Vacuum tube
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Reverted edits by [[Special:Contributions/OrgasGirl|OrgasGirl]] ([[User talk:OrgasGirl|talk]]) to last version by 213.130.226.4
[[Image:Diode tube schematic.svg|right|thumb|Structure of a vacuum tube diode]]
[[Image:Triode tube schematic.svg|right|thumb|Structure of a vacuum tube triode]]
[[Image:NEC vacuum tube.jpg|right|thumb|An old [[NEC]] vacuum tube]]
In [[electronics]], a '''vacuum tube''', '''electron tube''' (in North America), '''thermionic valve''', or just '''valve''' (elsewhere, especially in Britain) is a device used to [[amplifier|amplify]], [[switch]], otherwise modify, or create an [[Electricity| electrical]] [[signal (information theory)|signal]] by controlling the movement of [[electron]]s in a low-pressure space. Some special function vacuum tubes are filled with low-pressure gas: these are so-called soft valves (or tubes), as distinct from the hard [[vacuum]] type which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence [[Thermionic emission| thermionic]].
Vacuum tubes were critical to the development of electronics technology, which drove the expansion and commercialization of [[radio]] broadcasting, [[television]], [[radar]], [[high-end audio|sound reproduction]], large [[telephone]] networks, analog and digital [[computer]]s, and industrial [[process control]]. Some of these applications pre-dated electronics, but it was the vacuum tube that made them widespread and practical.
For most purposes, the vacuum tube has been replaced by [[Solid state (electronics)|solid-state]] [[semiconductor devices]] such as [[transistor]]s and solid-state [[diode]]s. Solid-state devices last much longer, are smaller, more efficient, more reliable, and cheaper than equivalent vacuum tube devices. However, tubes are still used in specialized applications: for engineering reasons, as in high-power radio frequency transmitters; or for their aesthetic appeal, as in [[tube sound|audio amplification]]. [[Cathode ray tube]]s are still used as display devices in [[television]] sets, [[computer monitor|video monitor]]s, and [[oscilloscope]]s, although they are being replaced by [[LCD]]s and other [[flat-panel display]]s. A specialized form of the electron tube, the [[magnetron]], is the source of microwave energy in [[microwave oven]]s and some radar systems.
==Explanation==
A vacuum tube consists of [[electrode]]s in a [[vacuum]] in a (usually tubular) insulating heat-resistant envelope. Many tubes have glass envelopes, though some types such as power tubes may have ceramic or metal envelopes. The electrodes are attached to leads which pass through the envelope via an airtight seal. On most tubes, the leads are designed to plug into a [[tube socket]] for easy replacement.
The simplest vacuum tubes resemble [[Incandescence|incandescent]] [[Incandescent light bulb|light bulb]]s in that they have a [[electrical filament|filament]] [[glass-to-metal seal|sealed in a glass envelope]] which has been evacuated of all air. When hot, the filament releases [[electron]]s into the vacuum: a process called [[thermionic emission]]. The resulting negatively-charged cloud of electrons is called a [[space charge]]. These electrons will be drawn to a metal plate inside the envelope, if the plate (also called the [[anode]]) is positively charged relative to the filament (or [[hot cathode|cathode]]). The result is a flow of electrons from filament to plate. This cannot work in the reverse direction because the plate is not heated and does not emit electrons. This very simple example described can thus be seen to operate as a [[diode]]: a device that conducts current only in one direction. The vacuum tube diode conducts [[conventional current]] from plate (anode) to the filament (cathode); this is the opposite direction to the flow of electrons (called electron current).
Vacuum tubes operate primarily on the function of the heat gradient difference between the hot cathode and the cold anode. Because of this operating requirement, vacuum tubes are inherently power-inefficient; enclosing the tube within a heat-retaining envelope of insulation would allow the entire tube to reach the same temperature, resulting in electron emission from the anode that would counter the normal one-way current flow. Because the tube requires a vacuum to operate, convection cooling of the anode is typically not possible. Instead anode cooling occurs primarily through [[black body|black-body radiation]] and conduction of heat to the outer glass envelope via the anode mounting frame. [[Cold cathode]] tubes do exist but are used primarily in lighting systems, where unidirectional power regulation is not the functional purpose of the tube.{{Facts|date=August 2007}}
The vacuum tube is a ''voltage-controlled device'', with the relationship between the input and output circuits determined by a [[transconductance]] function. The solid-state device most closely analogous to the vacuum tube is the [[junction gate field-effect transistor|JFET]], although the vacuum tube typically operates at far higher voltage (and power) levels than the JFET.
==History of development==
[[Image:VacuumTubeGuts.agr.jpg|thumb|left|upright|Inside of a vacuum tube with plate cut open.]]
The 19th century saw increasing research with evacuated tubes, such as the [[Geissler tube|Geissler]] and [[Crookes tube]]s. Scientists who experimented with such tubes included [[Eugen Goldstein]], [[Nikola Tesla]], [[Johann Wilhelm Hittorf]], [[Thomas Edison]], and many others. These tubes were mostly for specialized scientific applications, or were novelties, with the exception of the [[light bulb]]. The groundwork laid by these scientists and inventors, however, was critical to the development of vacuum tube technology.
Though the [[thermionic emission]] effect was originally reported in 1873 by [[Frederick Guthrie]], it is Thomas Edison's 1884 investigation of the "[[Edison Effect]]" that is more often mentioned. Edison patented what he found,<ref>{{US patent|307031}}</ref> but he did not understand the underlying physics, or the potential value of the discovery.
===Diodes and triodes===
The English physicist [[John Ambrose Fleming]] worked as an engineering consultant for technology firms, including Edison Telephone; in 1904, as a result of experiments conducted on Edison Effect bulbs imported from the USA and while working as scientific adviser to the [[Marconi Company|Marconi]] company, he developed a device he called an "oscillation valve" (because it passes current in only one direction) or '''kenotron''', which can also be used as part of a radio wave detector. Later known as the [[Fleming valve]] and then the [[diode]], it allowed [[electrical current]] to flow in only one direction, enabling the [[rectifier|rectification]] of alternating current.
In 1906 [[Robert von Lieben]] filed<ref>{{cite web | title = Robert von Lieben — Patent Nr 179807 Dated November 19, 1906 | publisher = Kaiserliches Patentamt | date = 1906-11-19 | url = http://www.hts-homepage.de/Lieben/DRP179807.pdf | format = [[PDF]] | accessdate = 2008-03-30 }}</ref> for a three electrode amplifying vacuum tube. His invention included also a beam focusing electromagnet.
In 1907 [[Lee De Forest]] placed a bent wire serving as a screen, later known as the [[control grid|"grid" electrode]], between the filament and [[plate electrode]]. As the voltage applied to the grid was varied from negative to positive, the number of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was therefore an excellent and very sensitive [[amplifier]] of voltages. DeForest called his invention the "[[Audion tube|Audion]]". In 1907, DeForest filed<ref>{{US patent|879532}}</ref> for a three-electrode version of the Audion for use in radio communications. The device is now known as the [[triode]]. De Forest's device was not strictly a vacuum tube, but clearly depended for its action on ionisation of the relatively high levels of gas remaining after evacuation. The De Forest company, in its Audion leaflets, warned against operation which might cause the vacuum to become too hard. The Finnish inventor [[Eric Tigerstedt]] significantly improved on the original triode design in 1914, while working on his [[sound film|sound-on-film]] process in Berlin, Germany. The first true vacuum triodes 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. Pliotrons were closely followed by the French 'R' Type which was in widespread use by the allied military by 1916. These two types were the first true ''vacuum'' tubes. Historically, vacuum levels in production vacuum tubes typically ranged between 10 µ[[Pa]] to 10 nPa.
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortions at low volumes. This is not to be confused with the [[Overdrive (music)|overdrive]] that tube amplifiers exhibit at high volume levels (known as the [[tube sound]]). To remedy the low-volume distortion problem, engineers plotted curves of the applied grid voltage and resulting plate currents, and discovered that there was a range of relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current". Today this current would be called the quiescent or [[standing current]]. The controlling voltage was superimposed onto this fixed voltage, resulting in linear swings of plate current for both positive and negative swings of the input voltage. This concept was called ''[[grid bias]]''.
[[Battery (electricity)|Batteries]] provided the voltages required by tubes in early radio sets. In North American terminology, the [[A battery (vacuum tubes)|"A" batteries]] provided the filament voltage. Although North American terminology calls this the A battery, most of the English-speaking world knows it by a descriptive label: the LT (low-tension) supply or battery. These were often rechargeable—usually of the [[lead-acid battery|lead-acid]] type ranging from 2 to 12 volts (1-6 cells) with single, double and triple cells being most common. Because these batteries produced 2 V, 4 V or 6 V, tube heaters were designed to operate at those voltages—a scheme which continues to be followed today. In portable radios, flashlight (torch) batteries were sometimes used.
The [[B battery (vacuum tubes)|"B" batteries]] (in North American English) provided the plate voltage. These were generally of [[dry cell]] construction, containing many small 1.5 volt cells in [[Series and parallel_circuits#Series_circuits|series]]. They typically came in ratings of 22.5, 45, 67.5, 90 or 135 volts. To this day, plate voltage is referred to as B<sup>+</sup>, but only in North America. The rest of the English-speaking world calls this the HT (high-tension) supply or battery.
Some sets used [[C battery (vacuum tubes)|"C" batteries]] (North American English) to provide grid bias, although many circuits used [[grid leak]] [[resistor]]s, [[voltage divider]]s or [[cathode bias]] to provide proper tube bias. Most of the English-speaking world calls this simply the 'grid bias battery'.
=== Direct and indirect heating ===
Many innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use this cathode as the source of electron flow in the tube rather than the filament itself. This minimized the introduction of hum when the filament was energized with [[alternating current]]. In such tubes, the filament is called a [[heater]] to distinguish it as an inactive element.
===Tetrodes and pentodes===
[[Image:valve.radio.arp.600pix.jpg|thumb|right|upright|A two-tube homemade radio from 1958. The tubes
are the two columns with the dark tops. The flying leads connect to the low-voltage filament and high-voltage anode supplies.]]
When triodes were first used in radio transmitters and receivers, it was found that they had a tendency to oscillate due to parasitic anode-to-grid capacitance. Many circuits were developed to reduce this problem (e.g. the [[Neutrodyne]] amplifier), but proved unsatisfactory over wide ranges of frequencies. It was discovered that the addition of a second grid, located between the control grid and the plate and called a ''[[screen grid]]'' could solve these problems. A positive voltage slightly lower than the plate voltage was applied to it, and the screen grid was bypassed (for high frequencies) to ground with a capacitor. This arrangement decoupled the anode and the first grid, completely eliminating the oscillation problem. An additional side effect of this second grid is that the [[Miller effect|Miller capacitance]] is also reduced, which improves gain at high frequency. This two-grid tube is called a ''[[tetrode]]'', meaning four active electrodes.
[[Image:Hp vacuum tube.jpg|thumb|left|Radio transmitter high-power vacuum tube. The braided copper leads provide heater current for the cathode. The tube also has a heat sink. [[Dubendorf]] Museum of Military Aviation.]]
However, the tetrode has some new problems. In any tube, electrons strike the anode hard enough to knock out [[secondary emission|secondary electrons]]. In a triode these (less energetic) electrons cannot reach the grid or cathode, and are re-captured by the anode. But in a tetrode, they can be captured by the second grid, reducing the plate current and the amplification of the circuit. Since secondary electrons can outnumber the primary electrons, in the worst case, particularly when the plate voltage dips below the screen voltage, the plate current can actually go down with increasing plate voltage.<ref>[http://www.cjseymour.plus.com/elec/valves/valves.htm Introduction to Thermionic Valves (Vacuum Tubes)], Colin J. Seymour</ref> This is the "tetrode kink" (see the reference for a plot of this effect in the RCA-235 tetrode). Another consequence of this effect is that under severe overload, the current collected by the screen grid can cause it to overheat and melt, destroying the tube.
Again the solution was to add another grid, called a [[suppressor grid]]. This third grid was biased at either ground or cathode voltage and its negative voltage (relative to the anode) electrostatically suppressed the secondary electrons by repelling them back toward the anode. This three-grid tube is called a [[pentode]], meaning five electrodes.
===Other variations===
<!-- NOTE: The apparent mismatch between names and numbers of grids is not an error. The name refers to the total number of electrodes, while the number is only the grids, not the emitter and collector. -->
[[Image:12SA7GT GridDetail.jpg|thumb|right|upright|12SA7 Pentagrid converter tube.]]
Frequency conversion can be accomplished by various methods in [[superheterodyne]] receivers. Tubes with 5 grids, called [[pentagrid converter]]s, were generally used, although alternatives such as using a combination of a [[triode]] with a [[pentagrid converter#the hexode|hexode]] were also used. Even [[pentagrid converter#the octode|octodes]] have been used for frequency conversion. The additional grids are either [[control grid]]s, with different signals applied to each one, or [[screen grid]]s. In many designs a special grid acted as a second 'leaky' plate to provide a built-in oscillator, which then coupled this signal with the incoming radio signal. These signals create a single, combined effect on the plate current (and thus the signal output) of the tube circuit. The heptode, or [[pentagrid converter]], was the most common of these. 6BE6 is an example of a heptode (note that the first number in the tube ID indicates the filament voltage).
To reduce the cost and complexity of radio equipment, by 1940 it was common practice to combine more than one function, or more than one set of elements in the bulb of a single tube. The only constraint was where patents, and other licencing considerations required the use of multiple tubes. See [[British Valve Association]]
For example, the RCA Type 55 was a [[double diode triode]] used as a detector, [[automatic gain control]] rectifier and audio [[preamplifier|preamp]] in early AC powered radios. The same set of tubes often included the 53 Dual Triode Audio Output.
Another early type of multi-section tube, the [[6SN7]], is a "dual triode" which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less.
[[Image:RCA12ax7.jpg|thumb|right|upright|An RCA 12AX7 dual-triode tube (1947)]]
The 12AX7 is a dual high-gain triode widely used in [[guitar amplifiers]], audio preamps, and instruments.
The invention of the 9-pin miniature tube base, besides allowing the 12AX7 family, also allowed many other multi section tubes, such as the [[6GH8]] triode pentode. Along with a host of similar tubes, the 6GH8 was quite popular in [[television]] receivers. Some color TV sets used exotic types like the [[6JH8]] which had two plates and beam deflection electrodes (known as 'sheet beam' tube). Vacuum tubes used like this were designed for demodulation of synchronous signals, an example of which is color [[demodulation]] for television receivers.
[[Image:Minaturevacuumtube.jpg|thumb|right|1960s era miniature tubes from Australia.]]
The desire to include many functions in one envelope resulted in the General Electric [[Compactron]]. A typical unit, the 6AG11 Compactron tube contained two triodes and two diodes, but many in the series had triple triodes.
An early example of multiple devices in one envelope was the [[Loewe 3NF]]. This 1920s device had 3 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 tubeholder, it was able to substantially undercut the competition since, in Germany, state tax was levied by the number of tubeholders. However, reliability was compromised, and production costs for the tube were much greater.
Loewe were to also offer the 2NF (two tetrodes plus passive components) and the WG38 (two pentodes, a triode and the passive components).
[[Image:Vacuumtuberadio.jpg|thumb|left|Vacuum tubes in an Australian radio of the late 1930s]]
The [[beam tetrode|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 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-grid capacitance, screen-grid current, and secondary emission effects 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.
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. [[6L6]] was the first popular beam power tube, introduced by [[RCA]] in 1936. Corresponding tubes in Europe were the [[KT66]], [[KT77]] and [[KT88]] by GEC (the KT standing for "Kinkless Tetrode").
Variations of the 6L6 design are still widely used in 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.
=== Special-purpose tubes ===
Some special-purpose tubes are constructed with particular gases in the envelope. For instance, [[voltage regulator]] [[gas filled tube|tube]]s contain various [[inert gas]]es such as [[argon]], [[helium]] or [[neon]], and take advantage of the fact that these gases will [[ion]]ize at predictable voltages. The [[thyratron]] is a special-purpose tube filled with low-pressure gas or mercury, some of which vaporizes. Like other 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, and the control electrode no longer can stop current flow; the tube "latches" into conduction. Removing plate (anode) voltage lets the gas de-ionize, restoring its non-conductive state. Some thyratrons can carry relatively large currents for their physical size. One example is the miniature type 2D21, often seen in 1950s [[jukebox]]es as control switches for [[relay]]s. A cold-cathode version of the thyratron, which uses a pool of mercury for its cathode, is called an Ignitron (tm). It can switch thousands of amperes in its largest versions. Thyratrons containing hydrogen have a very consistent time delay between their turn-on pulse and full conduction, and have long been used in radar transmitters. Thyratrons behave much like [[silicon controlled rectifier]]s.
Tubes usually have glass envelopes, but metal, fused quartz ([[silica]]), and [[ceramic]] are possible choices. The 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]] is a tiny tube made only of metal and ceramic. In some power tubes, the metal envelope is also the anode. 4CX800A 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]] 8974, a forced water-cooled power tetrode capable of dissipating 1.5 megawatts. (By comparison, the largest power transistor can only dissipate about 1 kilowatt.) A pair of 8974s is capable of producing 2 megawatts of audio power. The 8974 is used only in military and commercial radio-frequency installations.
==Reliability==
[[Image:Acremeter tube tester.jpg|thumb|upright|Tube tester manufactured in 1930]]
The chief reliability problem of a tube is that the filament or cathode is slowly "[[cathode poisoning|poisoned]]" by atoms from other elements in the tube, which damage its ability to emit electrons. Trapped gases or slow gas leaks can also damage the cathode or cause plate-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, due to its positive temperature coefficient, is generally associated with the power-up period as a result of the switch-on current surge. A negative temperature coefficient device, such as a [[thermistor]], was sometimes incorporated in the equipment heater supply to compensate.
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.
===Vacuum ===
The vacuum inside the envelope must be as perfect, or "hard", as possible. Any gas atoms remaining might be [[ionize]]d at operating voltages, and will conduct electricity between the elements in an uncontrolled manner. This can lead to erratic operation or even catastrophic destruction of the tube and associated circuitry. Unabsorbed free air sometimes ionizes and becomes visible as a pink-purple [[glow discharge]] between the tube elements.
To prevent any remaining [[gas]]es from remaining in a free state in the tube, modern tubes are constructed with "[[getter]]s", which are usually small, circular troughs filled with metals that oxidize quickly, with [[barium]] being the most common. While the tube envelope is being evacuated, the internal parts except the getter are heated by [[Radio frequency|RF]] [[induction heating]] to extract any remaining gases from the metal. The tube is then sealed and the getter is heated to a high temperature, again by radio frequency induction heating. This causes the material to evaporate, absorbing/reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside of the envelope of the tube. The getter continues to absorb any gas molecules that leak into the tube during its working life. If a tube develops a crack in the envelope, this deposit turns a white color when 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.
[[Image:P-120-row-220.jpg|thumb|right|Schematic diagram of a 1948 vacuum tube radio from Poland.]]
=== Transmitting tubes ===
Large transmitting tubes have [[tungsten]] filaments containing a small trace of [[thorium]]. A thin layer of thorium atoms forms on the outside of the wire when heated, serving as an efficient source of electrons. The thorium slowly evaporates from the wire surface, while new thorium atoms [[diffusion|diffuse]] to the surface to replace them. Such thoriated tungsten cathodes deliver lifetimes in the tens of thousands of hours. The claimed record is held by an Eimac power tetrode used in a Los Angeles radio station's transmitter, which was removed from service after 80,000 hours (~9 years) of operation. Transmitting tubes are also claimed to survive lightning strikes more often than transistor transmitters do. For RF power levels above 20 kilowatts, vacuum tubes are commonly more efficient and reliable than similar solid-state circuits.
=== 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 when heated to about 780 degrees Celsius, thus emitting electrons.
=== Computer vacuum tubes ===
{{see also|List of vacuum tube computers}}
==== Colossus ====
The [[Colossus computer]]'s designer, Dr [[Tommy Flowers]], had a theory that most of the unreliability was caused during power down and (mainly) power up. Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators (the wartime mains supply being considered too unreliable). The only time it was switched off was for conversion to the Colossus Mk2 and the addition of another 500 or so tubes. Another 9 Colossus Mk2s were built, and all 10 machines ran with a surprising degree of reliability. The 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year—nearly all of it for the tube heaters.
====Whirlwind====
To meet the reliability requirements of the early digital computer [[Whirlwind (computer)|Whirlwind]], it was necessary to build special "computer vacuum tubes" with extended cathode life. 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 the silicon from the heater wire alloy (and paying extra for more frequent replacement of the [[wire]] drawing [[Die (manufacturing)#Wire pulling|dies]]) allowed production of tubes that were reliable enough for the Whirlwind project. The tubes developed for Whirlwind later found their way into the giant [[Semi Automatic Ground Environment|SAGE]] air-defense computer system. High-purity [[nickel]] tubing and cathode coatings free of materials that can poison emission (such as [[silicate]]s and [[aluminium]]) also contribute to long cathode life. The first such "computer tube" was Sylvania's 7AK7 of 1948. 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 reliability made mid-cable amplifiers in [[Submarine communications cable|submarine cable]]s possible.
=== World War II ===
[[Image:CV4501.JPG|thumb|right|A CV4501 subminiature tube for use in a military radio set. The tube is a special quality type based on the EF72, 35 mm long and 10 mm in diameter (excluding leads).]]
Near the end of [[World War II]], to make radios more rugged, some aircraft and army radios began to integrate the tube envelopes into the radio's cast [[aluminium]] or [[zinc]] chassis. The radio became just a printed circuit with non-tube components, soldered to the chassis that contained all the tubes. Another WWII idea was to make very small and rugged glass tubes, originally for use in radio-frequency metal detectors built into [[artillery]] shells. These [[proximity fuze]]s made artillery more effective. Tiny tubes were later known as "subminiature" types. They were widely used in 1950s military and aviation electronics.
==Applications==
Tubes were heavily used in the early generations of electronic devices, such as [[radio]]s, [[television]]s, and early [[computer]]s such as the [[Colossus computer|Colossus]] which used 2000 tubes, the [[ENIAC]] which used nearly 18,000 tubes, and the [[IBM 700/7000 series|IBM 700 series]].
Vacuum tubes are less susceptible than corresponding solid-state components to the [[electromagnetic pulse]] effect of [[nuclear explosion]]s. This property kept them in use for certain [[military]] applications long after transistors had replaced them elsewhere. Vacuum tubes are still used for very high-powered applications such as [[microwave oven]]s, industrial radio-frequency heating, generating large amounts of RF energy for [[particle accelerator]]s, and power amplification for broadcasting.
Many [[audiophile]]s, professional audio engineers, and musicians prefer the [[tube sound]] of audio equipment based on vacuum tubes over electronics based on [[transistor]]s. There are companies which still make specialized audio hardware featuring tube technology.
Tubes are still being manufactured today in China: Shuguang Electron Group Co. (Specialised to manufacturing tubes primarily for audio applications), Tianjin tube factory (Very small manufacturer of high-end audio tubes), Nanjing Sanle Electronics Co. (Transmitting and industrial tubes, such like all new production 3-500ZG, 4-250A and 4-400C series tube types for used in radio amateur's linear amplifiers and professional broadcasters radio transmitters), JiangXi Jingguang Electronics Co. (Ceramic transmitting tubes, some of them marketed by Eimac), Huaguang Electric Power & Electronics Co. (Manufactures a huge catalogue of transmitting and industrial tubes, also good quality chinese made 833C), Russia: JSC Expo-pul (Audio tube factory of New Sensor Inc., Tubes is marketed by different kind of brand names; Sovtek, Electro Harmonix, Tung-Sol, Mullard and also Svetlana S-marked in USA), LLC Ryazan (Specialised to manufacturing all russian made SV811 and SV572 series tubes for audio applications and transmitting tubes like 811A, 572B and GU-81, marketed in western countries by Svetlana, Sovtek and Ryazan brands), MVSZ Svetlana St. Petersburg (High quality "winged-C" audio and ceramic transmitting tubes), JSC "Voskhod" KRLZ. (Specialised to manufacturing only some tube types for small signal RF and audio applications), USA: Western Electric Inc. (Legendary 300B triodes), Communications & Power Industries Inc. (Eimac and rebuilded Econco high power transmitting tubes), Burle Industries Inc. (Industrial and transmitting tubes, formerly factory of RCA), MPD Components Inc. (Specialised in microwave tubes, formerly Ken-Rad and later GE tube factory), France: Thales Electron Devices (High power transmitting tubes), Czech Republic: Emission Labs (High-end audio tubes), Tesla Electrontubes s.r.o. (Specialised to manufacturing only transmitting tubes), Slovakia: JJ-Electronic (Primarily for audio applications, factory was formerly part of Tesla Electrontubes), Serbia: Ei "Elektronska industrija" RC (out of production).
[[Image:Tubes.jpg|thumb|right|Russian 12AX7 tubes inside a modern guitar amplifier.]]
The sound produced by a tube based amplifier with the tubes overloaded ([[Overdrive (music)|overdriven]]) is widely used in [[electric guitar]] [[Instrument amplifier|amplification]], and has defined the texture of some genres of music such as classic rock and blues. Guitarists often prefer tube amplifiers for the perceived warmth of their tone and the natural compression effect they can apply to an input signal.
In 2002, computer motherboard maker [[AOpen]] brought back the vacuum tube for modern computer use by releasing the AX4GE Tube-G motherboard. This motherboard uses a Sovtek 6922 vacuum tube (a version of the [[6DJ8]]) as part of AOpen’s TubeSound Technology. AOpen claims that the vacuum tube brings superior sound.
==Cooling==
Like any electronic device, vacuum tubes produce heat while operating. Compared to semiconductor devices, larger tubes operate at higher power levels and hence dissipate more heat. The majority of the heat is dissipated at the anode, though some of the grids can also dissipate power. The tube's heater also contributes to the total (and is a heat source that is absent in semiconductors).
Various methods of cooling are used to remove generated heat. For low-power dissipation devices the heat is radiated from the anode, which often is blackened on the external surface to assist infrared radiation. Natural air circulation or [[convection]] is usually required to keep power tubes from overheating. For larger power dissipation, forced-air cooling (fans) may be required.
From the inception of this technology until the 1950s, the dominant approach to cooling low-power tubes remained aimed at avoiding immediate or very short term failures. For noncritical consumer applications, and in absence of technological alternatives, tube failures did not create major problems for equipment manufacturers, as the cost of tube replacements was borne by end users long accustomed to the experience. Some tubes for the US defense market featured a metal casing, as opposed to glass, and an opaque, black finish that facilitated both heat conduction and radiative cooling. In some highly specialized professional applications where replacement was out of the question, such as [[undersea cable]] repeaters, no failures were acceptable. Moreover, as vacuum tube based defence systems became increasingly complex and deployed in ever increasing numbers, it became clear that point failures which were individually easy to diagnose and rectify had a devastating effect on the [[uptime]] of systems that contained hundreds of tubes. This resulted in both the creation of special long lasting tubes for projects such as [[Whirlwind (computer)|Whirlwind]] and [[Semi Automatic Ground Environment|SAGE]], and also in special tube shields that aided heat dispersal and could be retrofitted on existing equipment. These shields act by improving heat conduction from the surface of the tube to the shield itself by means of tens of copper tongues in contact with the glass tube, and have an opaque, black outside finish for improved heat radiation.
High-power tubes in older, large transmitters or power amplifiers are liquid cooled, usually with deionised water for heat transfer to an external radiator, similar to the cooling system of an internal combustion engine. Since the anode is usually the cooled element, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator. Otherwise the high voltage can be conducted through the cooling water to the radiator system; hence the need for deionised water. Such systems usually have a built-in water-conductance monitor which will shut down the high-tension supply (often tens of kilovolts) if the conductance becomes too high. Some very high-power transmitters, such as those used in shortwave broadcasting and VLF communications, use pressurized steam for cooling. Modern transmitters using tubes mainly in the PA section are now largely cooled by forced air through a radiator or other heat-sinking device.
== Other vacuum tube devices ==
Many devices were built during the 1920–1960 period using vacuum-tube techniques. Most such tubes were rendered obsolete by semiconductors; some techniques for integrating multiple devices in a single module, sharing the same glass envelope have been discussed above, such as the [[Loewe 3NF]]. Vacuum-tube electronic devices still in common use include the [[magnetron]], [[klystron]], [[photomultiplier]], [[x-ray tube]] and [[cathode ray tube]]. The magnetron is the type of tube used in all [[microwave oven]]s. 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. Photomultipliers are still the most sensitive detectors of light. Many [[television]]s, [[oscilloscope]]s and computer monitors still use cathode ray tubes, though [[flat panel display]]s are becoming more popular as prices drop.
Many of the better tube radios had so-called "tuning eye" indicator tubes behind their front panels, with just the top of the tube showing. See External Links, below: "Good article about tuning eye tubes". This article describes tubes used in the USA; Europe had tubes that worked on the same general principles, but different-looking.
[[Secondary emission]] is the term for what happens when electrons in a vacuum strike certain materials, and the impacts cause electrons to be emitted. For some materials, more electrons are emitted than originally hit the surface. Such devices, called electron multipliers, amplify the current represented by the incoming electrons. Several stages (as many as 15 or so) can be cascaded for high gain, and are essential parts of very sensitive phototubes, usually called photomultipliers or multiplier photoubes. The image orthicon TV studio camera tubes also used multistage photomultipliers.
For decades, electron-tube designers tried to use secondary emission to obtain more amplification in vacuum tubes with hot cathodes, but they suffered from short life because the material used for the secondary-emission electrode (called a dynode) "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. However, transistors were rapidly improving, and eclipsed tubes in general.
A variant, called a channel electron multiplier, is simply a curved tube, such as a helix, coated on the inside with material with good secondary emission. One type had a little funnel to capture incoming electrons. The tube was resistive, and its ends were connected to enough voltage to create repeated cascades of electrons.
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.)
The fluorescent displays commonly used on VCRs and automotive dashboards are actually vacuum tubes, using [[phosphor]]-coated anodes to form the display characters, and a heated filamentary cathode as an electron source. These devices are properly called "VFDs", or [[Vacuum fluorescent display|Vacuum Fluorescent Displays]]. Because the filaments are in view, they must be operated at temperatures where the filament does not glow visibly. These devices are often found in automotive applications, where their high brightness allows reading the display in daylight.
Some tubes, like [[magnetron]]s, [[traveling wave tube]]s, [[carcinotron]]s, and [[klystron]]s, combine magnetic and electrostatic effects. These are efficient (usually narrow-band) RF producers and still find use in [[radar]], [[microwave oven]]s and industrial heating. Traveling-wave tubes (TWTs) are very good amplifiers; they are used in some communications satellites. High-powered klystron amplifier tubes can provide hundreds of kW in the UHF range.
[[Gyrotron]]s or vacuum masers, used to generate high-power millimetre band waves, are magnetic vacuum tubes in which a small [[relativity (physics)|relativistic]] effect, due to the high voltage, is used for bunching the electrons. Gyrotrons can generate very high powers (hundreds of kW).
[[Free electron laser]]s, used to generate high-power coherent light and perhaps even [[X ray]]s, are highly relativistic vacuum tubes driven by high-energy particle accelerators.
[[Particle accelerator]]s can be considered vacuum tubes that work backward, the electric fields driving the electrons, or other charged particles. In this respect, a cathode ray tube is a particle accelerator.
A tube in which electrons move through a vacuum (or gaseous medium) within a gas-tight envelope is generically called an ''electron tube''.
Some [[condenser microphone]] designs use built-in vacuum tube [[preamplifier]]s.
[[Image:JolidaJD502B.JPG|thumb|right|Jolida JD502B, a modern tube [[integrated amplifier]], with preamp tubes in front, power tubes in back.]]
As of 2008, scores of small companies are manufacturing [[audiophile]] amplifiers and preamps that use vacuum tubes.<ref>{{cite web |url=http://www.worldtubeaudio.com/directory/directory.htm |title=World Tube Audio Portal }}</ref>
''Vacuum tube'' can also mean a tube with a [[vacuum]]. It is e.g. used for demonstration of, and experiments with, [[free-fall]].
==Field emitter vacuum tubes==
In the early years of the 21st century there has been renewed interest in vacuum tubes, this time in the form of [[integrated circuit]]s. The most common design uses a [[cold cathode]] [[field emission|field emitter]], with electrons emitted from a number of sharp [[Spindt tip|nano-scale tips]] formed on the surface of a metal cathode.
Their advantages include greatly enhanced robustness combined with the ability to provide high power outputs at low power consumptions. Operating on the same principles as traditional tubes, prototype device cathodes have been constructed with emitter tips formed using [[Inorganic nanotube|nanotube]]s, and by etching electrodes as hinged flaps (similar to the technology used to create the microscopic mirrors used in [[Digital Light Processing]]) that are stood upright by an [[electrostatic charge]].
Such integrated microtubes may find application in [[microwave]] devices including [[mobile phone]]s, for [[Bluetooth]] and [[Wi-Fi]] transmission, in [[radar]] and for [[satellite]] communication. Presently they are being studied for possible application to flat-panel display construction.
==See also==
{{portalpar|Electronics|Nuvola_apps_ksim.png}}
{{multicol}}
*[[List of vacuum tubes]]
*[[List of vacuum tube computers]]
*[[All American Five]]
*[[Gas filled tube]]
*[[Irving Langmuir]]
*[[Nixie tube]], a [[display device]] that is sometimes mistaken for a vacuum tube but contains [[neon]] gas
{{multicol-break}}
*[[Pneumatic tube]]
*[[Tube sound]]
*[[Valve amplifier]]
*[[RETMA tube designation]]
*[[RMA tube designation]]
*[[Mullard-Philips tube designation]]
{{multicol-end}}
==Patents==
* {{US patent|803684}} - Instrument for converting alternating electric currents into continuous currents ([[Fleming valve]] patent)
* {{US patent|841387}} - Device for amplifying feeble electrical currents
* {{US patent|879532}} - De Forest's [[Audion tube|Audion]]
* {{US patent|2141059}} - Television system
==Notes==
{{reflist}}
==References==
*{{cite book |last=Spangenberg |first=Karl R. |title=Vacuum Tubes |publisher=McGraw-Hill |date = 1948 |id={{LCC|TK7872.V3}} {{OCLC|567981}} }}
*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. A range of books published in the UK in the 1940s and 50s by Cleaver Hume Press on all aspects of the design and application of vacuum tubes. They were originally published in Dutch in Holland. French and German editions were probably also published.
*RCA "Radiotron Designer's Handbook" 1953(4th Edition) Contains very useful 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 at the time.
<references/>
==External links==
{{commonscat|Vacuum tubes}}
*[http://www.john-a-harper.com/tubes201/ How vacuum tubes really work] - John Harper's webpage about thermionic emission and vacuum tube theory, using introductory college-level mathematics.
*[http://www.ken-gilbert.com/techstuff/vtf.html The Vacuum Tube FAQ] - Henry Pasternack's FAQ from rec.audio
* [http://www.marconicalling.com/museum/html/events/events-i=39-s=0.html The invention of the thermionic valve]. Fleming discovers the thermionic (or oscillation) valve, or 'diode'.
* [http://www.milbert.com/tstxt.htm Tubes Vs. Transistors] : Is There An Audible Difference? - 1972 AES paper on audible differences in sound quality between vacuum tubes and transistors.
* [http://www.tubecollector.org/ The Virtual Valve Museum]
* [http://members.chello.nl/~h.dijkstra19 The cathode ray tube site]
* [http://www.oneillselectronicmuseum.com/page10.html O'Neill's Electronic Museum - Vacuum tube museum]
* [http://www.atatan.com/~s-ito/vacuum/vacuum.html Vacuum Tubes for Beginners] -- [http://www.atatan.com/~s-ito/vacuum/vacuum-j.html Japanese Version]
* [http://www.nj7p.org/Tube.php NJ7P Tube Database] - Searchable online electronic data manual for tubes used in North America.
*[http://tdsl.duncanamps.com/ Vacuum tube data sheet locator]
*[http://www.classiccmp.org/rtellason/tubes.html Large tube chart listing some characteristics and giving access to datasheets for most of them]
*[http://blog.makezine.com/archive/2008/01/make_your_own_vaccum_tube.html A video of a French amateur radio operator making his own vacuum tube triodes]
*[http://everything2.com/title/tuning%2520eye%2520tubes Good article about tuning eye tubes.]
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