Vacuum tube

Fleming's first diodes

The English physicist John Ambrose Fleming worked as an engineering consultant for firms including Edison Swan,[15] Edison Telephone and the Marconi Company. In 1904, as a result of experiments conducted on Edison effect bulbs imported from the United States, he developed a device he called an "oscillation valve" (because it passes current in only one direction). The heated filament, 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 heated 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 While firmly established by history, the term "detector" is not of itself descriptive and should be considered outdated.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.

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[16] 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 help refine the triode vacuum tube.

However, Lee De Forest is credited with inventing the triode tube in 1907 while experimenting to improve his original (diode) Audion. By placing an additional electrode between the filament (cathode) and plate (anode), he discovered the ability of the resulting device to amplify signals. As the voltage applied to the 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 the passage of emitted electrons and reduce the current to the plate. Thus, a few volt 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 for use as an electronic amplifier in radio communications. This eventually became known as the triode.

General Electric Company Pliotron, Science History Institute

De Forest's original device was made with conventional vacuum technology. The vacuum was not a "hard vacuum" but rather left a very small amount of residual gas. The physics behind the device's operation was also not settled. The residual gas would cause a blue glow (visible ionization) when the plate voltage was high (above about 60 volts). In 1912, De Forest brought the Audion to Harold Arnold in AT&T's engineering department. Arnold recommended that AT&T purchase the patent, and AT&T followed his recommendation. Arnold developed high-vacuum tubes which were tested in the summer of 1913 on AT&T's long-distance network.[17] The high-vacuum tubes could operate at high plate voltages without a blue glow.

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.[18]

Irving Langmuir at the General Electric research laboratory (Schenectady, New York) had improved Wolfgang Gaede's high-vacuum diffusion pump and used it to settle the question of thermionic emission and conduction in a vacuum. Consequently, General Electric started producing hard vacuum triodes (which were branded Pliotrons) in 1915.[19] Langmuir patented the hard vacuum triode, but De Forest and AT&T successfully asserted priority and invalidated the patent.

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. Historically, vacuum levels in production vacuum tubes typically ranged from 10 µPa down to 10 nPa.[20]

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 (g_m) 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 g_m 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 R_p or R_a. The Van der Bijl equation defines their relationship as follows: ${\displaystyle g_{m}={\mu \over R_{p}}}$

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 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 battery'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.

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.[21] 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. The screen's constant voltage also reduced the anode voltage's influence on the space charge. Where the ratio of plate voltage control of plate current to grid control of the plate current (amplification factor) commonly ranges from below ten to perhaps 100, tetrode amplification factors readily exceeded 500. 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.[22] The otherwise undesirable negative resistance was exploited to produce a 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[23] 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.

The pentagrid converter contains 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 control grids (at a low potential) and screen grids (at a high voltage). Many designs use such a screen grid as an additional anode to provide feedback for the oscillator function, whose current adds 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 United States, 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) can be combined in the bulb of a single multisection tube. An early example is the Loewe 3NF. This 1920s device has 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 United States, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also has a single tube socket, but because it uses a four-pin base, the additional element connections are 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 is a double diode triode used as a detector, automatic gain control rectifier and audio preamplifier in early AC powered radios. These sets often include 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[24][25][26]) 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 can 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 has 12 pins. A typical example, the 6AG11, contains two triodes and two diodes.

Some otherwise conventional tubes do not fall into standard categories; the 6AR8, 6JH8 and 6ME8 have several common grids, followed by a pair of beam deflection electrodes which deflected the current towards either of two anodes. They were sometimes known as the 'sheet beam' tubes and used in some color TV sets for color demodulation. The similar 7360 was popular as a balanced SSB (de)modulator.

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 windings of the control and screen grid wires are aligned such that the screen grid is in the "shadow" of the control grid. The two grids are positioned so that the control grid creates "sheets" of electrons that pass between the screen-grid wires.

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. While they are not strictly pentodes, their overall electrical behavior is similar.

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.

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.

Miniature tube (right) compared to the older octal style. Not including pins, the larger tube, a 5U4GB, is 93 mm high with a 35 mm diameter base, while the smaller, a 9-pin 12AX7, is 45 mm high, and 20.4 mm in diameter.

Subminiature CV4501 tube (SQ version of EF72), 35 mm long x 10 mm diameter (excluding leads)

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[27] 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 seven or nine 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, c. 20 mm high by 11 mm diameter

Sub-miniature tubes with a size roughly that of half a cigarette were used in one of the very earliest general-purpose digital computers, the Jaincomp-B, produced by the Jacobs Instrument Company,[28][lower-alpha 1] and consumer applications as 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 aircraft radio transceivers, UHF television tuners, and some HiFi FM radio tuners (Sansui 500A) until replaced by high-frequency capable transistors.

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. Over time vacuum tubes became much smaller.

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 an 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.[29] 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.

The 1946 ENIAC computer used 17,468 vacuum tubes and consumed 150 kW of power

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.[30] "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".[31] 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".[31] 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.[32] 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 (one of the first commercially available electronic computers), and UNIVAC I, also available commercially.

Advances using subminiature tubes included the Jaincomp series of machines produced by the Jacobs Instrument Company of Bethesda, Maryland. Models such as its Jaincomp-B employed just 300 such tubes in a desktop-sized unit that offered performance to rival many of the then room-sized machines.[28]

Vacuum tubes seen on end in recreation of World War II-era Colossus computer at Bletchley Park, England

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.[31] Colossus was the first use of vacuum tubes working in concert on such a large scale for a single machine.[31]

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, which added more tubes. Another nine Colossus Mk2s were built. Each Mk2 consumed 15 kilowatts; most of the power was 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.[33]

Circuitry from core memory unit of Whirlwind

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 largely to evaporation of silicon, used in the tungsten alloy to make the heater wire easier to draw. The silicon forms barium orthosilicate at the interface between the nickel sleeve and the cathode barium oxide coating.[7]^:301 This "cathode interface" is a high-resistance layer (with some parallel capacitance) which greatly reduces the cathode current when the tube is switched into conduction mode.[34]^:224 Elimination of silicon from the heater wire alloy (and more frequent replacement of the wire drawing dies) allowed the production of tubes that were reliable enough for the Whirlwind project. High-purity nickel tubing and cathode coatings free of materials such as silicates and aluminum that can reduce emissivity also contribute to long cathode life.

The first such "computer tube" was Sylvania's 7AK7 pentode of 1948 (these replaced the 7AD7, which was supposed to be better quality than the standard 6AG7 but proved too unreliable).[35]^:59 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. Running in cutoff with the heater on accelerates cathode poisoning and the output current of the tube will be greatly reduced when switched into conduction mode.[34]^:224 The 7AK7 tubes improved the cathode poisoning problem, but that alone was insufficient to achieve the required reliability.[35]^:60 Further measures included switching off the heater voltage when the tubes were not required to conduct for extended periods, turning on and off the heater voltage with a slow ramp to avoid thermal shock on the heater element,[34]^:226 and stress testing the tubes during offline maintenance periods to bring on early failure of weak units.[35]^:60–61

The tubes developed for Whirlwind were later used in the giant SAGE air-defense computer system. 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.

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.[39]

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. After the use of B-batteries was phased out and rectified line-power was employed to produce the high voltage needed by tubes' plates, the term "B+" persisted in the US when referring to the high voltage source, most of the rest of the English speaking world refers to this supply as just HT (high tension).

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 no current flows through a tube's grid connection, these batteries had no current drain and lasted the longest, usually limited by their own shelf life. The supply from the grid bias battery was rarely, if ever, disconnected when the radio was otherwise switched off. 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 along with the development of resistor/capacitor coupling which replaced earlier interstage transformers.

The "C battery" for bias is a designation having no relation to the "C cell" battery size.

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 100–120 V 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.

Getter in opened tube; silvery deposit from getter

Dead vacuum fluorescent display (air has leaked in and the getter spot has become white)

A vacuum tube needs an extremely good ("hard") vacuum to avoid the consequences of generating positive ions within the tube. With a small amount of residual gas, some of those atoms may ionize when struck by an electron and create fields that adversely affect the tube characteristics. Larger amounts of residual gas can create a self-sustaining visible glow discharge between the tube elements. To avoid these effects, the residual pressure within the tube must be low enough that the mean free path of an electron is much longer than the size of the tube (so an electron is unlikely to strike a residual atom and very few ionized atoms will be present). Commercial vacuum tubes are evacuated at manufacture to about 0.000001 mmHg (1.0×10⁻⁶ Torr; 130 μPa; 1.3×10⁻⁶ mbar; 1.3×10⁻⁹ atm).[43]

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 that 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.

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 (18.6 years) 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.[44] 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 approximately 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 approximately 15 kW also show better overall power efficiency than tube-based power amplifiers.

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 aluminum 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.

A catastrophic failure is one that 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.[45]

Tube rectifiers have limited current capability and exceeding ratings will eventually destroy a tube.

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.[46] Usable life of this expensive component was sometimes extended by fitting a boost transformer to increase heater voltage.

Vacuum tubes may 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 (with greater than unity gain) 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.[47] Some of these effects make tubes unsuitable for small-signal audio use, although unobjectionable for other purposes. Selecting the best of a batch of nominally identical tubes for critical applications can produce better results.

Tube pins can develop non-conducting or high resistance surface films due to heat or dirt. Pins can be cleaned to restore conductance.

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.

The X-ray tube is a type of cathode ray tube that generates X-rays 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.

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.)

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,[48] or geomagnetic storms produced by giant solar flares.[49] 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.[48] In that aircraft, 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 devices 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. Solid-state devices such as gallium nitride are promising replacements, but are very expensive and still in development.

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 both the U.S. and Russia.

70-watt tube-hybrid audio amplifier selling for US$2,680[50] in 2011, about 10 times the price of a comparable model using transistors.[51]

Enough people prefer tube sound to make tube amplifiers commercially viable in three areas: musical instrument (e.g., guitar) amplifiers, devices used in recording studios, and audiophile equipment.[52]

Many guitarists prefer using valve amplifiers to solid-state models, often due to the way they tend to distort when overdriven.[53] 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.

The cathode ray tube was the dominant display technology for televisions and computer monitors at the start of the 21st century. However, rapid advances and falling prices of LCD flat panel technology soon took the place of CRTs in these devices.[54] By 2010, most CRT production had ended.[55]

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.)

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.[56] 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, and in radar and 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 to be working on vacuum-channel transistors produced using CMOS techniques.[57]

The space between the Cathode and the Anode form a cloud which is known as the "space charge".

The V-I characteristic depends upon the size and material of the plate and cathode.[58] Express the ratio between voltage plate and plate current.[59]

• V-I curve (Voltage across filaments, plate current)
• Plate current, plate voltage characteristics
• DC plate resistance of the plate—resistance of the path between anode and cathode of direct current
• AC plate resistance of the plate—resistance of the path between anode and cathode of alternating current

Size of electrostatic field is the size between two or more plates in the tube.