A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls
electric current flow in a high
vacuum between
electrodes to which an electric
potential difference
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
has been applied.
The type known as a thermionic tube or thermionic valve utilizes
thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
of electrons from a
hot cathode
In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating elemen ...
for fundamental electronic functions such as signal
amplification and current
rectification. Non-thermionic types such as a vacuum
phototube
A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light. Such a tube is more correctly called a 'photoemissive cell' to distinguish it from photovoltaic or photoconductive cells. Phototubes were previ ...
, however, achieve electron emission through the
photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
, and are used for such purposes as the detection of light intensities. In both types, the electrons are accelerated from the cathode to the
anode by the
electric field in the tube.
The simplest vacuum tube, the
diode (i.e.
Fleming valve), invented in 1904 by
John Ambrose Fleming, contains only a heated electron-emitting cathode and an anode. Electrons can only flow in one direction through the device—from the cathode to the anode. Adding one or more
control grid
The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consist ...
s within the tube allows the current between the cathode and anode to be controlled by the voltage on the grids.
These devices became a key component of electronic circuits for the first half of the twentieth century. They were crucial to the development of
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmi ...
,
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
,
radar,
sound recording and reproduction
Sound recording and reproduction is the electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording ...
, long-distance
telephone networks, and analog and early digital
computers. Although some applications had used earlier technologies such as the
spark gap transmitter
A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the main type use ...
for radio or
mechanical computer
A mechanical computer is a computer built from mechanical components such as levers and gears rather than electronic components. The most common examples are adding machines and mechanical counters, which use the turning of gears to increment out ...
s for computing, it was the invention of the thermionic vacuum tube that made these technologies widespread and practical, and created the discipline of
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
.
In the 1940s, the invention of
semiconductor devices made it possible to produce
solid-state devices, which are smaller, more efficient, reliable, durable, safer, and more economical than thermionic tubes. Beginning in the mid-1960s, thermionic tubes were being replaced by the
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
. However, the
cathode-ray tube (CRT) remained the basis for television monitors and
oscilloscopes
An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
until the early 21st century.
Thermionic tubes are still used in some applications, such as the
magnetron
The cavity magnetron is a high-power vacuum tube used in early radar systems and currently in microwave ovens and linear particle accelerators. It generates microwaves using the interaction of a stream of electrons with a magnetic field while ...
used in microwave ovens, certain high-frequency
amplifiers, amplifiers for electric musical instruments such as guitars, as well as high end audio amplifiers, which many audio enthusiasts prefer for their "warmer"
tube sound
Tube sound (or valve sound) is the characteristic sound associated with a vacuum tube amplifier (valve amplifier in British English), a vacuum tube-based audio amplifier. At first, the concept of ''tube sound'' did not exist, because practically ...
.
Not all electronic circuit valves / electron tubes are vacuum tubes.
Gas-filled tube
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric ...
s are similar devices, but containing a gas, typically at low pressure, which exploit phenomena related to
electric discharge in gases, usually without a heater.
Classifications
One classification of thermionic vacuum tubes is by the number of active electrodes. A device with two active elements is a
diode, usually used for
rectification. Devices with three elements are
triodes used for amplification and switching. Additional electrodes create
tetrode
A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). ...
s,
pentodes, and so forth, which have multiple additional functions made possible by the additional controllable electrodes.
Other classifications are:
* by frequency range (
audio, radio,
VHF,
UHF,
microwave)
* by power rating (small-signal, audio power, high-power radio transmitting)
* by
cathode/
filament type (indirectly heated, directly heated) and
warm-up time (including "bright-emitter" or "dull-emitter")
* by characteristic curves design (e.g., sharp- versus remote-
cutoff in some pentodes)
* by application (receiving tubes, transmitting tubes, amplifying or switching, rectification, mixing)
* specialized parameters (long life, very low
microphonic sensitivity and low-noise audio amplification, rugged or military versions)
* specialized functions (light or radiation detectors, video imaging tubes)
* tubes used to display
information (
"magic eye" tubes,
vacuum fluorescent displays,
CRTs)
Tubes have different functions, such as
cathode-ray tubes which create a beam of electrons for display purposes (such as the television picture tube) in addition to more specialized functions such as
electron microscopy and
electron beam lithography
Electron-beam lithography (often abbreviated as e-beam lithography, EBL) is the practice of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist (exposing). The electron ...
.
X-ray tube
An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast ...
s are also vacuum tubes.
Phototube
A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light. Such a tube is more correctly called a 'photoemissive cell' to distinguish it from photovoltaic or photoconductive cells. Phototubes were previ ...
s and
photomultipliers rely on electron flow through a vacuum, though in those cases electron emission from the cathode depends on energy from
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
s rather than
thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
. Since these sorts of "vacuum tubes" have functions other than electronic amplification and rectification they are described elsewhere.
Description
A vacuum tube consists of two or more
electrodes in a vacuum inside an airtight envelope. Most tubes have glass envelopes with a
glass-to-metal seal based on
kovar
Kovar (trademark of CRS Holdings, inc., Delaware) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K betwe ...
sealable
borosilicate glass
Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...
es, though ceramic and metal envelopes (atop insulating bases) have been used. The electrodes are attached to leads which pass through the envelope via an airtight seal. Most vacuum tubes have a limited lifetime, due to the filament or heater burning out or other failure modes, so they are made as replaceable units; the electrode leads connect to pins on the tube's base which plug into a
tube socket
Tube sockets are electrical sockets into which vacuum tubes (electronic valves) can be plugged, holding them in place and providing terminals, which can be soldered into the circuit, for each of the pins. Sockets are designed to allow tubes to b ...
. Tubes were a frequent cause of failure in electronic equipment, and consumers were expected to be able to replace tubes themselves. In addition to the base terminals, some tubes had an electrode terminating at a
top cap
A spinning top, or simply a top, is a toy with a squat body and a sharp point at the bottom, designed to be spun on its vertical axis, balancing on the tip due to the gyroscopic effect.
Once set in motion, a top will usually wobble for a few ...
. The principal reason for doing this was to avoid leakage resistance through the tube base, particularly for the high impedance grid input.
[ The bases were commonly made with phenolic insulation which performs poorly as an insulator in humid conditions. Other reasons for using a top cap include improving stability by reducing grid-to-anode capacitance, improved high-frequency performance, keeping a very high plate voltage away from lower voltages, and accommodating one more electrode than allowed by the base. There was even an occasional design that had two top cap connections.
The earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases ]electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
s into the vacuum, a process called thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
, originally known as the ''Edison effect''. A second electrode, the anode or ''plate'', will attract those electrons if it is at a more positive voltage. The result is a net flow of electrons from the filament to plate. However, electrons cannot flow in the reverse direction because the plate is not heated and does not emit electrons. The filament ('' cathode'') has a dual function: it emits electrons when heated; and, together with the plate, it creates an electric field due to the potential difference between them. Such a tube with only two electrodes is termed a diode, and is used for rectification. Since current can only pass in one direction, such a diode (or '' rectifier'') will convert alternating current (AC) to pulsating DC. Diodes can therefore be used in a DC power supply, as a demodulator of amplitude modulated (AM) radio signals and for similar functions.
Early tubes used the filament as the cathode; this is called a "directly heated" tube. Most modern tubes are "indirectly heated" by a "heater" element inside a metal tube that is the cathode. The heater is electrically isolated from the surrounding cathode and simply serves to heat the cathode sufficiently for thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
of electrons. The electrical isolation allows all the tubes' heaters to be supplied from a common circuit (which can be AC without inducing hum) while allowing the cathodes in different tubes to operate at different voltages. H. J. Round invented the indirectly heated tube around 1913.
The filaments require constant and often considerable power, even when amplifying signals at the microwatt level. Power is also dissipated when the electrons from the cathode slam into the anode (plate) and heat it; this can occur even in an idle amplifier due to the quiescent current necessary to ensure linearity and low distortion. In a power amplifier, this heating can be considerable and can destroy the tube if driven beyond its safe limits. Since the tube contains a vacuum, the anodes in most small and medium power tubes are cooled by radiation through the glass envelope. In some special high power applications, the anode forms part of the vacuum envelope to conduct heat to an external heat sink, usually cooled by a blower, or water-jacket.
Klystron
A klystron is a specialized linear-beam vacuum tube, invented in 1937 by American electrical engineers Russell and Sigurd Varian,Pond, Norman H. "The Tube Guys". Russ Cochran, 2008 p.31-40 which is used as an amplifier for high radio frequen ...
s and magnetrons often operate their anodes (called ''collectors'' in klystrons) at ground potential to facilitate cooling, particularly with water, without high-voltage insulation. These tubes instead operate with high negative voltages on the filament and cathode.
Except for diodes, additional electrodes are positioned between the cathode and the plate (anode). These electrodes are referred to as grids as they are not solid electrodes but sparse elements through which electrons can pass on their way to the plate. The vacuum tube is then known as a triode, tetrode
A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). ...
, pentode, etc., depending on the number of grids. A triode has three electrodes: the anode, cathode, and one grid, and so on. The first grid, known as the control grid, (and sometimes other grids) transforms the diode into a ''voltage-controlled device'': the voltage applied to the control grid affects the current between the cathode and the plate. When held negative with respect to the cathode, the control grid creates an electric field that repels electrons emitted by the cathode, thus reducing or even stopping the current between cathode and anode. As long as the control grid is negative relative to the cathode, essentially no current flows into it, yet a change of several volts on the control grid is sufficient to make a large difference in the plate current, possibly changing the output by hundreds of volts (depending on the circuit). The solid-state device which operates most like the pentode tube is the junction field-effect transistor
The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifiers. ...
(JFET), although vacuum tubes typically operate at over a hundred volts, unlike most semiconductors in most applications.
History and development
The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes tube
A Crookes tube (also Crookes–Hittorf tube) is an early experimental electrical discharge tube, with partial vacuum, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were ...
s. The many scientists and inventors who experimented with such tubes include Thomas Edison, Eugen Goldstein, Nikola Tesla, and Johann Wilhelm Hittorf. With the exception of early light bulbs, such tubes were only used in scientific research or as novelties. The groundwork laid by these scientists and inventors, however, was critical to the development of subsequent vacuum tube technology.
Although thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mater ...
was originally reported in 1873 by Frederick Guthrie, it was Thomas Edison's apparently independent discovery of the phenomenon in 1883 that became well known. Although Edison was aware of the unidirectional property of current flow between the filament and the anode, his interest (and patent) concentrated on the sensitivity of the anode current to the current through the filament (and thus filament temperature). It was years later that John Ambrose Fleming applied the rectifying property of the Edison effect to detection {{Unreferenced, date=March 2018
In general, detection is the action of accessing information without specific cooperation from with the sender.
In the history of radio communications, the term " detector" was first used for a device that detected ...
of radio signals, as an improvement over the magnetic detector.[Fleming, J. A. (1934)]
''Memories of a Scientific Life''
London, UK: Marshall, Morgan & Scott, Ltd. pp. 136 - 143. Retrieved Nov. 2021.
Amplification by vacuum tube became practical only with Lee de Forest's 1907 invention of the three-terminal "audion
The Audion was an electronic detecting or amplifying vacuum tube invented by American electrical engineer Lee de Forest in 1906.De Forest patented a number of variations of his detector tubes starting in 1906. The patent that most clearly covers ...
" tube, a crude form of what was to become the triode. Being essentially the first electronic amplifier, such tubes were instrumental in long-distance telephony (such as the first coast-to-coast telephone line in the US) and public address system
A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...
s, and introduced a far superior and versatile technology for use in radio transmitters and receivers. The electronics revolution of the 20th century arguably began with the invention of the triode vacuum tube.
Diodes
At the end of the 19th century, radio or wireless technology was in an early stage of development and the Marconi Company was engaged in development and construction of radio communication systems. Guglielmo Marconi appointed English physicist John Ambrose Fleming as scientific advisor in 1899. Fleming had been engaged as scientific advisor to Edison Telephone (1879), as scientific advisor at Edison Electric Light (1882), and was also technical consultant to Edison-Swan. One of Marconi's needs was for improvement of the detector
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
. Marconi had developed a magnetic detector
The magnetic detector or Marconi magnetic detector, sometimes called the "Maggie", was an early radio wave detector used in some of the first radio receivers to receive Morse code messages during the wireless telegraphy era around the turn of the ...
, which was less responsive to natural sources of radio frequency interference than the coherer, but the magnetic detector only provided an audio frequency signal to a telephone receiver. A reliable detector that could drive a printing instrument was needed. As a result of experiments conducted on Edison effect bulbs,[ Fleming developed a vacuum tube that he termed the ''oscillation valve'' because it passed current in only one direction. The cathode was a carbon lamp filament, heated by passing current through it, that produced thermionic emission of electrons. Electrons that had been emitted from the cathode were attracted to the '']plate
Plate may refer to:
Cooking
* Plate (dishware), a broad, mainly flat vessel commonly used to serve food
* Plates, tableware, dishes or dishware used for setting a table, serving food and dining
* Plate, the content of such a plate (for example: ...
'' ('' anode'') when the plate was at a positive voltage with respect to the cathode. Electrons could not pass in the reverse direction because the plate was not heated and not capable of thermionic emission of electrons. Fleming filed a patent for these tubes, assigned to the Marconi company, in the UK in November 1904 and this patent was issued in September 1905. Later known as the Fleming valve, the oscillation valve was developed for the purpose of rectifying radio frequency current as the detector component of radio receiver circuits.[
While offering no advantage over the electrical sensitivity of crystal detectors,][Robison, S. S. (1911)]
Manual of Wireless Telegraphy for the use of Naval Electricians''
Annapolis, MD: United States Naval Institute. p. 124 fig. 84; pp. 131, 132. Retrieved Nov 2021 the Fleming valve offered advantage, particularly in shipboard use, over the difficulty of adjustment of the crystal detector and the susceptibility of the crystal detector to being dislodged from adjustment by vibration or bumping.
The first vacuum tube diodes designed for rectifier application in power supply circuits were introduced in April 1915 by Saul Dushman of General Electric.
Triodes
In the 19th century, telegraph and telephone engineers had recognized the need to extend the distance that signals could be transmitted. In 1906, Robert von Lieben
Robert von Lieben (September 5, 1878, in Vienna – February 20, 1913, in Vienna) was an Austrian entrepreneur, and self-taught physicist and inventor. Lieben and his associates Eugen Reisz and Siegmund Strauss invented and produced a ga ...
filed for a patent for a cathode-ray tube which used an external magnetic deflection coil and was intended for use as an amplifier in telephony equipment. This von Lieben magnetic deflection tube was not a successful amplifier, however, because of the power used by the deflection coil. Von Lieben would later make refinements to triode vacuum tubes.
Lee de Forest is credited with inventing the triode tube in 1907 while experimenting to improve his original (diode) Audion
The Audion was an electronic detecting or amplifying vacuum tube invented by American electrical engineer Lee de Forest in 1906.De Forest patented a number of variations of his detector tubes starting in 1906. The patent that most clearly covers ...
. By placing an additional electrode between the filament ( cathode) and plate
Plate may refer to:
Cooking
* Plate (dishware), a broad, mainly flat vessel commonly used to serve food
* Plates, tableware, dishes or dishware used for setting a table, serving food and dining
* Plate, the content of such a plate (for example: ...
(anode), he discovered the ability of the resulting device to amplify signals. As the voltage applied to the control grid
The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consist ...
(or simply "grid") was lowered from the cathode's voltage to somewhat more negative voltages, the amount of current from the filament to the plate would be reduced. The negative electrostatic field created by the grid in the vicinity of the cathode would inhibit the passage of emitted electrons and reduce the current to the plate. With the voltage of the grid less than that of the cathode, no direct current could pass from the cathode to the grid.
Thus a change of voltage applied to the grid, requiring very little power input to the grid, could make a change in the plate current and could lead to a much larger voltage change at the plate; the result was voltage and power amplification. In 1908, de Forest was granted a patent () for such a three-electrode version of his original Audion for use as an electronic amplifier in radio communications. This eventually became known as the triode.
De Forest's original device was made with conventional vacuum technology. The vacuum was not a "hard vacuum" but rather left a very small amount of residual gas. The physics behind the device's operation was also not settled. The residual gas would cause a blue glow (visible ionization) when the plate voltage was high (above about 60 volts). In 1912, de Forest and John Stone Stone brought the Audion for demonstration to AT&T's engineering department. Dr. Harold D. Arnold of AT&T recognized that the blue glow was caused by ionized gas. Arnold recommended that AT&T purchase the patent, and AT&T followed his recommendation. Arnold developed high-vacuum tubes which were tested in the summer of 1913 on AT&T's long-distance network. The high-vacuum tubes could operate at high plate voltages without a blue glow.
Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his sound-on-film
Sound-on-film is a class of sound film processes where the sound accompanying a picture is recorded on photographic film, usually, but not always, the same strip of film carrying the picture. Sound-on-film processes can either record an analog ...
process in Berlin, Germany. Tigerstedt's innovation was to make the electrodes concentric cylinders with the cathode at the centre, thus greatly increasing the collection of emitted electrons at the anode.
Irving Langmuir at the General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable en ...
research laboratory (Schenectady, New York
Schenectady () is a city in Schenectady County, New York, United States, of which it is the county seat. As of the 2020 census, the city's population of 67,047 made it the state's ninth-largest city by population. The city is in eastern New Y ...
) had improved Wolfgang Gaede's high-vacuum diffusion pump and used it to settle the question of thermionic emission and conduction in a vacuum. Consequently, General Electric started producing hard vacuum triodes (which were branded Pliotrons) in 1915. Langmuir patented the hard vacuum triode, but de Forest and AT&T successfully asserted priority and invalidated the patent.
Pliotrons were closely followed by the French type ' TM' and later the English type 'R' which were in widespread use by the allied military by 1916. Historically, vacuum levels in production vacuum tubes typically ranged from 10 µPa down to 10 nPa ( down to ).
The triode and its derivatives (tetrodes and pentodes) are transconductance devices, in which the controlling signal applied to the grid is a ''voltage'', and the resulting amplified signal appearing at the anode is a ''current''. Compare this to the behavior of the bipolar junction transistor
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...
, in which the controlling signal is a current and the output is also a current.
For vacuum tubes, transconductance or mutual conductance () is defined as the change in the plate(anode)/cathode current divided by the corresponding change in the grid to cathode voltage, with a constant plate(anode) to cathode voltage. Typical values of for a small-signal vacuum tube are 1 to 10 millisiemens. It is one of the three 'constants' of a vacuum tube, the other two being its gain μ and plate resistance or . The Van der Bijl equation defines their relationship as follows:
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortion at low volumes. Plotting plate current as a function of applied grid voltage, it was seen that there was a range of grid voltages for which the transfer characteristics were approximately linear.
To use this range, a negative bias voltage had to be applied to the grid to position the DC operating point in the linear region. This was called the idle condition, and the plate current at this point the "idle current". The controlling voltage was superimposed onto the bias voltage, resulting in a linear variation of plate current in response to positive and negative variation of the input voltage around that point.
This concept is called ''grid bias
In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing ...
''. Many early radio sets had a third battery called the "C battery" (unrelated to the present-day C cell, for which the letter denotes its size and shape). The C battery's positive terminal was connected to the cathode of the tubes (or "ground" in most circuits) and whose negative terminal supplied this bias voltage to the grids of the tubes.
Later circuits, after tubes were made with heaters isolated from their cathodes, used cathode biasing, avoiding the need for a separate negative power supply. For cathode biasing, a relatively low-value resistor is connected between the cathode and ground. This makes the cathode positive with respect to the grid, which is at ground potential for DC.
However C batteries continued to be included in some equipment even when the "A" and "B" batteries had been replaced by power from the AC mains. That was possible because there was essentially no current draw on these batteries; they could thus last for many years (often longer than all the tubes) without requiring replacement.
When triodes were first used in radio transmitters and receivers, it was found that tuned amplification stages had a tendency to oscillate unless their gain was very limited. This was due to the parasitic capacitance between the plate (the amplifier's output) and the control grid (the amplifier's input), known as the Miller capacitance.
Eventually the technique of ''neutralization'' was developed whereby the RF transformer connected to the plate (anode) would include an additional winding in the opposite phase. This winding would be connected back to the grid through a small capacitor, and when properly adjusted would cancel the Miller capacitance. This technique was employed and led to the success of the Neutrodyne radio during the 1920s.
However, neutralization required careful adjustment and proved unsatisfactory when used over a wide range of frequencies.
Tetrodes and pentodes
To combat the stability problems of the triode as a radio frequency amplifier due to grid-to-plate capacitance, the physicist Walter H. Schottky invented the tetrode or ''screen grid tube'' in 1919. He showed that the addition of an electrostatic shield between the control grid and the plate could solve the problem. This design was refined by Hull and Williams. The added grid became known as the ''screen grid
A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate (called ''anode'' in British English). T ...
'' or ''shield grid''. The screen grid is operated at a positive voltage significantly less than the plate voltage and it is bypassed to ground with a capacitor of low impedance at the frequencies to be amplified.[Landee, Davis, Albrecht (1957]
''Electronic Designers' Handbook''
New York: McGraw-Hill. pp. 3-34 - 3-38.
This arrangement substantially decouples the plate and the control grid
The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consist ...
, eliminating the need for neutralizing circuitry at medium wave broadcast frequencies. The screen grid also largely reduces the influence of the plate voltage on the space charge near the cathode, permitting the tetrode to produce greater voltage gain than the triode in amplifier circuits. While the amplification factors of typical triodes commonly range from below ten to around 100, tetrode amplification factors of 500 are common. Consequently, higher voltage gains from a single tube amplification stage became possible, reducing the number of tubes required. Screen grid tubes were put on the market in late 1927.[K. R. Thrower, (2009) ''British Radio Valves The Classic Years: 1926-1946'', Reading, UK: Speedwell, p. 3]
However, the useful region of operation of the screen grid tube as an amplifier was limited to plate voltages greater than the screen grid voltage, due to secondary emission from the plate. In any tube, electrons strike the plate with sufficient energy to cause the emission of electrons from its surface. In a triode this secondary emission of electrons is not important since they are simply re-captured by the plate. But in a tetrode they can be captured by the screen grid since it is also at a positive voltage, robbing them from the plate current and reducing the amplification of the tube. Since secondary electrons can outnumber the primary electrons over a certain range of plate voltages, the plate current can decrease with increasing plate voltage. This is the ''dynatron region'' or ''tetrode kink'' and is an example of negative resistance
In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.
This is in contrast to an ordina ...
which can itself cause instability. Another undesirable consequence of secondary emission is that screen current is increased, which may cause the screen to exceed its power rating.
The otherwise undesirable negative resistance region of the plate characteristic was exploited with the dynatron oscillator circuit to produce a simple oscillator only requiring connection of the plate to a resonant LC circuit to oscillate. The dynatron oscillator operated on the same principle of negative resistance as the tunnel diode oscillator many years later.
The dynatron region of the screen grid tube was eliminated by adding a grid between the screen grid and the plate to create the pentode. The suppressor grid of the pentode was usually connected to the cathode and its negative voltage relative to the anode repelled secondary electrons so that they would be collected by the anode instead of the screen grid. The term ''pentode'' means the tube has five electrodes. The pentode was invented in 1926 by Bernard D. H. Tellegen and became generally favored over the simple tetrode. Pentodes are made in two classes: those with the suppressor grid wired internally to the cathode (e.g. EL84/6BQ5) and those with the suppressor grid wired to a separate pin for user access (e.g. 803, 837). An alternative solution for power applications is the beam tetrode
A beam tetrode, sometimes called a beam power tube, is a type of vacuum tube or thermionic valve that has two grids and forms the electron stream from the cathode into multiple partially collimated beams to produce a low potential space cha ...
or ''beam power tube'', discussed below.
Multifunction and multisection tubes
Superheterodyne receivers require a local oscillator
In electronics, a local oscillator (LO) is an electronic oscillator used with a mixer to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequenc ...
and mixer, combined in the function of a single pentagrid converter
The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.
The pentagrid was part of a line of development of valves that were able to take an ...
tube. Various alternatives such as using a combination of a triode with a hexode
The pentagrid converter is a type of radio receiving valve (vacuum tube) with five grids used as the frequency mixer stage of a superheterodyne radio receiver.
The pentagrid was part of a line of development of valves that were able to take an ...
and even an octode have been used for this purpose. The additional grids include control grid
The control grid is an electrode used in amplifying thermionic valves (vacuum tubes) such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consist ...
s (at a low potential) and screen grids (at a high voltage). Many designs use such a screen grid as an additional anode to provide feedback for the oscillator function, whose current adds to that of the incoming radio frequency signal. The pentagrid converter thus became widely used in AM receivers, including the miniature tube version of the "All American Five
The term All American Five (abbreviated AA5) is a colloquial name for mass-produced, superheterodyne radio receivers that used five vacuum tubes in their design. These radio sets were designed to receive amplitude modulation (AM) broadcasts in the ...
". Octodes, such as the 7A8, were rarely used in the United States, but much more common in Europe, particularly in battery operated radios where the lower power consumption was an advantage.
To further reduce the cost and complexity of radio equipment, two separate structures (triode and pentode for instance) can be combined in the bulb of a single ''multisection tube''. An early example is the Loewe 3NF
The Loewe 3NF was an early attempt to combine several functions in one electronic device.
Produced by the German Loewe-Audion GmbH as early as 1926, the device consisted of three triode valves (tubes) in a single glass envelope together with ...
. This 1920s device has three triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver. As the Loewe set had only one tube socket, it was able to substantially undercut the competition, since, in Germany, state tax was levied by the number of sockets. However, reliability was compromised, and production costs for the tube were much greater. In a sense, these were akin to integrated circuits. In the United States, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also has a single tube socket, but because it uses a four-pin base, the additional element connections are made on a "mezzanine" platform at the top of the tube base.
By 1940 multisection tubes had become commonplace. There were constraints, however, due to patents and other licensing considerations (see British Valve Association
The British Radio and Valve Manufacturers' Association (BVA) was a 20th-century cartel of vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high va ...
). Constraints due to the number of external pins (leads) often forced the functions to share some of those external connections such as their cathode connections (in addition to the heater connection). The RCA Type 55 is a double diode triode used as a detector, automatic gain control rectifier and audio preamplifier
A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier a ...
in early AC powered radios. These sets often include the 53 Dual Triode Audio Output. Another early type of multi-section tube, the 6SN7, is a "dual triode" which performs the functions of two triode tubes while taking up half as much space and costing less.
The 12AX7 is a dual "high mu" (high voltage gain) triode in a miniature enclosure, and became widely used in audio signal amplifiers, instruments, and guitar amplifier
A guitar amplifier (or amp) is an electronic device or system that strengthens the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which ar ...
s.
The introduction of the miniature tube base (see below) which can have 9 pins, more than previously available, allowed other multi-section tubes to be introduced, such as the 6GH8
The 6GH8 (More commonly labeled as the 6GH8A)is a nine pin miniature vacuum tube, produced as a combination of medium-mu Triode and sharp-cutoff Pentode. It follows that the tube is divided into two sections - anode and cathode. Each of the sectio ...
/ECF82 triode-pentode, quite popular in television receivers. The desire to include even more functions in one envelope resulted in the General Electric Compactron which has 12 pins. A typical example, the 6AG11, contains two triodes and two diodes.
Some otherwise conventional tubes do not fall into standard categories; the 6AR8, 6JH8 and 6ME8 have several common grids, followed by a pair of beam deflection electrodes which deflected the current towards either of two anodes.They were sometimes known as the 'sheet beam' tubes and used in some color TV sets for color demodulation. The similar 7360 was popular as a balanced SSB (de)modulator.
Beam power tubes
A beam power tube forms the electron stream from the cathode into multiple partially collimated beams to produce a low potential space charge
Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. Thi ...
region between the anode and screen grid to return anode secondary emission electrons to the anode when the anode potential is less than that of the screen grid. Formation of beams also reduces screen grid current. In some cylindrically symmetrical beam power tubes, the cathode is formed of narrow strips of emitting material that are aligned with the apertures of the control grid, reducing control grid current. This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes.
Manufacturer's data sheets often use the terms ''beam pentode'' or ''beam power pentode'' instead of ''beam power tube'', and use a pentode graphic symbol instead of a graphic symbol showing beam forming plates.
Beam power tubes offer the advantages of a longer load line, less screen current, higher transconductance and lower third harmonic distortion than comparable power pentodes. Beam power tubes can be connected as triodes for improved audio tonal quality but in triode mode deliver significantly reduced power output.
Gas-filled tubes
Gas-filled tube
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric ...
s such as discharge tube
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric ...
s and cold cathode
A cold cathode is a cathode that is not electrically heated by a filament.A negatively charged electrode emits electrons or is the positively charged terminal. For more, see field emission. A cathode may be considered "cold" if it emits more el ...
tubes are not ''hard'' vacuum tubes, though are always filled with gas at less than sea-level atmospheric pressure. Types such as the voltage-regulator tube
A voltage-regulator tube (VR tube) is an electronic component used as a shunt regulator to hold a voltage constant at a pre-determined level.
Physically, these devices resemble vacuum tubes, but there are two main differences:
* Their glass env ...
and thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
resemble hard vacuum tubes and fit in sockets designed for vacuum tubes. Their distinctive orange, red, or purple glow during operation indicates the presence of gas; electrons flowing in a vacuum do not produce light within that region. These types may still be referred to as "electron tubes" as they do perform electronic functions. High-power rectifiers use mercury vapor to achieve a lower forward voltage drop than high-vacuum tubes.
Miniature tubes
Early tubes used a metal or glass envelope atop an insulating bakelite base. In 1938 a technique was developed to use an all-glass construction with the pins fused in the glass base of the envelope. This was used in the design of a much smaller tube outline, known as the miniature tube, having seven or nine pins. Making tubes smaller reduced the voltage where they could safely operate, and also reduced the power dissipation of the filament. Miniature tubes became predominant in consumer applications such as radio receivers and hi-fi amplifiers. However, the larger older styles continued to be used especially as higher-power rectifiers, in higher-power audio output stages and as transmitting tubes.
Sub-miniature tubes
Sub-miniature tubes with a size roughly that of half a cigarette were used in consumer applications as hearing-aid amplifiers. These tubes did not have pins plugging into a socket but were soldered in place. The " acorn tube" (named due to its shape) was also very small, as was the metal-cased RCA nuvistor from 1959, about the size of a thimble
A thimble is a small pitted cup worn on the finger that protects it from being pricked or poked by a needle while sewing. The Old English word , the ancestor of thimble, is derived from Old English , the ancestor of the English word ''thumb''.
...
. The nuvistor was developed to compete with the early transistors and operated at higher frequencies than those early transistors could. The small size supported especially high-frequency operation; nuvistors were used in aircraft radio transceivers, UHF television tuners, and some HiFi FM radio tuners (Sansui 500A) until replaced by high-frequency capable transistors.
Improvements in construction and performance
The earliest vacuum tubes strongly resembled incandescent light bulbs and were made by lamp manufacturers, who had the equipment needed to manufacture glass envelopes and the vacuum pumps required to evacuate the enclosures. de Forest used Heinrich Geissler Heinrich may refer to:
People
* Heinrich (given name), a given name (including a list of people with the name)
* Heinrich (surname), a surname (including a list of people with the name)
*Hetty (given name), a given name (including a list of peo ...
's mercury displacement pump, which left behind a partial vacuum. The development of the diffusion pump in 1915 and improvement by Irving Langmuir led to the development of high-vacuum tubes. After World War I, specialized manufacturers using more economical construction methods were set up to fill the growing demand for broadcast receivers. Bare tungsten filaments operated at a temperature of around 2200 °C. The development of oxide-coated filaments in the mid-1920s reduced filament operating temperature to a dull red heat (around 700 °C), which in turn reduced thermal distortion of the tube structure and allowed closer spacing of tube elements. This in turn improved tube gain, since the gain of a triode is inversely proportional to the spacing between grid and cathode. Bare tungsten filaments remain in use in small transmitting tubes but are brittle and tend to fracture if handled roughly—e.g. in the postal services. These tubes are best suited to stationary equipment where impact and vibration is not present.
Indirectly heated cathodes
The desire to power electronic equipment using AC mains power faced a difficulty with respect to the powering of the tubes' filaments, as these were also the cathode of each tube. Powering the filaments directly from a power transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's c ...
introduced mains-frequency (50 or 60 Hz) hum into audio stages. The invention of the "equipotential cathode" reduced this problem, with the filaments being powered by a balanced AC power transformer winding having a grounded center tap.
A superior solution, and one which allowed each cathode to "float" at a different voltage, was that of the indirectly heated cathode: a cylinder of oxide-coated nickel acted as an electron-emitting cathode and was electrically isolated from the filament inside it. Indirectly heated cathodes enable the cathode circuit to be separated from the heater circuit. The filament, no longer electrically connected to the tube's electrodes, became simply known as a "heater", and could as well be powered by AC without any introduction of hum. In the 1930s, indirectly heated cathode tubes became widespread in equipment using AC power. Directly heated cathode tubes continued to be widely used in battery-powered equipment as their filaments required considerably less power than the heaters required with indirectly heated cathodes.
Tubes designed for high gain audio applications may have twisted heater wires to cancel out stray electric fields, fields that could induce objectionable hum into the program material.
Heaters may be energized with either alternating current (AC) or direct current (DC). DC is often used where low hum is required.
Use in electronic computers
Vacuum tubes used as switches made electronic computing possible for the first time, but the cost and relatively short mean time to failure
There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value ( magnitude and sign) of a given data set.
For a data set, the ''arithm ...
of tubes were limiting factors. "The common wisdom was that valves—which, like light bulbs, contained a hot glowing filament—could never be used satisfactorily in large numbers, for they were unreliable, and in a large installation too many would fail in too short a time".[ ]Tommy Flowers
Thomas Harold Flowers MBE (22 December 1905 – 28 October 1998) was an English engineer with the British General Post Office. During World War II, Flowers designed and built Colossus, the world's first programmable electronic computer, to help ...
, who later designed '' Colossus'', "discovered that, so long as valves were switched on and left on, they could operate reliably for very long periods, especially if their 'heaters' were run on a reduced current".[From part of Copeland's "Colossus" available online]
In 1934 Flowers built a successful experimental installation using over 3,000 tubes in small independent modules; when a tube failed, it was possible to switch off one module and keep the others going, thereby reducing the risk of another tube failure being caused; this installation was accepted by the Post Office (who operated telephone exchanges). Flowers was also a pioneer of using tubes as very fast (compared to electromechanical devices) electronic switches. Later work confirmed that tube unreliability was not as serious an issue as generally believed; the 1946 ENIAC
ENIAC (; Electronic Numerical Integrator and Computer) was the first programmable, electronic, general-purpose digital computer, completed in 1945. There were other computers that had these features, but the ENIAC had all of them in one pac ...
, with over 17,000 tubes, had a tube failure (which took 15 minutes to locate) on average every two days. The quality of the tubes was a factor, and the diversion of skilled people during the Second World War lowered the general quality of tubes. During the war Colossus was instrumental in breaking German codes. After the war, development continued with tube-based computers including, military computers ENIAC
ENIAC (; Electronic Numerical Integrator and Computer) was the first programmable, electronic, general-purpose digital computer, completed in 1945. There were other computers that had these features, but the ENIAC had all of them in one pac ...
and Whirlwind
A whirlwind is a weather phenomenon in which a vortex of wind (a vertically oriented rotating column of air) forms due to instabilities and turbulence created by heating and flow (current) gradients. Whirlwinds occur all over the world and i ...
, the Ferranti Mark 1
The Ferranti Mark 1, also known as the Manchester Electronic Computer in its sales literature, and thus sometimes called the Manchester Ferranti, was produced by British electrical engineering firm Ferranti Ltd. It was the world's first commer ...
(one of the first commercially available electronic computers), and UNIVAC I, also available commercially.
Advances using subminiature tubes included the Jaincomp series of machines produced by the Jacobs Instrument Company of Bethesda, Maryland. Models such as its Jaincomp-B employed just 300 such tubes in a desktop-sized unit that offered performance to rival many of the then room-sized machines.[Pentagon symposium: ''Commercially Available General Purpose Electronic Digital Computers of Moderate Price'', Washington, D.C., 14 MAY 1952]
/ref>
Colossus
Flowers's Colossus and its successor Colossus Mk2 were built by the British during World War II to substantially speed up the task of breaking the German high level Lorenz encryption. Using about 1,500 vacuum tubes (2,400 for Mk2), Colossus replaced an earlier machine based on relay and switch logic (the Heath Robinson). Colossus was able to break in a matter of hours messages that had previously taken several weeks; it was also much more reliable.[ Colossus was the first use of vacuum tubes ''working in concert'' on such a large scale for a single machine.][
]
Whirlwind and "special-quality" tubes
To meet the reliability requirements of the 1951 US digital computer Whirlwind, "special-quality" tubes with extended life, and a long-lasting cathode in particular, were produced. The problem of short lifetime was traced largely to evaporation of silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
, used in the tungsten alloy to make the heater wire easier to draw. The silicon forms barium orthosilicate
In chemistry, orthosilicate is the anion , or any of its salts and esters. It is one of the silicate anions. It is occasionally called the silicon tetroxide anion or group.C. A. Kumins, and A. E. Gessler (1953), "Short-Cycle Syntheses of Ultram ...
at the interface between the nickel sleeve and the cathode barium oxide
Barium oxide, also known as baria, is a white hygroscopic non-flammable compound with the formula BaO. It has a cubic structure and is used in cathode ray tubes, crown glass, and catalysts. It is harmful to human skin and if swallowed in larg ...
coating.[Morgan Jones, ''Valve Amplifiers'', Elsevier, 2012 .] This "cathode interface" is a high-resistance layer (with some parallel capacitance) which greatly reduces the cathode current when the tube is switched into conduction mode.[ Elimination of silicon from the heater wire alloy (and more frequent replacement of the ]wire
Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm
A wire is a flexible strand of metal.
Wire is c ...
drawing dies) allowed the production of tubes that were reliable enough for the Whirlwind project. High-purity nickel tubing and cathode coatings free of materials such as silicates and aluminum that can reduce emissivity also contribute to long cathode life.
The first such "computer tube" was Sylvania's 7AK7 pentode of 1948 (these replaced the 7AD7, which was supposed to be better quality than the standard 6AG7 but proved too unreliable).[ Computers were the first tube devices to run tubes at cutoff (enough negative grid voltage to make them cease conduction) for quite-extended periods of time. Running in cutoff with the heater on accelerates cathode poisoning and the output current of the tube will be greatly reduced when switched into conduction mode.][ The 7AK7 tubes improved the cathode poisoning problem, but that alone was insufficient to achieve the required reliability.][ Further measures included switching off the heater voltage when the tubes were not required to conduct for extended periods, turning on and off the heater voltage with a slow ramp to avoid ]thermal shock
Thermal shock is a type of rapidly transient mechanical load.
By definition, it is a mechanical load caused by a rapid change of temperature of a certain point.
It can be also extended to the case of a thermal gradient, which makes different pa ...
on the heater element,[E.S. Rich, N.H. Taylor, "Component failure analysis in computers", ''Proceedings of Symposium on Improved Quality Electronic Components'', vol. 1, pp. 222–233, Radio-Television Manufacturers Association, 1950.] and stress testing the tubes during offline maintenance periods to bring on early failure of weak units.[Bernd Ulmann, ''AN/FSQ-7: The Computer that Shaped the Cold War'', Walter de Gruyter GmbH, 2014 .]
The tubes developed for Whirlwind were later used in the giant SAGE air-defense computer system. By the late 1950s, it was routine for special-quality small-signal tubes to last for hundreds of thousands of hours if operated conservatively. This increased reliability also made mid-cable amplifiers in submarine cable Submarine cable is any electrical cable that is laid on the seabed, although the term is often extended to encompass cables laid on the bottom of large freshwater bodies of water.
Examples include:
*Submarine communications cable
*Submarine power ...
s possible.
Heat generation and cooling
A considerable amount of heat is produced when tubes operate, from both the filament (heater) and the stream of electrons bombarding the plate. In power amplifiers, this source of heat is greater than cathode heating. A few types of tube permit operation with the anodes at a dull red heat; in other types, red heat indicates severe overload.
The requirements for heat removal can significantly change the appearance of high-power vacuum tubes. High power audio amplifiers and rectifiers required larger envelopes to dissipate heat. Transmitting tubes could be much larger still.
Heat escapes the device by black-body radiation from the anode (plate) as infrared radiation, and by convection of air over the tube envelope. Convection is not possible inside most tubes since the anode is surrounded by vacuum.
Tubes which generate relatively little heat, such as the 1.4-volt filament directly heated tubes designed for use in battery-powered equipment, often have shiny metal anodes. 1T4, 1R5 and 1A7 are examples. Gas-filled tubes such as thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
s may also use a shiny metal anode since the gas present inside the tube allows for heat convection from the anode to the glass enclosure.
The anode is often treated to make its surface emit more infrared energy. High-power amplifier tubes are designed with external anodes that can be cooled by convection, forced air or circulating water. The water-cooled 80 kg, 1.25 MW 8974
8974 / X-2159 is a power tetrode designed for megawatt power levels in industrial and broadcast applications.
Specifications
The 8974 is an external anode tube made of metal and ceramic. It has an overall height of , a diameter of , and weigh ...
is among the largest commercial tubes available today.
In a water-cooled tube, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator to prevent high voltage leakage through the cooling water to the radiator system. Water as usually supplied has ions that conduct electricity; deionized water, a good insulator, is required. Such systems usually have a built-in water-conductance monitor which will shut down the high-tension supply if the conductance becomes too high.
The screen grid may also generate considerable heat. Limits to screen grid dissipation, in addition to plate dissipation, are listed for power devices. If these are exceeded then tube failure is likely.
Tube packages
Most modern tubes have glass envelopes, but metal, fused quartz (silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
) and ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
have also been used. A first version of the 6L6 used a metal envelope sealed with glass beads, while a glass disk fused to the metal was used in later versions. Metal and ceramic are used almost exclusively for power tubes above 2 kW dissipation. The nuvistor was a modern receiving tube using a very small metal and ceramic package.
The internal elements of tubes have always been connected to external circuitry via pins at their base which plug into a socket. Subminiature tubes were produced using wire leads rather than sockets, however, these were restricted to rather specialized applications. In addition to the connections at the base of the tube, many early triodes connected the grid using a metal cap at the top of the tube; this reduces stray capacitance between the grid and the plate leads. Tube caps were also used for the plate (anode) connection, particularly in transmitting tubes and tubes using a very high plate voltage.
High-power tubes such as transmitting tubes have packages designed more to enhance heat transfer. In some tubes, the metal envelope is also the anode. The 4CX1000A is an external anode tube of this sort. Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation. Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac Eimac is a trade mark of Eimac Products, part of the Microwave Power Products Division of Communications & Power Industries. It produces power vacuum tubes for radio frequency applications such as broadcast and radar transmitters. The company name ...
, a forced water-cooled power tetrode capable of dissipating 2.5 megawatts. By comparison, the largest power transistor can only dissipate about 1 kilowatt.
Names
The generic name " hermionicvalve" used in the UK derives from the unidirectional current flow allowed by the earliest device, the thermionic diode emitting electrons from a heated filament, by analogy with a non-return valve in a water pipe. The US names "vacuum tube", "electron tube", and "thermionic tube" all simply describe a tubular envelope which has been evacuated ("vacuum"), has a heater and controls electron flow.
In many cases, manufacturers and the military gave tubes designations that said nothing about their purpose (e.g., 1614). In the early days some manufacturers used proprietary names which might convey some information, but only about their products; the KT66 and KT88 were "kinkless tetrodes". Later, consumer tubes were given names that conveyed some information, with the same name often used generically by several manufacturers. In the US, Radio Electronics Television Manufacturers' Association (RETMA) designations comprise a number, followed by one or two letters, and a number. The first number is the (rounded) heater voltage; the letters designate a particular tube but say nothing about its structure; and the final number is the total number of electrodes (without distinguishing between, say, a tube with many electrodes, or two sets of electrodes in a single envelope—a double triode, for example). For example, the 12AX7 is a double triode (two sets of three electrodes plus heater) with a 12.6V heater (which, as it happens, can also be connected to run from 6.3V). The "AX" has no meaning other than to designate this particular tube according to its characteristics. Similar, but not identical, tubes are the 12AD7, 12AE7...12AT7, 12AU7, 12AV7, 12AW7 (rare!), 12AY7, and the 12AZ7.
A system widely used in Europe known as the Mullard–Philips tube designation, also extended to transistors, uses a letter, followed by one or more further letters, and a number. The type designator specifies the heater voltage or current (one letter), the functions of all sections of the tube (one letter per section), the socket type (first digit), and the particular tube (remaining digits). For example, the ECC83 (equivalent to the 12AX7) is a 6.3V (E) double triode (CC) with a miniature base (8). In this system special-quality tubes (e.g., for long-life computer use) are indicated by moving the number immediately after the first letter: the E83CC is a special-quality equivalent of the ECC83, the E55L a power pentode with no consumer equivalent.
Special-purpose tubes
Some special-purpose tubes are constructed with particular gases in the envelope. For instance, voltage-regulator tube
A voltage-regulator tube (VR tube) is an electronic component used as a shunt regulator to hold a voltage constant at a pre-determined level.
Physically, these devices resemble vacuum tubes, but there are two main differences:
* Their glass env ...
s contain various inert gases such as argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
, helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
or neon, which will ionize
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
at predictable voltages. The thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
is a special-purpose tube filled with low-pressure gas or mercury vapor. Like vacuum tubes, it contains a hot cathode and an anode, but also a control electrode which behaves somewhat like the grid of a triode. When the control electrode starts conduction, the gas ionizes, after which the control electrode can no longer stop the current; the tube "latches" into conduction. Removing anode (plate) voltage lets the gas de-ionize, restoring its non-conductive state.
Some thyratrons can carry large currents for their physical size. One example is the miniature type 2D21, often seen in 1950s jukebox
A jukebox is a partially automated music-playing device, usually a coin-operated machine, that will play a patron's selection from self-contained media. The classic jukebox has buttons, with letters and numbers on them, which are used to sele ...
es as control switches for relays. A cold-cathode version of the thyratron, which uses a pool of mercury for its cathode, is called an ignitron
An ignitron is a type of gas-filled tube used as a controlled rectifier and dating from the 1930s. Invented by Joseph Slepian while employed by Westinghouse, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignit ...
; some can switch thousands of amperes. Thyratrons containing hydrogen have a very consistent time delay between their turn-on pulse and full conduction; they behave much like modern silicon-controlled rectifier
A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The principle of four ...
s, also called thyristors due to their functional similarity to thyratrons. Hydrogen thyratrons have long been used in radar transmitters.
A specialized tube is the krytron
The krytron is a cold-cathode gas-filled tube intended for use as a very high-speed switch, somewhat similar to the thyratron. It consists of a sealed glass tube with four electrodes. A small triggering pulse on the grid electrode switches the ...
, which is used for rapid high-voltage switching. Krytrons are used to initiate the detonations used to set off a nuclear weapon; krytrons are heavily controlled at an international level.
X-ray tube
An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast ...
s are used in medical imaging among other uses. X-ray tubes used for continuous-duty operation in fluoroscopy and CT imaging equipment may use a focused cathode and a rotating anode to dissipate the large amounts of heat thereby generated. These are housed in an oil-filled aluminum housing to provide cooling.
The photomultiplier tube is an extremely sensitive detector of light, which uses the photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
and secondary emission, rather than thermionic emission, to generate and amplify electrical signals. Nuclear medicine imaging equipment and liquid scintillation counters use photomultiplier tube arrays to detect low-intensity scintillation due to ionizing radiation.
The Ignatron tube was used in resistance welding equipment in the early 1970s. The Ignatron had a cathode, anode and an igniter. The tube base was filled with mercury and the tube was used as a very high current switch. A large current potential was placed between the anode and cathode of the tube but was only permitted to conduct when the igniter in contact with the mercury had enough current to vaporize the mercury and complete the circuit. Because this was used in resistance welding there were two Ignatrons for the two phases of an AC circuit. Because of the mercury at the bottom of the tube they were extremely difficult to ship. These tubes were eventually replaced by SCRs (Silicon Controlled Rectifiers).
Powering the tube
Batteries
Batteries provided the voltages required by tubes in early radio sets. Three different voltages were generally required, using three different batteries designated as the A, B, and C battery. The "A" battery or LT (low-tension) battery provided the filament voltage. Tube heaters were designed for single, double or triple-cell lead-acid batteries, giving nominal heater voltages of 2 V, 4 V or 6 V. In portable radios, dry batteries were sometimes used with 1.5 or 1 V heaters. Reducing filament consumption improved the life span of batteries. By 1955 towards the end of the tube era, tubes using only 50 mA down to as little as 10 mA for the heaters had been developed.
The high voltage applied to the anode (plate) was provided by the "B" battery or the HT (high-tension) supply or battery. These were generally of dry cell construction and typically came in 22.5-, 45-, 67.5-, 90-, 120- or 135-volt versions. After the use of B-batteries was phased out and rectified line-power was employed to produce the high voltage needed by tubes' plates, the term "B+" persisted in the US when referring to the high voltage source. Most of the rest of the English speaking world refers to this supply as just HT (high tension).
Early sets used a grid bias battery or "C" battery which was connected to provide a ''negative'' voltage. Since no current flows through a tube's grid connection, these batteries had no current drain and lasted the longest, usually limited by their own shelf life. The supply from the grid bias battery was rarely, if ever, disconnected when the radio was otherwise switched off. Even after AC power supplies became commonplace, some radio sets continued to be built with C batteries, as they would almost never need replacing. However more modern circuits were designed using cathode biasing, eliminating the need for a third power supply voltage; this became practical with tubes using indirect heating of the cathode along with the development of resistor/capacitor coupling which replaced earlier interstage transformers.
The "C battery" for bias is a designation having no relation to the " C cell" battery size.
AC power
Battery replacement was a major operating cost for early radio receiver users. The development of the battery eliminator
A battery eliminator is a device powered by an electrical source other than a battery, which then converts the source to a suitable DC voltage that may be used by a second device designed to be powered by batteries. A battery eliminator does awa ...
, and, in 1925, batteryless receivers operated by household power, reduced operating costs and contributed to the growing popularity of radio. A power supply using a transformer with several windings, one or more rectifiers (which may themselves be vacuum tubes), and large filter capacitors provided the required direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or eve ...
voltages from the alternating current source.
As a cost reduction measure, especially in high-volume consumer receivers, all the tube heaters could be connected in series across the AC supply using heaters requiring the same current and with a similar warm-up time. In one such design, a tap on the tube heater string supplied the 6 volts needed for the dial light. By deriving the high voltage from a half-wave rectifier directly connected to the AC mains, the heavy and costly power transformer was eliminated. This also allowed such receivers to operate on direct current, a so-called AC/DC receiver design
An AC/DC receiver design is a style of power supply of vacuum tube radio or television receivers that eliminated the bulky and expensive mains transformer. A side-effect of the design was that the receiver could in principle operate from a DC su ...
. Many different US consumer AM radio manufacturers of the era used a virtually identical circuit, given the nickname All American Five
The term All American Five (abbreviated AA5) is a colloquial name for mass-produced, superheterodyne radio receivers that used five vacuum tubes in their design. These radio sets were designed to receive amplitude modulation (AM) broadcasts in the ...
.
Where the mains voltage was in the 100–120 V range, this limited voltage proved suitable only for low-power receivers. Television receivers either required a transformer or could use a voltage doubling circuit. Where 230 V nominal mains voltage was used, television receivers as well could dispense with a power transformer.
Transformer-less power supplies required safety precautions in their design to limit the shock hazard to users, such as electrically insulated cabinets and an interlock
An interlock is a feature that makes the state of two mechanisms or functions mutually dependent. It may be used to prevent undesired states in a finite-state machine, and may consist of any electrical, electronic, or mechanical devices or system ...
tying the power cord to the cabinet back, so the line cord was necessarily disconnected if the user or service person opened the cabinet. A ''cheater cord'' was a power cord ending in the special socket used by the safety interlock; servicers could then power the device with the hazardous voltages exposed.
To avoid the warm-up delay, "instant on" television receivers passed a small heating current through their tubes even when the set was nominally off. At switch on, full heating current was provided and the set would play almost immediately.
Reliability
One reliability problem of tubes with oxide cathodes is the possibility that the cathode may slowly become " poisoned" by gas molecules from other elements in the tube, which reduce its ability to emit electrons. Trapped gases or slow gas leaks can also damage the cathode or cause plate (anode) current runaway due to ionization
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
of free gas molecules. Vacuum hardness and proper selection of construction materials are the major influences on tube lifetime. Depending on the material, temperature and construction, the surface material of the cathode may also diffuse onto other elements. The resistive heaters that heat the cathodes may break in a manner similar to incandescent lamp filaments, but rarely do, since they operate at much lower temperatures than lamps.
The heater's failure mode is typically a stress-related fracture of the tungsten wire or at a weld point and generally occurs after accruing many thermal (power on-off) cycles. Tungsten wire has a very low resistance when at room temperature. A negative temperature coefficient device, such as a thermistor, may be incorporated in the equipment's heater supply or a ramp-up circuit may be employed to allow the heater or filaments to reach operating temperature more gradually than if powered-up in a step-function. Low-cost radios had tubes with heaters connected in series, with a total voltage equal to that of the line (mains). Some receivers made before World War II had series-string heaters with total voltage less than that of the mains. Some had a resistance wire running the length of the power cord to drop the voltage to the tubes. Others had series resistors made like regular tubes; they were called ballast tubes.
Following World War II, tubes intended to be used in series heater strings were redesigned to all have the same ("controlled") warm-up time. Earlier designs had quite-different thermal time constants. The audio output stage, for instance, had a larger cathode and warmed up more slowly than lower-powered tubes. The result was that heaters that warmed up faster also temporarily had higher resistance, because of their positive temperature coefficient. This disproportionate resistance caused them to temporarily operate with heater voltages well above their ratings, and shortened their life.
Another important reliability problem is caused by air leakage into the tube. Usually oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
in the air reacts chemically with the hot filament or cathode, quickly ruining it. Designers developed tube designs that sealed reliably. This was why most tubes were constructed of glass. Metal alloys (such as Cunife and Fernico) and glasses had been developed for light bulbs that expanded and contracted in similar amounts, as temperature changed. These made it easy to construct an insulating envelope of glass, while passing connection wires through the glass to the electrodes.
When a vacuum tube is overloaded or operated past its design dissipation, its anode (plate) may glow red. In consumer equipment, a glowing plate is universally a sign of an overloaded tube. However, some large transmitting tubes are designed to operate with their anodes at red, orange, or in rare cases, white heat.
"Special quality" versions of standard tubes were often made, designed for improved performance in some respect, such as a longer life cathode, low noise construction, mechanical ruggedness via ruggedized filaments, low microphony, for applications where the tube will spend much of its time cut off, etc. The only way to know the particular features of a special quality part is by reading the datasheet. Names may reflect the standard name (12AU7>12AU7A, its equivalent ECC82>E82CC, etc.), or be absolutely anything (standard and special-quality equivalents of the same tube include 12AU7, ECC82, B329, CV491, E2163, E812CC, M8136, CV4003, 6067, VX7058, 5814A and 12AU7A).
The longest recorded valve life was earned by a Mazda AC/P pentode valve (serial No. 4418) in operation at the BBC #REDIRECT BBC #REDIRECT BBC
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's main Northern Ireland transmitter at Lisnagarvey. The valve was in service from 1935 until 1961 and had a recorded life of 232,592 hours. The BBC maintained meticulous records of their valves' lives with periodic returns to their central valve stores.
A vacuum tube needs an extremely high vacuum (or ''hard'' vacuum, from X-ray terminology) to avoid the consequences of generating positive ions within the tube. Residual gas atoms
when struck by an electron and can adversely affect the cathode, reducing emission. Larger amounts of residual gas can create a visible
between the tube electrodes and cause overheating of the electrodes, producing more gas, damaging the tube and possibly other components due to excess current.
To avoid these effects, the residual pressure within the tube must be low enough that the
of an electron is much longer than the size of the tube (so an electron is unlikely to strike a residual atom and very few ionized atoms will be present). Commercial vacuum tubes are evacuated at manufacture to about .
To prevent gases from compromising the tube's vacuum, modern tubes are constructed with
being the most common.