Vacuum tube

This article is about the electronic device. For experi-ments in an evacuated pipe, see free fall. For the trans-port system, see pneumatic tube.In electronics, a vacuum tube, an electron tube,[1][2][3]

Modern vacuum tubes, mostly miniature style

or just a tube (North America), or valve (Britain andsome other regions) is a device that controls electric cur-rent between electrodes in an evacuated container. Vac-uum tubes mostly rely on thermionic emission of elec-trons from a hot filament or a cathode heated by thefilament. This type is called a thermionic tube orthermionic valve. A phototube, however, achieves elec-tron emission through the photoelectric effect. Not allelectronic circuit valves/electron tubes are vacuum tubes(evacuated); gas-filled tubes are similar devices contain-ing a gas, typically at low pressure, which exploit phe-nomena related to electric discharge in gases, usuallywithout a heater.The simplest vacuum tube, the diode, contains only aheater, a heated electron-emitting cathode (the filamentitself acts as the cathode in some diodes), and a plate (an-ode). Current can only flow in one direction through thedevice between the two electrodes, as electrons emittedby the cathode travel through the tube and are collectedby the anode. Adding one or more control grids withinthe tube allows the current between the cathode and an-ode to be controlled by the voltage on the grid or grids.[4]Tubes with grids can be used for many purposes, includ-ing amplification, rectification, switching, oscillation, anddisplay.Invented in 1904 by John Ambrose Fleming, vacuumtubes were a basic component for electronics through-out the first half of the twentieth century, which sawthe diffusion of radio, television, radar, sound reinforce-ment, sound recording and reproduction, large telephonenetworks, analog and digital computers, and industrialprocess control. Although some applications had coun-terparts using earlier technologies such as the spark gap

transmitter or mechanical computers, it was the inven-tion of the vacuum tube that made these technologieswidespread and practical. In the 1940s the invention ofsemiconductor devices made it possible to produce solid-state devices, which are smaller, more efficient, more reli-able, more durable, and cheaper than tubes. Hence, fromthemid-1950s solid-state devices such as transistors grad-ually replaced tubes. The cathode-ray tube (CRT) re-mained the basis for televisions and video monitors un-til superseded in the 21st century. However, there arestill a few applications for which tubes are preferred tosemiconductors; for example, the magnetron used in mi-crowave ovens, and certain high frequency amplifiers.

1 Classifications

One classification of vacuum tubes is by the number ofactive electrodes, (neglecting the filament or heater). Adevice with two active elements is a diode, usually usedfor rectification. Devices with three elements are triodesused for amplification and switching. Additional elec-trodes create tetrodes, pentodes, and so forth, which havemultiple additional functions made possible by the addi-tional controllable electrodes.Other classifications are:

• by frequency range (audio, radio, VHF, UHF,microwave)

• by power rating (small-signal, audio power, high-power radio transmitting)

• by cathode/filament type (indirectly heated, di-rectly heated) andWarm-up time (including “bright-emitter” or “dull-emitter”)

• by characteristic curves design (e.g., sharp- versusremote-cutoff in some pentodes)

• by application (receiving tubes, transmitting tubes,amplifying or switching, rectification, mixing)

• specialized parameters (long life, very lowmicrophonic sensitivity and low-noise audioamplification, rugged/military versions

• specialized functions (light or radiation detectors,video imaging tubes)

• tubes used to display information (Nixie tubes,“magic eye” tubes, Vacuum fluorescent displays,CRTs)

1

2 2 DESCRIPTION

Multiple classifications may apply to a device; for exam-ple similar dual triodes can be used for audio preampli-fication and as flip-flops in computers, although linearityis important in the former case and long life in the latter.Tubes have different functions, such as cathode ray tubeswhich create a beam of electrons for display purposes(such as the television picture tube) in addition to morespecialized functions such as electron microscopy andelectron beam lithography. X-ray tubes are also vac-uum tubes. Phototubes and photomultipliers rely on elec-tron flow through a vacuum, though in those cases elec-tron emission from the cathode depends on energy fromphotons rather than thermionic emission. Since thesesorts of “vacuum tubes” have functions other than elec-tronic amplification and rectification they are describedin their own articles.

2 Description

Glass tube

Anode

Heater

Heatedcathode

Diode: elec-trons from the hot cathode flow towards the positiveanode, but not vice versa

Glass tube

Anode

Heater

Heatedcathode

Grid

Triode: volt-age applied to the grid controls plate (anode) current.

A vacuum tube consists of two or more electrodes in avacuum inside an airtight enclosure. Most tubes haveglass envelopes, though ceramic and metal envelopes(atop insulating bases) have been used. The electrodesare attached to leads which pass through the envelopevia an airtight seal. On most tubes, the leads, in theform of pins, plug into a tube socket for easy replace-ment of the tube (tubes were a frequent cause of failurein electronic equipment, and consumers were expectedto be able to replace tubes themselves). Some tubeshad an electrode terminating at a top cap. The princi-pal reason for doing this was to avoid leakage resistancethrough the tube base, particularly for the high impedancegrid input.[5][6] The bases were commonly made withphenolic insulation which performs poorly as an insulatorin humid conditions. Other reasons for using a top capinclude reduced grid-to-anode capacitance,[7] improvedhigh-frequency performance, keeping a very high platevoltage away from lower voltages, and accommodatingone more electrode than allowed by the base. There waseven an occasional design that had two top cap connec-tions.The earliest vacuum tubes evolved from incandescentlight bulbs, containing a filament sealed in an evacuatedglass envelope. When hot, the filament releases electronsinto the vacuum, a process called thermionic emission,originally known as the “Edison Effect”. A second elec-trode, the anode or plate, will attract those electrons if itis at a more positive voltage. The result is a net flow ofelectrons from the filament to plate. However, electronscannot flow in the reverse direction because the plate isnot heated and does not emit electrons. The filament(cathode) has a dual function: it emits electrons whenheated; and, together with the plate, it creates an elec-tric field due to the potential difference between them.Such a tube with only two electrodes is termed a diode,and is used for rectification. Since current can only pass

3

in one direction, such a diode (or rectifier) will convertalternating current (AC) to pulsating DC. This can there-fore be used in a DC power supply, and is also used as ademodulator of amplitude modulated (AM) radio signalsand similar functions.Early tubes used the directly heated filament as the cath-ode. Many more modern tubes employ indirect heat-ing, with a separate electrically isolated “heater” inside atubular cathode. The heater is not an electrode, but sim-ply serves to heat the cathode sufficiently for thermionicemission of electrons. This allowed all the tubes to beheated through a common circuit (which can as well beAC) while allowing each cathode to arrive at a voltage in-dependently of the others, removing an unwelcome con-straint on circuit design.The filaments require constant and often considerablepower, even when amplifying signals at the microwattlevel. Power is also dissipated when the electrons fromthe cathode slam into the anode (plate) and heat it; thiscan occur even in an idle amplifier due to quiescent cur-rents necessary to ensure linearity and low distortion. In apower amplifier, this heating can be considerable and candestroy the tube if driven beyond its safe limits. Sincethe tube contains a vacuum, the anodes in most small andmedium power tubes are cooled by radiation through theglass envelope. In some special high power applications,the anode forms part of the vacuum envelope to conductheat to an external heat sink, usually cooled by a blower,or water-jacket.Klystrons and magnetrons often operate their anodes(called collectors in klystrons) at ground potential to fa-cilitate cooling, particularly with water, without high-voltage insulation. These tubes instead operate with highnegative voltages on the filament and cathode.Except for diodes, additional electrodes are positionedbetween the cathode and the plate (anode). These elec-trodes are referred to as grids as they are not solid elec-trodes but sparse elements through which electrons canpass on their way to the plate. The vacuum tube is thenknown as a triode, tetrode, pentode, etc., depending onthe number of grids. A triode has three electrodes: theanode, cathode, and one grid, and so on. The first grid,known as the control grid, (and sometimes other grids)transforms the diode into a voltage-controlled device: thevoltage applied to the control grid affects the current be-tween the cathode and the plate. When held negative withrespect to the cathode, the control grid creates an electricfield which repels electrons emitted by the cathode, thusreducing or even stopping the current between cathodeand anode. As long as the control grid is negative rela-tive to the cathode, essentially no current flows into it, yeta change of several volts on the control grid is sufficientto make a large difference in the plate current, possiblychanging the output by hundreds of volts (depending onthe circuit). The solid-state device which operates mostlike the pentode tube is the junction field-effect transistor

(JFET), although vacuum tubes typically operate at overa hundred volts, unlike most semiconductors in most ap-plications.

3 History and development

One of Edison’s experimental bulbs

The 19th century saw increasing research with evacu-ated tubes, such as the Geissler and Crookes tubes. Themany scientists and inventors who experimented withsuch tubes include Thomas Edison, Eugen Goldstein,Nikola Tesla, and Johann Wilhelm Hittorf. With the ex-ception of early light bulbs, such tubes were only used inscientific research or as novelties. The groundwork laidby these scientists and inventors, however, was critical tothe development of subsequent vacuum tube technology.Although thermionic emission was originally reported in1873 by Frederick Guthrie,[8] it was Thomas Edison’sapparently independent discovery of the phenomenonin 1883 that became well known. Although Edisonwas aware of the unidirectional property of current flow

4 3 HISTORY AND DEVELOPMENT

between the filament and the anode, his interest (andpatent[9]) concentrated on the sensitivity of the anode cur-rent to the current through the filament (and thus filamenttemperature). Little practical use was ever made of thisproperty (however early radios often implemented vol-ume controls through varying the filament current of am-plifying tubes). It was only years later that John AmbroseFleming utilized the rectifying property of the diode tubeto detect (demodulate) radio signals, a substantial im-provement on the early cat’s-whisker detector alreadyused for rectification.However actual amplification by a vacuum tube only be-came practical with Lee De Forest's 1907 invention ofthe three-terminal "audion" tube, a crude form of whatwas to become the triode.[10] Being essentially the firstelectronic amplifier, [11] such tubes were instrumental inlong-distance telephony (such as the first coast-to-coasttelephone line in the US) and public address systems, andintroduced a far superior and versatile technology for usein radio transmitters and receivers. The electronics revo-lution of the 20th century arguably began with the inven-tion of the triode vacuum tube.

3.1 Diodes

Fleming’s first diodes

Main article: Diode

The English physicist John Ambrose Fleming workedas an engineering consultant for firms including EdisonTelephone and the Marconi Company. In 1904, as a re-sult of experiments conducted on Edison effect bulbs im-ported from the USA, he developed a device he called an“oscillation valve” (because it passes current in only onedirection). The heated filament, or cathode, was capa-ble of thermionic emission of electrons that would flowto the plate (or anode) when it was at a higher voltage.Electrons, however, could not pass in the reverse direc-tion because the plate was not heated and thus not capableof thermionic emission of electrons.Later known as the Fleming valve, it could be used asa rectifier of alternating current and as a radio wavedetector. This greatly improved the crystal set which

rectified the radio signal using an early solid-state diodebased on a crystal and a so-called cat’s whisker. Unlikemodern semiconductors, such a diode required painstak-ing adjustment of the contact to the crystal in order for itto rectify. The tube was relatively immune to vibration,and thus vastly superior on shipboard duty, particularlyfor navy ships with the shock of weapon fire commonlyknocking the sensitive but delicate galena off its sensitivepoint (the tube was in general no more sensitive as a radiodetector, but was adjustment free). The diode tube wasa reliable alternative for detecting radio signals. Higherpower diode tubes or power rectifiers found their way intopower supply applications until they were eventually re-placed by silicon rectifiers in the 1960s.

3.2 Triodes

Main article: TriodeOriginally, the only use for tubes in radio circuits was

The first triode, the De Forest Audion, invented in 1906

Triodes as they evolved over 40 years of tube manufacture, fromthe RE16 in 1918 to a 1960s era miniature tube.

for rectification, not amplification. In 1906, Robert vonLieben filed for a patent[12] for a cathode ray tube whichincluded magnetic deflection. This could be used for am-plifying audio signals and was intended for use in tele-phony equipment. He would later go on to help refine thetriode vacuum tube.

3.2 Triodes 5

Triode symbol. From top to bottom: plate (anode), control grid,cathode, heater (filament)

However, it was Lee De Forest who is credited with in-venting the triode tube in 1907 while continuing experi-ments to improve his original Audion tube, a crude fore-runner of the triode. By placing an additional electrodebetween the filament (cathode) and plate (anode), he dis-covered the ability of the resulting device to amplify sig-nals of all frequencies. As the voltage applied to the so-called control grid (or simply “grid”) was lowered fromthe cathode’s voltage to somewhat more negative volt-ages, the amount of current from the filament to the platewould be reduced. The negative electrostatic field createdby the grid in the vicinity of the cathode would inhibitthermionic emission and reduce the current to the plate.Thus, a few volts’ difference at the grid would make alarge change in the plate current and could lead to a muchlarger voltage change at the plate; the result was voltageand power amplification. In 1908, De Forest filed for apatent (U.S. Patent 879,532) for such a three-electrodeversion of his original Audion tube for use as an elec-tronic amplifier in radio communications. This eventu-ally became known as the triode.De Forest’s device was not a hard vacuum tube, as he er-roneously believed that it depended on the presence ofresidual gas remaining after evacuation. In its Audionleaflets, the De Forest company even warned against anyoperation which might lead to too high a vacuum. In1912 De Forest brought the audion to Harold Arnold inAT&T’s engineering department. Arnold recommendedthat AT&T purchase the patent. He developed high-vacuum tubes which were tested in the summer of 1913on AT&T’s long distance network.[13]

The Finnish inventor Eric Tigerstedt significantly im-proved on the original triode design in 1914, while work-ing on his sound-on-film process in Berlin, Germany.Tigerstedt’s innovation was to make the electrodes con-

General Electric Company Pliotron, Chemical Heritage Founda-tion

centric cylinders with the cathode at the centre, thusgreatly increasing the collection of emitted electrons atthe anode.[14] The first true vacuum triodes in productionwere the Pliotrons developed by Irving Langmuir at theGeneral Electric research laboratory (Schenectady, NewYork) in 1915. Langmuir was one of the first scientists torealize that a harder vacuum would improve the amplify-ing behaviour of the triode, having improved Gaede’s dif-fusion vacuum pump. Pliotrons were closely followed bythe French type 'TM' and later the English type 'R' whichwere in widespread use by the allied military by 1916.These types were the first true hard vacuum tubes; earlydiodes and triodes performed as such despite a rather highresidual gas pressure. Techniques to produce and main-tain better vacua in tubes were then developed. Histori-cally, vacuum levels in production vacuum tubes typicallyranged from 10 µPa down to 10 nPa.

6 3 HISTORY AND DEVELOPMENT

The triode and its derivatives (tetrodes and pentodes) aretransconductance devices, in which the controlling signalapplied to the grid is a voltage, and the resulting ampli-fied signal appearing at the anode is a current. Comparethis to the behavior of the bipolar junction transistor, inwhich the controlling signal is a current and the outputis also a current. For vacuum tubes, transconductance ormutual conductance (gm) is defined as the change in theplate(anode)/cathode current divided by the correspond-ing change in the grid/cathode voltage, with a constantplate(anode)/cathode voltage. Typical values of gm for asmall-signal vacuum tube are 1 to 10 millisiemens. It isone of the three 'constants’ of a vacuum tube, the othertwo being its gain μ and plate resistance R or Rₐ. TheVan der Bijl equation defines their relationship as follows:gm = µ

Rp

The non-linear operating characteristic of the triodecaused early tube audio amplifiers to exhibit harmonicdistortion at low volumes. Plotting plate current as a func-tion of applied grid voltage, it was seen that there was arange of grid voltages for which the transfer characteris-tics were approximately linear. To use this range, a neg-ative bias voltage had to be applied to the grid to posi-tion the DC operating point in the linear region. This wascalled the idle condition, and the plate current at this pointthe “idle current”. The controlling voltage was superim-posed onto the bias voltage, resulting in a linear variationof plate current in response to both positive and negativevariation of the input voltage around that point. This con-cept is called grid bias. Many early radio sets had a thirdbattery called the “C battery” (unrelated to the present-day C cell) whose positive terminal was connected to thecathode of the tubes (or “ground” in most circuits) andwhose negative terminal supplied this bias voltage to thegrids of the tubes. Later circuits, after tubes were madewith heaters isolated from their cathodes, used cathodebiasing, avoiding the need for a separate negative powersupply. However C batteries continued to be included insome equipment even when the “A” and “B” batteries hadbeen replaced by power from the AC mains. That waspossible because there was essentially no current draw onthese batteries; they could thus last for many years (oftenlonger than all the tubes) without requiring replacement.When triodes were first used in radio transmitters and re-ceivers, it was found that tuned amplification stages hada tendency to oscillate unless their gain was very lim-ited. This was due to the parasitic capacitance betweenthe plate (the amplifier’s output) and the control grid(the amplifier’s input), known as the Miller capacitance.Eventually the technique of neutralization was developedwhereby the RF transformer connected to the plate (an-ode) would include an additional winding in the oppositephase. This winding would be connected back to the gridthrough a small capacitor, and when properly adjustedwould cancel the Miller capacitance. This technique wasemployed and led to the success of the Neutrodyne radioduring the 1920s. However, neutralization required care-

ful adjustment and proved unsatisfactory when used overa wide ranges of frequencies.

3.3 Tetrodes and pentodes

Main articles: Tetrode and PentodeTo combat the stability problems and limited voltage gain

Tetrode symbol. From top to bottom: plate (anode), screen grid,control grid, cathode, heater (filament)

due to the Miller effect, the physicist Walter H. Schot-tky invented the tetrode tube in 1919.[15] He showed thatthe addition of a second grid, located between the con-trol grid and the plate (anode), known as the screen grid,could solve these problems. (“Screen” in this case refersto electrical “screening” or shielding, not physical con-struction: all “grid” electrodes in between the cathodeand plate are “screens” of some sort rather than solid elec-trodes since they must allow for the passage of electronsdirectly from the cathode to the plate). A positive voltageslightly lower than the plate (anode) voltage was appliedto it, and was bypassed (for high frequencies) to groundwith a capacitor. This arrangement decoupled the an-ode and the control grid, essentially eliminating theMillercapacitance and its associated problems. Consequently,higher voltage gains from a single tube became possible,reducing the number of tubes required in many circuits.This two-grid tube is called a tetrode, meaning four activeelectrodes, and was common by 1926.However, the tetrode had one new problem. In any tube,electrons strike the anode with sufficient energy to causethe emission of electrons from its surface. In a triode thisso-called secondary emission of electrons is not impor-tant since they are simply re-captured by the more posi-tive anode (plate). But in a tetrode they can be capturedby the screen grid (thus also acting as an anode) sinceit is also at a high voltage, thus robbing them from the

3.4 Multifunction and multisection tubes 7

At certain values of plate voltage and current, the tetrode char-acteristic curves are kinked due to secondary emission.

plate current and reducing the amplification of the de-vice. Since secondary electrons can outnumber the pri-mary electrons, in the worst case, particularly as the platevoltage dips below the screen voltage, the plate currentcan decrease with increasing plate voltage. This is theso-called “tetrode kink” and is an example of negative re-sistance which can itself cause instability.[16] The other-wise undesirable negative resistance was exploited to pro-duce an extremely simple oscillator circuit only requiringconnection of the plate to a resonant LC circuit to oscil-late; this was effective over a wide frequency range. Theso-called dynatron oscillator thus operated on the sameprinciple of negative resistance as the tunnel diode oscil-lator many years later. Another undesirable consequenceof secondary emission is that in extreme cases enoughcharge can flow to the screen grid to overheat and destroyit. Later tetrodes had anodes treated to reduce secondaryemission; earlier ones such as the type 77 sharp-cutoffpentode connected as a tetrode made better dynatrons.The solution was to add another grid between the screengrid and the main anode, called the suppressor grid(since it suppressed secondary emission current towardthe screen grid). This grid was held at the cathode (or“ground”) voltage and its negative voltage (relative to theanode) electrostatically repelled secondary electrons sothat they would be collected by the anode after all. Thisthree-grid tube is called a pentode, meaning five elec-trodes. The pentode was invented in 1926 by BernardD. H. Tellegen[17] and became generally favored over thesimple tetrode. Pentodes are made in two classes: thosewith 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 beamtetrode or “beam power tube”, discussed below.

3.4 Multifunction and multisection tubes

The pentagrid converter contained five grids between the cathodeand the plate (anode).

Superheterodyne receivers require a local oscillator andmixer, combined in the function of a single pentagrid con-verter tube. Various alternatives such as using a combi-nation of a triode with a hexode and even an octode havebeen used for this purpose. The additional grids includeboth control grids (at a low potential) and screen grids (ata high voltage). Many designs used such a screen grid asan additional anode to provide feedback for the oscilla-tor function, whose current was added to that of the in-coming radio frequency signal. The pentagrid converterthus became widely used in AM receivers including theminiature tube version of the "All American Five". Oc-todes such as the 7A8 were rarely used in the US, butmuch more common in Europe, particularly in batteryoperated radios where the lower power consumption wasan advantage.To further reduce the cost and complexity of radio equip-ment, two separate structures (triode and pentode for in-stance) could be combined in the bulb of a single mul-

8 3 HISTORY AND DEVELOPMENT

tisection tube. An early example was the Loewe 3NF.This 1920s device had three triodes in a single glass en-velope together with all the fixed capacitors and resis-tors required to make a complete radio receiver. As theLoewe set had only one tube socket, it was able to substan-tially undercut the competition since, in Germany, statetax was levied by the number of sockets. However, re-liability was compromised, and production costs for thetube were much greater. In a sense, these were akin tointegrated circuits. In the US, Cleartron briefly producedthe “Multivalve” triple triode for use in the Emerson BabyGrand receiver. This Emerson set also had a single tubesocket, but because it used a four-pin base, the additionalelement connections were made on a “mezzanine” plat-form at the top of the tube base.By 1940 multisection tubes had become commonplace.There were constraints, however, due to patents and otherlicensing considerations (see British Valve Association).Constraints due to the number of external pins (leads) of-ten forced the functions to share some of those externalconnections such as their cathode connections (in addi-tion to the heater connection). The RCA Type 55 wasa double diode triode used as a detector, automatic gaincontrol rectifier and audio preamplifier in early AC pow-ered radios. These sets often included the 53 Dual TriodeAudio Output. Another early type of multi-section tube,the 6SN7, is a “dual triode” which performs the func-tions of two triode tubes, while taking up half as muchspace and costing less. The 12AX7 is a dual “high mu”(high voltage gain[18][19][20]) triode in a miniature enclo-sure, and became widely used in audio signal amplifiers,instruments, and guitar amplifiers.The introduction of the miniature tube base (see below)which could have 9 pins, more than previously avail-able, allowed other multi-section tubes to be introduced,such as the 6GH8/ECF82 triode-pentode, quite popularin television receivers. The desire to include even morefunctions in one envelope resulted in the General ElectricCompactron which had 12 pins. A typical example, the6AG11, contained two triodes and two diodes.Some otherwise conventional tubes do not fall into stan-dard categories; the 6JH8 had several common grids, fol-lowed by a pair of beam deflection electrodes which de-flected the current towards either of two anodes. It wassometimes known as the 'sheet beam' tube, and was usedin some color TV sets for demodulation of synchronoussignals, as for example for color demodulation.

3.5 Beam power tubes

Main article: Beam tetrode

The beam power tube is usually a tetrode with the ad-dition of beam-forming electrodes, which take the placeof the suppressor grid. These angled plates (not to beconfused with the anode) focus the electron stream onto

6L6 tubes in glass envelopes

certain spots on the anode which can withstand the heatgenerated by the impact of massive numbers of electrons,while also providing pentode behavior. The position-ing of the elements in a beam power tube uses a designcalled “critical-distance geometry”, which minimizes the“tetrode kink”, plate to control grid capacitance, screengrid current, and secondary emission from the anode, thusincreasing power conversion efficiency. The control gridand screen grid are also wound with the same pitch, ornumber of wires per inch.Aligning the grid wires also helps to reduce screen cur-rent, which represents wasted energy. This design helpsto overcome some of the practical barriers to design-ing high-power, high-efficiency power tubes. EMI engi-neers Cabot Bull and Sidney Rodda developed the de-sign which became the 6L6, the first popular beam powertube, introduced by RCA in 1936 and later correspond-ing tubes in Europe the KT66, KT77 and KT88 madeby the Marconi-Osram Valve subsidiary of GEC (the KTstanding for “Kinkless Tetrode”).“Pentode operation” of beam power tubes is often de-scribed in manufacturers’ handbooks and data sheets, re-sulting in some confusion in terminology.Variations of the 6L6 design are still widely used in tubeguitar amplifiers, making it one of the longest-lived elec-tronic device families in history. Similar design strate-gies are used in the construction of large ceramic powertetrodes used in radio transmitters.Beam power tubes can be connected as triodes for im-proved audio tonal quality but in triode mode deliver sig-nificantly reduced power output.

3.7 Miniature tubes 9

3.6 Gas-filled tubes

Gas-filled tubes such as discharge tubes and cold cath-ode tubes are not hard vacuum tubes, though are alwaysfilled with gas at less than sea-level atmospheric pressure.Types such as the voltage-regulator tube and thyratronresemble hard vacuum tubes and fit in sockets designedfor vacuum tubes. Their distinctive orange, red, or pur-ple glow during operation indicates the presence of gas;electrons flowing in a vacuum do not produce light withinthat region. These types may still be referred to as “elec-tron tubes” as they do perform electronic functions. High-power rectifiers use mercury vapor to achieve a lower for-ward voltage drop than high-vacuum tubes.

Subminiature CV4501 tube (SQ version of EF72), 35 mm long x10 mm diameter (excluding leads)

3.7 Miniature tubes

Early tubes used a metal or glass envelope atop an insulat-ing bakelite base. In 1938 a technique was developed touse an all-glass construction[21] with the pins fused in theglass base of the envelope. This was used in the designof a much smaller tube outline, known as the miniaturetube, having 7 or 9 pins. Making tubes smaller reducedthe voltage where they could safely operate, and also re-duced the power dissipation of the filament. Miniaturetubes became predominant in consumer applications suchas radio receivers and hi-fi amplifiers. However the largerolder styles continued to be used especially as higherpower rectifiers, in higher power audio output stages andas transmitting tubes.Subminiature tubes with a size roughly that of half acigarette were used in hearing-aid amplifiers. These tubesdid not have pins plugging into a socket but were sol-dered in place. The “acorn” valve (named due to itsshape) was also very small, as was the metal-cased RCAnuvistor from 1959, about the size of a thimble. The nu-vistor was developed to compete with the early transistorsand operated at higher frequencies than those early tran-sistors could. The small size supported especially high-frequency operation; nuvistors were used in UHF televi-sion tuners and someHiFi FM radio tuners (Sansui 500A)until replaced by high-frequency capable transistors.

RCA 6DS4 “Nuvistor” triode, circa 20 mm high by 11 mm diam-eter

Commercial packaging for vacuum tubes used in the latter halfof the 20th century including boxes for individual tubes (bottomright), sleeves for rows of the boxes (left), and bags that smallertubes would be put in by a store upon purchase (top right)

10 3 HISTORY AND DEVELOPMENT

3.8 Improvements in construction andperformance

The earliest vacuum tubes strongly resembled incandes-cent light bulbs and were made by lamp manufacturers,who had the equipment needed to manufacture glass en-velopes and the vacuum pumps required to evacuate theenclosures. De Forest used Heinrich Geissler's mercurydisplacement pump, which left behind a partial vacuum.The development of the diffusion pump in 1915 andimprovement by Irving Langmuir led to the develop-ment of high-vacuum tubes. After World War I, spe-cialized manufacturers using more economical construc-tion methods were set up to fill the growing demand forbroadcast receivers. Bare tungsten filaments operated ata temperature of around 2200 °C. The development ofoxide-coated filaments in themid-1920s reduced filamentoperating temperature to a dull red heat (around 700 °C),which in turn reduced thermal distortion of the tube struc-ture and allowed closer spacing of tube elements. This inturn improved tube gain, since the gain of a triode is in-versely proportional to the spacing between grid and cath-ode. Bare tungsten filaments remain in use in small trans-mitting tubes but are brittle and tend to fracture if han-dled roughly – e.g. in the postal services. These tubesare best suited to stationary equipment where impact andvibration is not present.

3.9 Indirectly heated cathodes

The desire to power electronic equipment using ACmains power faced a difficulty with respect to the pow-ering of the tubes’ filaments, as these were also the cath-ode of each tube. Powering the filaments directly froma power transformer introduced mains-frequency (50 or60 Hz) hum into audio stages. The invention of the“equipotential cathode” reduced this problem, with thefilaments being powered by a balanced AC power trans-former winding having a grounded center tap.A superior solution, and one which allowed each cathodeto “float” at a different voltage, was that of the indirectlyheated cathode: a cylinder of oxide-coated nickel actedas electron-emitting cathode, and was electrically isolatedfrom the filament inside it. Indirectly heated cathodes en-able the cathode circuit to be separated from the heatercircuit. The filament, no longer electrically connected tothe tube’s electrodes, became simply known as a “heater”,and could as well be powered by AC without any intro-duction of hum.[22] In the 1930s indirectly heated cath-ode tubes became widespread in equipment using ACpower. Directly heated cathode tubes continued to bewidely used in battery-powered equipment as their fila-ments required considerably less power than the heatersrequired with indirectly heated cathodes.Tubes designed for high gain audio applications may havetwisted heater wires to cancel out stray electric fields,

fields that could induce objectionable hum into the pro-gram material.Heaters may be energized with either alternating current(AC) or direct current (DC). DC is often used where lowhum is required.

3.10 Use in electronic computers

See also: List of vacuum tube computersVacuum tubes used as switches made electronic com-

The 1946 ENIAC computer used 17,468 vacuum tubes and con-sumed 150 kW of power

puting possible for the first time, but the cost andrelatively short mean time to failure of tubes werelimiting factors.[23] “The common wisdom was thatvalves—which, like light bulbs, contained a hot glowingfilament—could never be used satisfactorily in large num-bers, for they were unreliable, and in a large installationtoomanywould fail in too short a time”.[24] TommyFlow-ers, who later designed Colossus, “discovered that, so longas valves were switched on and left on, they could operatereliably for very long periods, especially if their 'heaters’were run on a reduced current”.[24] In 1934 Flowers builta successful experimental installation using over 3,000tubes in small independent modules; when a tube failed,it was possible to switch off one module and keep theothers going, thereby reducing the risk of another tubefailure being caused; this installation was accepted by thePost Office (who operated telephone exchanges). Flow-ers was also a pioneer of using tubes as very fast (com-pared to electromechanical devices) electronic switches.Later work confirmed that tube unreliability was not asserious an issue as generally believed; the 1946 ENIAC,with over 17,000 tubes, had a tube failure (which took 15minutes to locate) on average every two days. The qualityof the tubes was a factor, and unfortunately the diversionof skilled people during the Second World War loweredthe general quality of tubes.[25] During the war Colossus

11

was instrumental in breaking German codes. After thewar, development continued with tube-based computersincluding, military computers ENIAC and Whirlwind,the FerrantiMark 1 (the first commercially available elec-tronic computer), andUNIVAC I, also available commer-cially.

3.10.1 Colossus

Main article: Colossus computer

Flowers’s Colossus and its successor Colossus Mk2 werebuilt by the British during World War II to substantiallyspeed up the task of breaking the German high levelLorenz encryption. Using about 1,500 vacuum tubes(2,400 for Mk2), Colossus replaced an earlier machinebased on relay and switch logic (the Heath Robinson).Colossus was able to break in a matter of hours messagesthat had previously taken several weeks; it was also muchmore reliable.[24] Colossus was the first use of vacuumtubes working in concert on such a large scale for a singlemachine.[24]

Once Colossus was built and installed, it ran continuously,powered by dual redundant diesel generators, the wartimemains supply being considered too unreliable. The onlytime it was switched off was for conversion to Mk2, withthe addition of more tubes. Another nine Colossus Mk2swere built, and all ten machines were surprisingly reli-able. The ten machines drew 15 kilowatts of power eachcontinuously, largely for the tube heaters.A Colossus reconstruction was switched on in 1996; itwas upgraded to Mk2 configuration in 2004; it found thekey for a wartime German ciphertext in 2007.[26]

3.10.2 Whirlwind and “special-quality” tubes

Main article: Whirlwind (computer)

To meet the reliability requirements of the 1951 US digi-tal computer Whirlwind, “special-quality” tubes with ex-tended life, and a long-lasting cathode in particular, wereproduced. The problem of short lifetime was traced toevaporation of silicon, used in the tungsten alloy to makethe heater wire easier to draw. Elimination of siliconfrom the heater wire alloy (and more frequent replace-ment of the wire drawing dies) allowed production oftubes that were reliable enough for theWhirlwind project.The tubes developed for Whirlwind were later used in thegiant SAGE air-defense computer system. High-puritynickel tubing and cathode coatings free of materials thatcan poison emission (such as silicates and aluminium)also contribute to long cathode life. The first such “com-puter tube” was Sylvania’s 7AK7 of 1948. By the late1950s it was routine for special-quality small-signal tubesto last for hundreds of thousands of hours, if operated

conservatively. This increased reliability also made mid-cable amplifiers in submarine cables possible.

4 Heat generation and cooling

The anode (plate) of this transmitting triode has been designed todissipate up to 500 W of heat

A considerable amount of heat is produced when tubesoperate, both from the filament (heater) but also from thestream of electrons bombarding the plate. In power am-plifiers this source of heat will exceed the power due tocathode heating. A few types of tube permit operationwith the anodes at a dull red heat; in other types, red heatindicates severe overload.The requirements for heat removal can significantlychange the appearance of high-power vacuum tubes.High power audio amplifiers and rectifiers required largerenvelopes to dissipate heat. Transmitting tubes could bemuch larger still.Heat escapes the device by black body radiation from theanode (plate) as infrared radiation, and by convection ofair over the tube envelope.[27] Convection is not possiblein most tubes since the anode is surrounded by vacuum.

12 6 NAMES

Tubes which generate relatively little heat, such as the1.4-volt filament directly heated tubes designed for use inbattery-powered equipment, often have shiny metal an-odes. 1T4, 1R5 and 1A7 are examples. Gas-filled tubessuch as thyratrons may also use a shiny metal anode, sincethe gas present inside the tube allows for heat convectionfrom the anode to the glass enclosure.The anode is often treated to make its surface emit moreinfrared energy. High-power amplifier tubes are designedwith external anodes which can be cooled by convection,forced air or circulating water. The water-cooled 80 kg,1.25 MW 8974 is among the largest commercial tubesavailable today.In a water-cooled tube, the anode voltage appears directlyon the cooling water surface, thus requiring the water tobe an electrical insulator to prevent high voltage leakagethrough the cooling water to the radiator system. Wa-ter as usually supplied has ions which conduct electricity;deionized water, a good insulator, is required. Such sys-tems usually have a built-in water-conductance monitorwhich will shut down the high-tension supply if the con-ductance becomes too high.The screen grid may also generate considerable heat.Limits to screen grid dissipation, in addition to plate dissi-pation, are listed for power devices. If these are exceededthen tube failure is likely.

5 Tube packages

Metal-cased tubes with octal bases

Most modern tubes have glass envelopes, but metal, fusedquartz (silica) and ceramic have also been used. A firstversion of the 6L6 used ametal envelope sealed with glassbeads, while a glass disk fused to the metal was used inlater versions. Metal and ceramic are used almost ex-clusively for power tubes above 2 kW dissipation. Thenuvistor was a modern receiving tube using a very smallmetal and ceramic package.The internal elements of tubes have always been con-nected to external circuitry via pins at their base which

High power GS-9B triode transmitting tube with heat sink at bot-tom.

plug into a socket. Subminiature tubes were producedusing wire leads rather than sockets, however these wererestricted to rather specialized applications. In additionto the connections at the base of the tube, many early tri-odes connected the grid using a metal cap at the top ofthe tube; this reduces stray capacitance between the gridand the plate leads. Tube caps were also used for the plate(anode) connection, particularly in transmitting tubes andtubes using a very high plate voltage.High-power tubes such as transmitting tubes have pack-ages designed more to enhance heat transfer. In sometubes, the metal envelope is also the anode. The4CX1000A is an external anode tube of this sort. Airis blown through an array of fins attached to the anode,thus cooling it. Power tubes using this cooling scheme areavailable up to 150 kW dissipation. Above that level, wa-ter or water-vapor cooling are used. The highest-powertube currently available is the Eimac 4CM2500KG, aforced water-cooled power tetrode capable of dissipating2.5 megawatts. (By comparison, the largest power tran-sistor can only dissipate about 1 kilowatt.)

6 Names

The generic name "[thermionic] valve” used in the UKderives from the unidirectional current flow allowed bythe earliest device, the thermionic diode emitting elec-trons from a heated filament, by analogy with a non-returnvalve in a water pipe.[28] The US names “vacuum tube”,“electron tube”, and “thermionic tube” all simply describea tubular envelope which has been evacuated (“vacuum”),has a heater, and controls electron flow.In many cases manufacturers and the military gave tubes

13

designations which said nothing about their purpose (e.g.,1614). In the early days some manufacturers used pro-prietary names which might convey some information,but only about their products; the KT66 and KT88 were"Kinkless Tetrodes”. Later, consumer tubes were givennames which conveyed some information, with the samename often used generically by several manufacturers.In the US, Radio Electronics Television Manufacturers’Association (RETMA) designations comprise a number,followed by one or two letters, and a number. The firstnumber is the (rounded) heater voltage; the letters desig-nate a particular tube but say nothing about its structure;and the final number is the total number of electrodes(without distinguishing between, say, a tube with manyelectrodes, or two sets of electrodes in a single envelope—a double triode, for example). For example, the 12AX7 isa double triode (two sets of three electrodes plus heater)with a 12.6V heater (which, as it happens, can also beconnected to run from 6.3V). The “AX” has no meaningother than to designate this particular tube according toits characteristics. Similar, but not identical, tubes arethe 12AD7, 12AE7...12AT7, 12AU7, 12AV7, 12AW7(rare!), 12AY7, and the 12AZ7.A system widely used in Europe known as the Mullard-Philips tube designation, also extended to transistors, usesa letter, followed by one or more further letters, and anumber. The type designator specifies the heater voltageor current (one letter), the functions of all sections of thetube (one letter per section), the socket type (first digit),and the particular tube (remaining digits). For example,the ECC83 (equivalent to the 12AX7) is a 6.3V (E) dou-ble triode (CC) with a miniature base (8). In this systemspecial-quality tubes (e.g., for long-life computer use) areindicated by moving the number immediately after thefirst letter: the E83CC is a special-quality equivalent ofthe ECC83, the E55L a power pentode with no consumerequivalent.

7 Special-purpose tubes

Some special-purpose tubes are constructed with par-ticular gases in the envelope. For instance, voltage-regulator tubes contain various inert gases such as argon,helium or neon, which will ionize at predictable voltages.The thyratron is a special-purpose tube filled with low-pressure gas or mercury vapor. Like vacuum tubes, itcontains a hot cathode and an anode, but also a con-trol electrode which behaves somewhat like the grid ofa triode. When the control electrode starts conduction,the gas ionizes, after which the control electrode can nolonger stop the current; the tube “latches” into conduc-tion. Removing anode (plate) voltage lets the gas de-ionize, restoring its non-conductive state. Some thyra-trons can carry large currents for their physical size. Oneexample is the miniature type 2D21, often seen in 1950sjukeboxes as control switches for relays. A cold-cathode

Voltage-regulator tube in operation. Low pressure gas within tubeglows due to current flow.

version of the thyratron, which uses a pool of mercuryfor its cathode, is called an ignitron; some can switchthousands of amperes. Thyratrons containing hydrogenhave a very consistent time delay between their turn-onpulse and full conduction; they behave much like modernsilicon-controlled rectifiers, also called thyristors due totheir functional similarity to thyratrons. Thyratrons havelong been used in radar transmitters.An extremely specialized tube is the krytron, which isused for extremely precise and rapid high-voltage switch-ing. Krytrons with certain specifications are suitable toinitiate the precise sequence of detonations used to setoff a nuclear weapon, and are heavily controlled at an in-ternational level.X-ray tubes are used in medical imaging among otheruses. X-ray tubes used for continuous-duty operation influoroscopy and CT imaging equipment may use a fo-cused cathode and a rotating anode to dissipate the largeamounts of heat thereby generated. These are housed inan oil-filled aluminium housing to provide cooling.The photomultiplier tube is an extremely sensitive de-tector of light, which uses the photoelectric effectand secondary emission, rather than thermionic emis-sion, to generate and amplify electrical signals. Nu-clear medicine imaging equipment and liquid scintillationcounters use photomultiplier tube arrays to detect low-intensity scintillation due to ionizing radiation.

14 8 POWERING THE TUBE

8 Powering the tube

8.1 Batteries

Main article: Battery (vacuum tube)

Batteries provided the voltages required by tubes in earlyradio sets. Three different voltages were generally re-quired, using three different batteries designated as theA, B, andC battery. The “A” battery or LT (low-tension)battery provided the filament voltage. Tube heaters weredesigned for single, double or triple-cell lead-acid batter-ies, giving nominal heater voltages of 2 V, 4 V or 6 V. Inportable radios, dry batteries were sometimes used with1.5 or 1 V heaters. Reducing filament consumption im-proved the life span of batteries. By 1955 towards theend of the tube era, tubes using only 50 mA down to aslittle as 10 mA for the heaters had been developed.[29]

The high voltage applied to the anode (plate) was pro-vided by the “B” battery or the HT (high-tension) supplyor battery. These were generally of dry cell constructionand typically came in 22.5-, 45-, 67.5-, 90-, 120- or 135-volt versions.

Batteries for a vacuum tube circuit. The C battery is highlighted.

Early sets used a grid bias battery or “C” battery whichwas connected to provide a negative voltage. Since vir-tually no current flows through a tube’s grid connection,these batteries had very low drain and lasted the longest.Even after AC power supplies became commonplace,some radio sets continued to be built with C batteries,as they would almost never need replacing. Howevermore modern circuits were designed using cathode bias-ing, eliminating the need for a third power supply voltage;this became practical with tubes using indirect heating ofthe cathode.

The “C battery” for bias is a designation having no relationto the "C cell" battery size.

8.2 AC power

“Cheater cord” redirects here. For the three-prong totwo-prong mains plug adapter, see Cheater plug.

Battery replacement was a major operating cost for earlyradio receiver users. The development of the batteryeliminator, and, in 1925, batteryless receivers operatedby household power, reduced operating costs and con-tributed to the growing popularity of radio. A powersupply using a transformer with several windings, one ormore rectifiers (which may themselves be vacuum tubes),and large filter capacitors provided the required directcurrent voltages from the alternating current source.As a cost reduction measure, especially in high-volumeconsumer receivers, all the tube heaters could be con-nected in series across theAC supply using heaters requir-ing the same current and with a similar warm-up time. Inone such design, a tap on the tube heater string suppliedthe 6 volts needed for the dial light. By deriving the highvoltage from a half-wave rectifier directly connected tothe AC mains, the heavy and costly power transformerwas eliminated. This also allowed such receivers to oper-ate on direct current, a so-called AC/DC receiver design.Many different US consumer AM radio manufacturers ofthe era used a virtually identical circuit, given the nick-name All American Five.Where the mains voltage was in the 100-120V range, thislimited voltage proved suitable only for low-power re-ceivers. Television receivers either required a transformeror could use a voltage doubling circuit. Where 230 Vnominal mains voltage was used, television receivers aswell could dispense with a power transformer.Transformer-less power supplies required safety precau-tions in their design to limit the shock hazard to users,such as electrically insulated cabinets and an interlock ty-ing the power cord to the cabinet back, so the line cordwas necessarily disconnected if the user or service per-son opened the cabinet. A cheater cord was a power cordending in the special socket used by the safety interlock;servicers could then power the device with the hazardousvoltages exposed.To avoid the warm-up delay, “instant on” television re-ceivers passed a small heating current through their tubeseven when the set was nominally off. At switch on, fullheating current was provided and the set would play al-most immediately.

9.1 Vacuum 15

Tube tester manufactured in 1930

9 Reliability

One reliability problem of tubes with oxide cathodesis the possibility that the cathode may slowly become"poisoned" by gas molecules from other elements in thetube, which reduce its ability to emit electrons. Trappedgases or slow gas leaks can also damage the cathode orcause plate (anode) current runaway due to ionization offree gas molecules. Vacuum hardness and proper selec-tion of construction materials are the major influences ontube lifetime. Depending on the material, temperatureand construction, the surface material of the cathode mayalso diffuse onto other elements. The resistive heatersthat heat the cathodes may break in a manner similar toincandescent lamp filaments, but rarely do, since they op-erate at much lower temperatures than lamps.The heater’s failure mode is typically a stress-related frac-ture of the tungsten wire or at a weld point and generallyoccurs after accruing many thermal (power on-off) cy-cles. Tungsten wire has a very low resistance when atroom temperature. A negative temperature coefficientdevice, such as a thermistor, may be incorporated in theequipment’s heater supply or a ramp-up circuit may beemployed to allow the heater or filaments to reach oper-ating temperature more gradually than if powered-up ina step-function. Low-cost radios had tubes with heatersconnected in series, with a total voltage equal to that ofthe line (mains). Following World War II, tubes intendedto be used in series heater strings were redesigned to all

have the same (“controlled”) warm-up time. Earlier de-signs had quite-different thermal time constants. The au-dio output stage, for instance, had a larger cathode, andwarmed up more slowly than lower-powered tubes. Theresult was that heaters that warmed up faster also tem-porarily had higher resistance, because of their positivetemperature coefficient. This disproportionate resistancecaused them to temporarily operate with heater voltageswell above their ratings, and shortened their life.Another important reliability problem is caused by airleakage into the tube. Usually oxygen in the air reactschemically with the hot filament or cathode, quickly ruin-ing it. Designers developed tube designs that sealed reli-ably. This was why most tubes were constructed of glass.Metal alloys (such as Cunife and Fernico) and glasseshad been developed for light bulbs that expanded andcontracted in similar amounts, as temperature changed.These made it easy to construct an insulating envelope ofglass, while passing connection wires through the glass tothe electrodes.When a vacuum tube is overloaded or operated past itsdesign dissipation, its anode (plate) may glow red. In con-sumer equipment, a glowing plate is universally a sign ofan overloaded tube. However, some large transmittingtubes are designed to operate with their anodes at red, or-ange, or in rare cases, white heat.“Special quality” versions of standard tubes were of-ten made, designed for improved performance in somerespect, such as a longer life cathode, low noise con-struction, mechanical ruggedness via ruggedized fila-ments, low microphony, for applications where the tubewill spend much of its time cut off, etc. The onlyway to know the particular features of a special qual-ity part is by reading the data sheet. Names may re-flect the standard name (12AU7==>12AU7A, its equiv-alent ECC82==>E82CC, etc.), or be absolutely anything(standard and special-quality equivalents of the sametube include 12AU7, ECC82, B329, CV491, E2163,E812CC, M8136, CV4003, 6067, VX7058, 5814A and12AU7A).[30]

The longest recorded valve life was earned by a MazdaAC/P pentode valve (serial No. 4418) in operation at theBBC's main Northern Ireland transmitter at Lisnagarvey.The valve was in service from 1935 until 1961 and hada recorded life of 232,592 hours. The BBC maintainedmeticulous records of their valves’ lives with periodic re-turns to their central valve stores.[31][32]

9.1 Vacuum

The highest possible vacuum is desired in a tube. Re-maining gas atoms will ionize and conduct electricity be-tween the elements in an undesiredmanner. In a defectivetube residual air pressure will lead to ionization, becom-ing visible as a pink-purple glow discharge between thetube elements.

16 9 RELIABILITY

Getter in opened tube; silvery deposit from getter

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

To prevent gases from compromising the tube’s vacuum,modern tubes are constructed with "getters", which areusually small, circular troughs filled with metals that ox-idize quickly, barium being the most common. Whilethe tube envelope is being evacuated, the internal partsexcept the getter are heated by RF induction heating toevolve any remaining gas from the metal parts. The tubeis then sealed and the getter is heated to a high temper-ature, again by radio frequency induction heating, whichcauses the getter material to vaporize and react with anyresidual gas. The vapor is deposited on the inside ofthe glass envelope, leaving a silver-colored metallic patchwhich continues to absorb small amounts of gas that mayleak into the tube during its working life. Great care istaken with the valve design to ensure this material is notdeposited on any of the working electrodes. If a tube de-velops a serious leak in the envelope, this deposit turns awhite color as it reacts with atmospheric oxygen. Largetransmitting and specialized tubes often use more exoticgetter materials, such as zirconium. Early gettered tubes

used phosphorus-based getters, and these tubes are eas-ily identifiable, as the phosphorus leaves a characteristicorange or rainbow deposit on the glass. The use of phos-phorus was short-lived and was quickly replaced by thesuperior barium getters. Unlike the barium getters, thephosphorus did not absorb any further gases once it hadfired.Getters act by chemically combining with residual or infil-trating gases, but are unable to counteract (non-reactive)inert gases. A known problem, mostly affecting valveswith large envelopes such as Cathode Ray Tubes and cam-era tubes such as Iconoscopes and Orthicons/Image Or-thicons, comes from helium infiltration. The effect ap-pears as impaired or absent functioning, and as a diffuseglow along the electron stream inside the tube. This effectcannot be rectified (short of re-evacuation and resealing),and is responsible for working examples of such tubes be-coming rarer and rarer. Unused (“New Old Stock”) tubescan also exhibit inert gas infiltration, so there is no long-term guarantee of these tube types surviving into the fu-ture.

9.2 Transmitting tubes

Large transmitting tubes have carbonized tungsten fila-ments containing a small trace (1% to 2%) of thorium.An extremely thin (molecular) layer of thorium atomsforms 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 replacethem. Such thoriated tungsten cathodes usually deliverlifetimes in the tens of thousands of hours. The end-of-life scenario for a thoriated-tungsten filament is when thecarbonized layer has mostly been converted back into an-other form of tungsten carbide and emission begins todrop off rapidly; a complete loss of thorium has neverbeen found to be a factor in the end-of-life in a tube withthis type of emitter. WAAY-TV in Huntsville, Alabamaachieved 163,000 hours of service from an Eimac exter-nal cavity klystron in the visual circuit of its transmitter;this is the highest documented service life for this typeof tube.[33] It has been said that transmitters with vacuumtubes are better able to survive lightning strikes than tran-sistor transmitters do. While it was commonly believedthat at RF power levels above approx. 20 kilowatts, vac-uum tubes were more efficient than solid state circuits,this is no longer the case especially in medium wave (AMbroadcast) service where solid state transmitters at nearlyall power levels have measurably higher efficiency. FMbroadcast transmitters with solid state power amplifiersup to approx. 15 kW also show better overall mains-power efficiency than tube-based power amplifiers.

9.4 Failure modes 17

9.3 Receiving tubes

Cathodes in small “receiving” tubes are coated with amixture of barium oxide and strontium oxide, sometimeswith addition of calcium oxide or aluminium oxide. Anelectric heater is inserted into the cathode sleeve, and in-sulated from it electrically by a coating of aluminium ox-ide. This complex construction causes barium and stron-tium atoms to diffuse to the surface of the cathode andemit electrons when heated to about 780 degrees Celsius.

9.4 Failure modes

9.4.1 Catastrophic failures

A catastrophic failure is one which suddenly makes thevacuum tube unusable. A crack in the glass envelope willallow air into the tube and destroy it. Cracks may resultfrom stress in the glass, bent pins or impacts; tube sock-ets must allow for thermal expansion, to prevent stress inthe glass at the pins. Stress may accumulate if a metalshield or other object presses on the tube envelope andcauses differential heating of the glass. Glass may alsobe damaged by high-voltage arcing.Tube heaters may also fail without warning, especiallyif exposed to over voltage or as a result of manufactur-ing defects. Tube heaters do not normally fail by evap-oration like lamp filaments, since they operate at muchlower temperature. The surge of inrush current when theheater is first energized causes stress in the heater, andcan be avoided by slowly warming the heaters, gradu-ally increasing current with a NTC thermistor included inthe circuit. Tubes intended for series-string operation ofthe heaters across the supply have a specified controlledwarm-up time to avoid excess voltage on some heatersas others warm up. Directly heated filament-type cath-odes as used in battery-operated tubes or some rectifiersmay fail if the filament sags, causing internal arcing. Ex-cess heater-to-cathode voltage in indirectly heated cath-odes can break down the insulation between elements anddestroy the heater.Arcing between tube elements can destroy the tube. Anarc 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, whichdamages the emission coating. Arcs can also be initiatedby any loose material inside the tube, or by excess screenvoltage. An arc inside the tube allows gas to evolve fromthe tube materials, and may deposit conductive materialon internal insulating spacers.[34]

Tube rectifiers have limited current capability and ex-ceeding ratings will eventually destroy a tube.

9.4.2 Degenerative failures

Degenerative failures are those caused by the slow dete-rioration of performance over time.Overheating of internal parts, such as control grids ormica spacer insulators, can result in trapped gas escap-ing into the tube; this can reduce performance. A getteris used to absorb gases evolved during tube operation,but has only a limited ability to combine with gas. Con-trol of the envelope temperature prevents some types ofgassing. A tube with an unusually high level of internalgas may exhibit a visible blue glow when plate voltageis applied. The getter (being a highly reactive metal) iseffective against many atmospheric gases, but has no (orvery limited) chemical reactivity to inert gases such ashelium. One progressive type of failure, especially withphysically large envelopes such as those used by cameratubes and cathode-ray tubes, comes from helium infiltra-tion. The exact mechanism not clear: the metal-to-glasslead-in seals are one possible infiltration site.Gas and ions within the tube contribute to grid cur-rent which can disturb operation of a vacuum tube cir-cuit. Another effect of overheating is the slow depositof metallic vapors on internal spacers, resulting in inter-element leakage.Tubes on standby for long periods, with heater voltageapplied, may develop high cathode interface resistanceand display poor emission characteristics. This effectoccurred especially in pulse and digital circuits, wheretubes had no plate current flowing for extended times.Tubes designed specifically for this mode of operationwere made.Cathode depletion is the loss of emission after thousandsof hours of normal use. Sometimes emission can berestored for a time by raising heater voltage, either fora short time or a permanent increase of a few percent.Cathode depletion was uncommon in signal tubes butwas a frequent cause of failure of monochrome televisioncathode-ray tubes.[35] Usable life of this expensive com-ponent was sometimes extended by fitting a boost trans-former to increase heater voltage.

9.4.3 Other failures

Vacuum tubes may have or develop defects in operationthat make an individual tube unsuitable in a given device,although it may perform satisfactorily in another applica-tion. Microphonics refers to internal vibrations of tubeelements which modulate the tube’s signal in an unde-sirable way; sound or vibration pick-up may affect thesignals, or even cause uncontrolled howling if a feedbackpath develops between a microphonic tube and, for exam-ple, a loudspeaker. Leakage current between AC heatersand the cathode may couple into the circuit, or electronsemitted directly from the ends of the heater may also in-ject hum into the signal. Leakage current due to internal

18 11 OTHER VACUUM TUBE DEVICES

contamination may also inject noise.[36] Some of theseeffects make tubes unsuitable for small-signal audio use,although unobjectionable for many purposes. Selectingthe best of a batch of nominally identical tubes for criti-cal applications can produce better results.Tube pins are designed to facilitate installation and re-moval from its socket but, due to the high operating tem-peratures of these devices and/or ingress of dirt and dustover time, pins can develop non-conducting or high resis-tance surface films. Pins can be easily cleaned to restoreconductance to normal standards.

10 Testing

Universal vacuum tube tester

Main article: Tube tester

Vacuum tubes can be tested outside of their circuitry us-ing a vacuum tube tester.

11 Other vacuum tube devices

Most small signal vacuum tube devices have been super-seded by semiconductors, but some vacuum tube elec-tronic devices are still in common use. The magnetronis the type of tube used in all microwave ovens. In spiteof the advancing state of the art in power semiconductortechnology, the vacuum tube still has reliability and costadvantages for high-frequency RF power generation.Some tubes, such as magnetrons, traveling-wave tubes,carcinotrons, and klystrons, combine magnetic and elec-trostatic effects. These are efficient (usually narrow-band) RF generators and still find use in radar, microwaveovens and industrial heating. Traveling-wave tubes(TWTs) are very good amplifiers and are even used insome communications satellites. High-powered klystronamplifier tubes can provide hundreds of kilowatts in theUHF range.

11.1 Cathode ray tubes

Main article: Cathode ray tube

The cathode ray tube (CRT) is a vacuum tube used par-ticularly for display purposes. Although there are stillmany televisions and computer monitors using cathoderay tubes, they are rapidly being replaced by flat paneldisplays whose quality has greatly improved even as theirprices drop. This is also true of digital oscilloscopes(based on internal computers and analog to digital con-verters), although traditional analog scopes (dependentupon CRTs) continue to be produced, are economical,and preferred by many technicians. At one time manyradios used "magic eye tubes", a specialized sort of CRTused in place of a meter movement to indicate signalstrength, or input level in a tape recorder. A modern in-dicator device, the vacuum fluorescent display (VFD) isalso a sort of cathode ray tube.X-ray tube is also a special type of Cathode ray tube,whilst x-ray emits when high voltage electron hit the an-ode.Gyrotrons or vacuum masers, used to generate high-powermillimeter band waves, are magnetic vacuum tubesin which a small relativistic effect, due to the high voltage,is used for bunching the electrons. Gyrotrons can gener-ate very high powers (hundreds of kilowatts). Free elec-tron lasers, used to generate high-power coherent lightand even X-rays, are highly relativistic vacuum tubesdriven by high-energy particle accelerators. Thus theseare sorts of cathode ray tubes.

11.2 Electron multipliers

A photomultiplier is a phototube whose sensitivity isgreatly increased through the use of electron multipli-cation. This works on the principle of secondary emis-sion, whereby a single electron emitted by the photocath-ode strikes a special sort of anode known as a dynodecausing more electrons to be released from that dynode.Those electrons are accelerated toward another dynodeat a higher voltage, releasing more secondary electrons;as many as 15 such stages provide a huge amplifica-tion. Despite great advances in solid state photodetectors,the single-photon detection capability of photomultipliertubes makes this vacuum tube device excel in certain ap-plications. Such a tube can also be used for detection ofionizing radiation as an alternative to the Geiger–Müllertube (itself not an actual vacuum tube). Historically, theimage orthicon TV camera tube widely used in televisionstudios prior to the development of modern CCD arraysalso used multistage electron multiplication.For decades, electron-tube designers tried to augmentamplifying tubes with electron multipliers in order to in-crease gain, but these suffered from short life becausethe material used for the dynodes “poisoned” the tube’s

12.2 Vacuum fluorescent display 19

hot cathode. (For instance, the interesting RCA 1630secondary-emission tube was marketed, but did not last.)However, eventually, Philips of the Netherlands devel-oped the EFP60 tube that had a satisfactory lifetime, andwas used in at least one product, a laboratory pulse gen-erator. By that time, however, transistors were rapidlyimproving, making such developments superfluous.One variant called a “channel electron multiplier” doesnot use individual dynodes but consists of a curved tube,such as a helix, coated on the inside with material withgood secondary emission. One type had a funnel of sortsto capture the secondary electrons. The continuous dyn-ode was resistive, and its ends were connected to enoughvoltage to create repeated cascades of electrons. Themicrochannel plate consists of an array of single stageelectron multipliers over an image plane; several of thesecan then be stacked. This can be used, for instance, as animage intensifier in which the discrete channels substitutefor focussing.Tektronix made a high-performance wideband oscillo-scope CRT with a channel electron multiplier plate be-hind the phosphor layer. This plate was a bundled arrayof a huge number of short individual c.e.m. tubes thataccepted a low-current beam and intensified it to providea display of practical brightness. (The electron optics ofthe wideband electron gun could not provide enough cur-rent to directly excite the phosphor.)

12 Vacuum tubes in the 21st cen-tury

12.1 Niche applications

Although vacuum tubes have been largely replaced bysolid-state devices in most amplifying, switching, andrectifying applications, there are certain exceptions. Inaddition to the special functions noted above, tubes stillhave some niche applications.In general, vacuum tubes are much less susceptible thancorresponding solid-state components to transient over-voltages, such as mains voltage surges or lightning, theelectromagnetic pulse effect of nuclear explosions[37] orgeomagnetic storms produced by giant solar flares.[38]This property kept them in use for certain military appli-cations long after more practical and less expensive solid-state technology was available for the same applications,as for example with the MiG-25.[37] In that plane, outputpower of their radia has reach about 1Kw and it can burnta channel under interference.Vacuum tubes are still practical alternatives to solid statein generating high power at radio frequencies in applica-tions such as industrial radio frequency heating, particleaccelerators, and broadcast transmitters. This is partic-ularly true at microwave frequencies where such devices

as the klystron and traveling-wave tube provide amplifica-tion at power levels unattainable using current semicon-ductor devices. The household microwave oven uses amagnetron tube to efficiently generate hundreds of wattsof microwave power.In military application, high power Vacuum tube can gen-eral signal above 10M - 100M Watt that can burn thereceiver without protection. it is considered as a non-nuclear electromagnetic weapon has been introduced inlate 90’s of the 20 century by US and Russia.

12.1.1 Audiophiles

Main article: tube soundEnough people prefer tube sound to make tube amplifiers

70-watt tube-hybrid audio amplifier selling for US$2,680[39] in2011, about 10 times the price of a comparable model usingtransistors.[40]

commercially viable in three areas: musical instrument(guitar) amplifiers, devices used in recording studios, andaudiophile equipment.[41]

Many guitarists prefer using valve amplifiers to solid-statemodels. Most popular vintage models use vacuum tubes.

12.2 Vacuum fluorescent display

Main article: Vacuum fluorescent displayA modern display technology using a variation of cath-ode ray tube is often used in videocassette recorders,DVD players and recorders, microwave oven control pan-els, and automotive dashboards. Rather than raster scan-ning, these vacuum fluorescent displays (VFD) switchcontrol 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. Be-cause the filaments are in view, they must be operatedat 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 volt-ages (often less than 50 volts) unlike cathode ray tubes.

20 15 REFERENCES

Typical VFD used in a videocassette recorder

Their high brightness allows reading the display in brightdaylight. VFD tubes are flat and rectangular, as wellas relatively thin. Typical VFD phosphors emit a broadspectrum of greenish-white light, permitting use of colorfilters, though different phosphors can give other colorseven within the same display. The design of these tubesprovides a bright glow despite the low energy of the inci-dent electrons. This is because the distance between thecathode and anode is relatively small. (This technology isdistinct from fluorescent lighting, which uses a dischargetube.)

12.3 Vacuum tubes using field electronemitters

In the early years of the 21st century there has beenrenewed interest in vacuum tubes, this time with theelectron emitter formed on a flat silicon substrate, as inintegrated circuit technology. This subject is now calledvacuum nanoelectronics.[42] The most common designuses a cold cathode in the form of a large-area field elec-tron source (for example a field emitter array). With thesedevices, electrons are field-emitted from a large numberof closely spaced individual emission sites.Their claimed advantages include much greater robust-ness and the ability to provide high power output at lowpower consumption. Operating on the same principlesas traditional tubes, prototype device cathodes have beenfabricated in several different ways. Although a commonapproach is to use a field emitter array, one interestingidea is to etch electrodes to form hinged flaps – similarto the technology used to create the microscopic mirrorsused in digital light processing – that are stood upright byan electrostatic charge.Such integrated microtubes may find applicationin microwave devices including mobile phones, forBluetooth and Wi-Fi transmission, in radar and forsatellite communication. As of 2012 they were beingstudied for possible applications in field emission dis-play technology, but there were significant productionproblems.As of 2014, NASA’s Ames Research Center was reportedon working on vacuum-channel transistors produced us-

ing CMOS techniques.[43]

13 See also

14 Patents• U.S. Patent 803,684 – Instrument for converting al-ternating electric currents into continuous currents(Fleming valve patent)

• U.S. Patent 841,387 – Device for amplifying feebleelectrical currents

• U.S. Patent 879,532 – De Forest’s Audion

15 References[1] Herbert J. Reich (April 13, 2013). Principles of Electron

Tubes. Literary Licensing, LLC. ISBN 978-1258664060.

[2] Fundamental Amplifier Techniques with Electron Tubes:Theory and Practice with Design Methods for Self Con-struction. Elektor Electronics. January 1, 2011. ISBN978-0905705934.

[3] “RCA Electron Tube 6BN6/6KS6”. Retrieved 2015-04-13.

[4] Hoddeson, L. “The Vacuum Tube”. PBS. Retrieved 6May 2012.

[5] Morgan Jones, Valve Amplifiers, p. 580, Elsevier, 2012ISBN 0080966403.

[6] George Henry Olsen, Electronics: A General Introduc-tion for the Non-Specialist, p.391, Springer, 2013 ISBN1489965351.

[7] Rogers, D.C., “Triode amplifiers in the frequency range100 Mc/s to 420 Mc/s ", Journal of the British Institutionof Radio Engineers, vol. 11, iss. 12, pp. 569-575, p.571

[8] Guthrie, Frederick (1876). Magnetism and Electricity.London and Glasgow: William Collins, Sons, & Com-pany.

[9] Thomas A. Edison U.S. Patent 307,031 “Electrical Indi-cator”, Issue date: 1884

[10] Guarnieri, M. (2012). “The age of vacuum tubes:Early devices and the rise of radio communica-tions”. IEEE Ind. Electron. M. 6 (1): 41–43.doi:10.1109/MIE.2012.2182822.

[11] White, Thomas, United States Early Radio History

[12] “Robert von Lieben — Patent Nr 179807 Dated Novem-ber 19, 1906” (PDF). Kaiserliches Patentamt. 19 Novem-ber 1906. Retrieved 30 March 2008.

[13] http://www.corp.att.com/attlabs/reputation/timeline/15tel.html

21

[14] Räisänen, Antti V.; Lehto, Arto (2003). Radio Engineer-ing for Wireless Communication and Sensor Applications.Artech House. p. 7. ISBN 1580536697.

[15] Guarnieri, M. (2012). “The age of vacuum tubes: the con-quest of analog communications”. IEEE Ind. Electron. M.6 (2): 52–54. doi:10.1109/MIE.2012.2193274.

[16] Introduction to Thermionic Valves (Vacuum Tubes),Colin J. Seymour

[17] “Philips Historical Products: Philips Vacuum Tubes”.Retrieved 3 November 2013.

[18] Baker, Bonnie (2008). Analog circuits. Newnes. p. 391.ISBN 0-7506-8627-8.

[19] Modjeski, Roger A. “Mu, Gm and Rp and how Tubes arematched”. Välljud AB. Retrieved 22 April 2011.

[20] Ballou, Glen (1987). Handbook for Sound Engineers: TheNew Audio Cyclopedia (1st ed.). Howard W. Sams Co. p.250. ISBN 0-672-21983-2. Amplification factor or volt-age gain is the amount the signal at the control grid is in-creased in amplitude after passing through the tube, whichis also referred to as the Greek letter μ (mu) or voltage gain(V ) of the tube.

[21] C H Gardner (1965) The Story of the Valve, Radio Con-structor (See particularly the section “Glass Base Con-struction”)

[22] L.W. Turner (ed.) Electronics Engineer’s Reference Book,4th ed. Newnes-Butterworth, London 1976 ISBN 0-408-00168-2 pages 7–2 through 7-6

[23] Guarnieri, M. (2012). “The age of Vacuum Tubes: Merg-ing with Digital Computing”. IEEE Ind. Electron. M. 6(3): 52–55. doi:10.1109/MIE.2012.2207830.

[24] From part of Copeland’s “Colossus” available online

[25] Alexander Randall 5th (14 February 2006). “A lost inter-view with ENIAC co-inventor J. Presper Eckert”. Com-puter World. Retrieved 2011-04-25.

[26] The National Museum of Computing – RebuildingColossus

• The National Museum of Computing – The Colos-sus Gallery

[27] RCA “Transmitting Tubes Manual” TT-5 1962, p. 10

[28] The Oxford Companion to the History of Modern Sci-ence, J. L. Heilbron , Oxford University Press 2003,9780195112290, “valve, thermionic”

[29] Sōgo Okamura (1994). History of electron tubes. IOSPress. pp. 133–. ISBN 978-90-5199-145-1. Retrieved9 May 2011.

[30] National Valve Museum: audio double triodes ECC81, 2,and 3

[31] Certified by BBC central valve stores, Motspur Park

[32] Mazda Data Booklet 1968 Page 112.

[33] 31 Alumni. “The Klystron & Cactus”. Retrieved 29 De-cember 2013.

[34] Tomer, Robert B. (1960), Getting the most out of vacuumtubes, Indianapolis, IN, USA: Howard W. Sams, LCCN60-13843. available on the Internet Archive. Chapter 1

[35] Tomer 1960, 60, chapter 2

[36] Tomer 1960, 60, chapter 3

[37] Broad, William J. “Nuclear Pulse (I): Awakening to theChaos Factor”, Science. 29 May 1981 212: 1009–1012

[38] Y Butt, The Space Review, 2011 "... geomagnetic storms,on occasion, can induce more powerful pulses than the E3pulse from even megaton type nuclear weapons.”

[39] Price of $4,680 for the “super enhanced version.” Includes90-day warranty on tubes “under normal operation condi-tions.” See Model no: SE-300B-70W

[40] Rolls RA200 100 W RMS/Channel @ 4 Ohms PowerAmplifier. Full Compass. Retrieved on 2011-05-09.

[41] Barbour, E. (1998). “The cool sound of tubes – vacuumtubemusical applications”. Spectrum, IEEE 35 (8) (IEEE).pp. 24–35.

[42] Ackerman, Evan. “Vacuum tubes could be the future ofcomputing”. Dvice. Dvice. Retrieved 8 February 2013.

[43] Anthony, Sebastian. “The vacuum tube strikes back:NASA’s tiny 460GHz vacuum transistor that could oneday replace silicon FETs”. ExtremeTech.

• Spangenberg, Karl R. (1948). Vacuum Tubes.McGraw-Hill. OCLC 567981. LCC TK7872.V3.

• Millman, J. & Seely, S. Electronics, 2nd ed.McGraw-Hill, 1951.

• Shiers, George, “The First Electron Tube”, Scien-tific American, March 1969, p. 104.

• Tyne, Gerald, Saga of the Vacuum Tube, Ziff Pub-lishing, 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 to1940, MMA International, 1982, pp 9–13.

• Eastman, Austin V., Fundamentals of VacuumTubes, McGraw-Hill, 1949

• Philips Technical Library. Books published in theUK in the 1940s and 1950s by Cleaver Hume Presson design and application of vacuum tubes.

• RCA Radiotron Designer’s Handbook, 1953 (4thEdition). Contains chapters on the design and ap-plication of receiving tubes.

22 16 EXTERNAL LINKS

• Wireless World. “Radio Designer’s Handbook”.UK reprint of the above.

• RCA “Receiving Tube Manual” RC15, RC26(1947, 1968) Issued every two years, contains de-tails of the technical specs of the tubes that RCAsold.

16 External links• http://www.pentalabs.com/tubeworks.html – Thehistory of vacuum tubes

• http://www.cfp-radio.com/realisations/rea03/rea03.html – (FR) How to build a vacuum tubetester.

• The Thermionic Detector – HJ van der Bijl (October1919)

• How vacuum tubes really work – Thermionic emis-sion and vacuum tube theory, using introductorycollege-level mathematics.

• The Vacuum Tube FAQ – FAQ from rec.audio

• The invention of the thermionic valve. Flemingdiscovers the thermionic (or oscillation) valve, or'diode'.

• Tubes Vs. Transistors : Is There an Audible Differ-ence? – 1972 AES paper on audible differences insound quality between vacuum tubes and transistors.

• The Virtual Valve Museum

• The cathode ray tube site

• O'Neill’s Electronic museum – vacuum tube mu-seum

• Vacuum tubes for beginners – Japanese Version

• NJ7P Tube Database – Data manual for tubes usedin North America.

• Vacuum tube data sheet locator

• Characteristics and datasheets

• Video of amateur radio operator making his ownvacuum tube triodes

• Tuning eye tubes.

• Archive film of Mullard factory Blackburn

• Western Electric specifications sheets for 1940s and1950s electron and vacuum tubes

23

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