augustine, St. Louis, St. Petersburg, Steamboat Springs, Stockton, Sunny Isles Beach, Sunnyvale, Tacoma, Tallahassee, Tampa, Telluride, Tennessee, Texas, Thousand Oaks, Toledo, Tucson, Tulsa, Utah, Vail, Vermont, Virginia Beach, Virginia, Waikiki, Washington D.C., Washington, West Palm Beach, West Virginia, Wichita, Winston-Salem, Wisconsin, Wyoming, Yellowstone, Yonkers, Yosemite, Zion, etc.
Uruguay: Montevideo, Punta del Este, etc.
Uzbekistan: Bukhara, Fergana, Khiva, Kokand, Navoiy, Samarkand, Tashkent, Urgench, etc.
Vanuatu: Port Vila, etc.
Venezuela: Caracas, Isla Margarita, Maracaibo, Porlamar, etc.
Vietnam: Cần Thơ, Da Lat, Da Nang, Haiphong, Hanoi, Ho Chi Minh City, Huế, Hạ Long, Hội An, Long Hải, Mỹ Tho, Nha Trang, Ninh Bình, Phan Thiết, Phú Quốc, Qui Nhơn, Rạch Giá, Sa Pa, Vũng Tàu, Đồng Hới, etc.
Yemen: Aden, Sana'a, etc.
Zambia: Livingstone, Lusaka, etc.
Zimbabwe: Bulawayo, Harare, Mutare, Victoria Falls, etc.