Encyclopedia
In
electronics, a
vacuum tube or
valve is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of
electrons in an
evacuated space. For most purposes, the vacuum tube has been replaced by the much smaller and less expensive
transistor, either as a discrete
device or in an
integrated circuit. However, tubes are still used in several specialised applications such as audio systems and high power RF transmitters, as the display device in
cathode ray tube television sets, and to generate microwaves in
microwave ovens.
The vacuum tube is a
voltage-controlled device, which means that the relationship between the input and output circuits is determined by a
transconductance function. The solid-state device most closely analogous to the vacuum tube is the
JFET, although the vacuum tube typically operates at far higher voltage levels than the JFET.
Explanation
Vacuum tubes, or
thermionic valves, are arrangements of electrodes in a
vacuum within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a
tube socket for easy replacement.
The simplest vacuum tubes resemble
incandescent light bulbs in that they have a
filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases
electrons into the vacuum: a process called
thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal "
plate" inside the envelope if the plate is positively charged relative to the filament . The result is a current of electrons flowing from filament to plate. This cannot work in the reverse direction because the plate is not heated and cannot emit electrons. This very simple example described can thus be seen to operate as a
diode: a device that conducts current only in one direction.
History of development
The 19th century saw increasing research with evacuated tubes, such as the
Geissler and
Crookes tubes. Scientists who experimented with such tubes included Eugen Goldstein,
Nikola Tesla, Johann Wilhelm Hittorf,
Thomas Edison, and many others. These tubes were mostly for specialized scientific applications, or were novelties, with the exception of the
light bulb. The groundwork laid by these scientists and inventors, however, was critical to the development of vacuum tube technology.
Though the
thermionic emission effect was observed as early as 1873, it is Thomas Edison's 1883 investigation of the "
Edison Effect" that is the best known. He promptly patented it , but as the particle nature of the
electron was not known until 1897, he did not understand the process.
Diodes and triodes
John Ambrose Fleming had worked for Edison; in 1904, as scientific adviser to the
Marconi company, he developed the "oscillation valve" or
kenotron. Later known as the
diode, it allowed electric current to flow in only one direction, enabling the
rectification of alternating current. Its operation is described in greater detail in the previous section.
In 1906
Lee De Forest placed a bent wire serving as a screen between the filament and
plate electrode, later known as the "grid" electrode. As the voltage applied to the grid was varied from negative to positive, the amount of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was therefore an excellent and very sensitive amplifier of voltages. DeForest called his invention the "Audion". In 1907, DeForest filed for a three-electrode version of the Audion for use in radio communications. The device is now known as the
triode. De Forest's device was not strictly a vacuum tube, but clearly depended for its action on ionisation of the relatively high levels of gas remaining after evacuation. The De Forest company in its Audion leaflets warned against operation which might cause the vacuum to become too hard. The first true vacuum triodes were the Pliotrons developed by
Irving Langmuir at the
General Electric research laboratory in 1915. These were closely followed by the French 'R' Type which was in widespread use by the allied military by 1916. These two types were the first true
vacuum tubes.
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortions at low volumes. This is not to be confused with the overdrive that tube amplifiers exhibit at high volume levels . To remedy the low volume overdrive problem, engineers plotted curves of the applied grid voltage and resulting plate currents, and discovered that there was a range of relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current". Today this current would be called the quiescent or standing current. The controlling voltage was superimposed onto this fixed voltage, resulting in linear swings of plate current for both positive and negative swings of the input voltage. This concept was called
grid bias.
Batteries were designed to provide the various voltages required. "A" batteries provided the filament voltage. These were often rechargable - usually of the
lead-acid type ranging from 2 to 12 volts with single, double and triple cells being most common. In portable radios, flashlight bateries were sometimes used.
The "B" batteries provided the plate voltage. These were generally of Dry cell construction, containing many small 1.5 Volt cells in series and typically came in ratings of 22.5, 45, 60, 90 or 135 volts. To this day, plate voltage is referred to as B
+.
Some sets used "C" batteries were used to provide grid bias, although many circuits used grid leak
resistors,
voltage dividers or Cathode bias to provide proper tube bias.
Direct and indirect heating
Many further innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use the cathode as the source of electron flow in the tube rather than the filament itself. This minimized the introduction of hum when the filament was energized with
alternating current. In such tubes, the filament is called a heater to distinguish it as an inactive element.
Tetrodes and pentodes
When triodes were first used in radio transmitters and receivers, it was found that they were often unstable and had a tendency to oscillate due to parasitic anode to grid capacitance. Many complex circuits were developed to reduce this problem , but proved unsatisfactory over wide ranges of frequencies. It was discovered that the addition of a second grid, located between the control grid and the plate and called a
screen grid could solve these problems. A positive voltage slightly lower than the plate voltage was applied to it, and the screen grid was bypassed to ground with a capacitor. This arrangement decoupled the anode and the first grid, completely eliminating the oscillation problem. This two-grid tube is called a
tetrode, meaning four active electrodes.
However, the tetrode had a problem too: the positive voltage on the second grid accelerated the electrons, causing them to strike the anode hard enough to knock out secondary electrons. These could then be captured by the second grid, reducing the plate current and the amplification of the circuit. This effect was sometimes called "tetrode kink". Again the solution was to add another grid, called a suppressor grid. This third grid was biased at either ground or cathode voltage and its negative voltage electrostatically suppressed the secondary electrons by repelling them back toward the anode. This three-grid tube is called a pentode, meaning five electrodes.
Other variations
Frequency Conversion can be accomplished by many different methods in
superheterodyne receivers. Tubes with 5 grids, called pentagrid converters, were generally used although alternative such as using a combination of a
triode with a hexode were also used, even octodes have been used for frequency conversion The additional grids are either control grids, with different signals applied to each one, or screen grids. In many designs a special grid acted as a second 'leaky' plate to provide a built-in oscillator, which then coupled this signal with the incoming radio signal. These signals create a single, combined effect on the plate current of the tube circuit. The heptode, or pentagrid converter, was the most common of these. 6BE6 is an example of a heptode .
It was common practice almost everywhere in the world to combine more than one function, or more than one set of elements in the bulb of a single tube. The only constraint was where patents, and other licencing considerations required the use of multiple tubes. See British Valve Association
The RCA Type 55 for example was a double diode triode used as a detector, AVC rectifier and audio preamp in early AC powered radios. The same set of tubes often included the 53 Dual Triode Audio Output. A German firm actually built a multi-section tube with the coupling components inside the envelope. In that case the cost of individually sealing the parts in separate glass tubing to protect them from exposure to the vacuum ended up increasing the final cost.
Another early type of multi-section tube, the
6SN7, is a "dual triode" which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less.
Currently the world's most popular vacuum tube is the
12AX7, with estimated annual worldwide sales of greater than 2 million units. The 12AX7 is a dual high-gain triode widely used in
guitar amplifiers, audio preamps, and instruments.
The invention of the 9 pin miniature tube base, besides allowing the 12AX7 Family also allowed many other multi section tubes, such as the 6GH8 triode pentode which along with a host of simalar tubes was quite popular in
television receivers. Some color TV sets even used exotic types like the 6JH8 which had two plates and beam deflection electrodes . Vacuum tubes used like this were designed for demodulation of synchronous signals, an example of which is color demodulation for television receivers.
The desire to include many functions in one envelope resulted in the General Electric
Compactron A typical unit, the 6AG11 Compactron tube contained two triodes and two diodes, but many in the series had triple triodes.
An early example of multiple devices in one envelope was the Loewe 3NF. this device had 3 triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver.
As the Loewe set had only one tubeholder, it was able to substantially undercut the competition since, in Germany, state tax was levied by the number of tubeholders.
Loewe were to also offer the 2NF and the WG38 .
The
beam power tube is usually a tetrode with the addition of beam-forming electrodes, which take the place of the suppressor grid. These angled plates focus the electron stream onto certain spots on the anode which can withstand the heat generated by the impact of massive numbers of electrons, while also providing pentode behavior. The positioning of the elements in a beam power tube uses a design called "critical-distance geometry", which minimizes the "tetrode kink", plate-grid capacitance, screen-grid current, and secondary emission effects from the anode, thus increasing power conversion efficiency. The control grid and screen grid are also wound with the same pitch, or number of wires per inch.
Aligning the grid wires also helps to reduce screen current, which represents wasted energy. This design helps to overcome some of the practical barriers to designing high power, high efficiency power tubes.
6L6 was the first popular beam power tube, introduced by
RCA in 1936. Corresponding tubes in Europe were the
KT66, KT77 and KT88 by GEC .
Variations of the 6L6 design are still widely used in guitar amplifiers, making it one of the longest lived electronic device families in history. Similar design strategies are used in the construction of large ceramic power tetrodes used in radio transmitters.
Special-purpose tubes
Some special-purpose tubes are intentionally constructed with various gases in the envelope. For instance, voltage regulator tubes contain various inert gases such as
argon,
helium or
neon, and take advantage of the fact that these gases will ionize at predictable voltages. The
thyratron is a special-purpose tube filled with low-pressure gas, for use as a high-speed electronic switch.
Tubes usually have glass envelopes, but metal, fused quartz , and
ceramic are possible choices. The first version of the 6L6 used a metal envelope sealed with glass beads, later a glass disk fused to the metal was used. Metal and ceramic are used almost exclusively for power tubes above 2 kW dissipation. The
nuvistor is a tiny tube made only of metal and ceramic. In some power tubes, the metal envelope is also the anode. 4CX800A is an external anode tube of this sort. Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation. Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac 8974, a water-cooled tetrode capable of dissipating 1.5 megawatts. . A pair of 8974s is capable of producing 2 megawatts of audio power. The 8974 is used only in exotic military and commercial radio-frequency installations.
Reliability
The chief reliability problem of a tube is that the filament or cathode is slowly "poisoned" by atoms from other elements in the tube, which damage its ability to emit electrons. Trapped gases or slow gas leaks can also damage the cathode or cause plate-current runaway due to ionization of free gas molecules.
Vacuum hardness and proper selection of construction materials are the major influences on tube lifetime. Depending on the material, temperature and construction, the surface material of the cathode may also diffuse onto other elements. The resistive filaments that heat the cathodes may burn out as lamp filaments do, but usually not so quickly as they need not be so hot.
Another important reliability problem is that the tube fails when air leaks into the tube. Usually
oxygen in the air reacts chemically with the hot filament or cathode, quickly ruining it. Designers therefore worked hard to develop tube designs that sealed reliably. This was why most tubes were constructed of glass. Metal alloys 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, and pass wires through the glass to the electrodes.
When a vacuum tube is overloaded or operated past its design dissipation, its anode may glow red. In consumer equipment, a glowing plate is universally a sign of an overloaded tube and must be corrected immediately. However, some large transmitting tubes are designed to operate with their anodes at red, orange or in rare cases, white heat.
Vacuum
It is very important that the vacuum inside the envelope be as perfect, or "hard", as possible. Any gas atoms remaining will be ionized at operating voltages, and will conduct electricity between the elements in an uncontrolled manner. This can lead to erratic operation or even catastrophic destruction of the tube and associated circuitry. Unabsorbed free air sometimes ionizes and becomes visible as a pink-purple
glow discharge between the tube elements.
To prevent any remaining gases from remaining in a free state in the tube, modern tubes are constructed with "
getters", which are usually small, circular troughs filled with metals that oxidize quickly, with barium being the most common. While the tube envelope is being evacuated, the internal parts except the getter are heated by
RF induction heating to extract any remaining gases from the metal. The tube is then sealed and the getter is heated to a high temperature, again by Radio frequency induction heating causing the material to evaporate, absorbing/reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside of the envelope of the tube. The getter continues to absorb any gas molecules that leak into the tube during its working life. If a tube develops a crack in the envelope, this deposit turns a white color when it reacts with atmospheric
oxygen. Large transmitting and specialized tubes often use more exotic getters. Early gettered tubes used phosphorous based getters and these tubes are easily identifiable as the phosphorous leaves a characteristic orange deposit on the glass. The use of Phosphorous was short lived and was quickly replaced by the superior barium getters. Unlike the barium betters, the phosphorous did not absorb any further gasses once it had fired.
British Broadcasting Corporation research
Mazda of the UK produced a range of tubes for use in AC powered domestic receivers and other general purposes in around 1935 . The British Broadcasting Corporation used to maintain scrupulous records of equipment maintenence including the achieved life of all tubes. Their records show that a Mazda AC/HL was removed from its equipment having achieved over 250,000 hours of service. When tested, the tube performed to the manufacturer's specification. The BBC did not claim any record for this as this order of longevity of life was typical for this range of tubes. Repair shops stocked up on spares to meet the anticipated demand for replacement tubes, but few were ever required. Any AC/ series tube encountered today is most likely unused .
Transmitting tubes
Large transmitting tubes have tungsten filaments containing a small trace of
thorium. A thin layer of thorium atoms forms on the outside of the wire when heated, serving as an efficient source of electrons. The thorium slowly evaporates from the wire surface, while new thorium atoms
diffuse to the surface to replace them. Such thoriated tungsten cathodes routinely deliver lifetimes in the tens of thousands of hours. The claimed record is held by an Eimac power tetrode used in a Los Angeles radio station's transmitter, which was removed from service after 80,000 hours of uneventful operation. Transmitting tubes are claimed to survive lightning strikes more often than transistor transmitters do.
Receiving tubes
Cathodes in small "receiving" tubes are coated with a mixture of
barium oxide and strontium oxide, sometimes with addition of
calcium oxide or aluminium oxide. An electric heater is inserted into the cathode sleeve, and insulated from it electrically. This complex construction causes barium and strontium atoms to diffuse to the surface of the cathode when heated to about 780 degrees Celsius, thus emitting electrons.
'Computer' vacuum tubes
'Colossus'
Colossus's designer, Dr Tommy Flowers, had a theory that most of the unreliability was caused during power down and power up . Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators . The only time it was switched off was for conversion to the Colussus Mk2 and the addition of another 500 or so tubes. Another 9 Colossi Mk2 were built, and all 10 machines ran with a surprising degree of reliability. The only problem was that the 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year - nearly all of it for the tube heaters.
Whirlwind
To meet the unique reliability requirements of the early digital computer Whirlwind, it was found necessary to build special "computer vacuum tubes" with extended cathode life. The problem of short lifetime was traced to evaporation of
silicon, used in the tungsten alloy to make the wire easier to draw. Elimination of the silicon from the heater wire alloy allowed production of tubes that were reliable enough for the Whirlwind project. The tubes developed for Whirlwind later found their way into the giant
SAGE air-defense computer system. High-purity
nickel tubing and cathode coatings free of materials that can poison emission also contribute to long cathode life. The first such "computer tube" was Sylvania's 7AK7 of 1948. By the late 1950s it was routine for special-quality small-signal tubes to last for hundreds of thousands of hours rather than thousands, if operated conservatively. This reliability made mid-cable amplifiers in submarine cables possible.
World War II
Near the end of
World War II, to make radios more rugged, some aircraft and army radios began to integrate the tube envelopes into the radio's cast
aluminum or
zinc chassis. The radio became just a printed circuit with non-tube components, soldered to the chassis that contained all the tubes. Another WWII idea was to make very small and rugged glass tubes, originally for use in radio-frequency metal detectors built into
artillery shells. These
proximity fuzes made artillery more effective. Tiny tubes were later known as "subminiature" types. They were widely used in 1950s military and aviation electronics.
Applications
Tubes were ubiquitous in the early generations of electronic devices, such as
radios,
televisions, and early
computers such as the
Colossus which used 2000 tubes, the
ENIAC which used nearly 18,000 tubes, and the IBM 700 series. Vacuum tubes inherently have higher resistance to the
electromagnetic pulse effect of
nuclear explosions. This property kept them in use for certain
military applications long after transistors had replaced them elsewhere. Vacuum tubes are still used for very high-powered applications such as
microwave ovens, industrial radio-frequency heating, and power amplification for broadcasting.
Tubes are also considered by many people in the
audiophile, professional audio, and musician communities to have superior audio characteristics over
transistor electronics, due to their warmer, more natural tone. There are many companies which still make specialized audio hardware featuring tube technology.
Tubes' characteristic sound when overloaded is widely used in
electric guitar amplification, and has defined the sound of some genres of music, including classic rock and rhythm and blues. In this regard, tube amplifiers are typically desired for the warmth and natural compression they can add to an input signal.
Cooling
All vacuum tubes produce heat when in operation; this is one of the major inherent disadvantages of using vacuum tubes over solid state components. Depending on the power dissipation, various methods of cooling may be used.
For low power dissipation devices, natural air circulation or
convection may be sufficient to keep the tubes from overheating. For larger power dissipation, forced-air cooling may be required.
High power tubes in large transmitters are cooled by distilled water .
Other vacuum tube devices
A vast array of devices were built during the 1920-1960 period using vacuum-tube techniques. Most such tubes were rendered obsolete by semiconductors; some techniques for integrating multiple devices in a single module, sharing the same glass envelope have been discussed above, such as the Loewe 3NF. Vacuum-tube electronic devices still in common use include the
magnetron,
klystron,
photomultiplier,
x-ray tube and
cathode ray tube. The magnetron is the type of tube used in all
microwave ovens. In spite of the advancing state of the art in power semiconductor technology, the vacuum tube still has reliability and cost advantages for high-frequency RF power generation. Photomultipliers are still the most sensitive detectors of light. Many
televisions,
oscilloscopes and computer monitors still use cathode ray tubes, though
flat panel displays are becoming more popular as prices drop.
The fluorescent displays commonly used on VCRs and automotive dashboards are actually vacuum tubes, using
phosphor-coated anodes to form the display characters, and a heated filamentary cathode as an electron source. These devices are properly called "VFDs", or
Vacuum Fluorescent Displays. Because the filaments are in view, they must be operated at temperatures where the filament does not show a glow. Their big advantage is that it is relatively easy to create bespoke designs with all the legends required for a specific task. These devices are often found in automotive applications where their high brightness allows reading the display in daylight.
Some tubes, like
magnetrons,
traveling wave tubes, carcinotrons, and
klystrons, combine magnetic and electrostatic effects. These are efficient RF producers and still find use in
radar,
microwave ovens and industrial heating.
Gyrotrons or vacuum masers, used to generate high power millimetre band waves, are magnetic vacuum tubes in which a small relativistic effect, due to the high voltage, is used for bunching the electrons.
Free electron lasers, used to generate high power coherent light and perhaps even
X rays, are highly relativistic vacuum tubes driven by high energy particle accelerators.
Particle accelerators can be considered vacuum tubes that work backward, the electric fields driving the electrons, or other changed particles. In this respect, a cathode ray tube is a particle accelerator.
A tube in which electrons move through a vacuum within a gas-tight envelope is generically called an
electron tube.
Vacuum tube can also literally mean a tube with a
vacuum. It is e.g. used for demonstration of, and experiments with,
free-fall.
Field emitter vacuum tubes
In the early years of the 21st century there has been renewed interest in vacuum tubes, this time in the form of
integrated circuits. The most common design uses a cold cathode field emitter, with electrons emitted from a number of sharp nano-scale tips formed on the surface of a metal cathode.
Their advantages include greatly enhanced robustness combined with the ability to provide high power outputs at low power consumptions. Operating on the same principles as traditional tubes, prototype device cathodes have been constructed with emitter tips formed using nanotubes, and by etching electrodes as hinged flaps that are stood upright by a
magnetic field.
Such integrated microtubes may find application in
microwave devices including
mobile phones, for
Bluetooth and
Wi-Fi transmission, in
radar and for
satellite communication. Presently they are being studied for possible application to flat-panel display construction.
Vacuum tube solar heaters
The term
vacuum tube has recently been used to refer to the tubular elements of
solar panels used for heating water.
Vacuum tube solar heaters are becoming increasingly popular.
See also
Patents
- Electrical indicator
- Incandescent electric light
- Instrument for converting alternating electric currents into continuous currents
- Device for amplifying feeble electrical currents
- De Forest's Audion
- Television system
External links and references
Books and articles
- Spangenburg K.R., Vacuum Tubes, McGraw-Hill, 1948
- Millman, J. & Seely, S. Electronics, 2nd ed. McGraw-Hill, 1951.
- Shiers, George, "The First Electron Tube", Scientific American, March 1969, p. 104.
- Tyne, Gerald, Saga of The Vacuum Tube, Ziff Publishing, 1943, , pp. 30-83.
- Stokes, John, 70 Years of Radio Tubes and Valves, Vestal Press, NY, 1982, pp. 3-9.
- Thrower, Keith, History of The British Radio Valve to 1940, MMA International, 1982, pp 9-13.
- Eastman, Austin V., Fundamentals of Vacuum Tubes, McGraw-Hill, 1949
- Philips Technical Library. A range of books published in the UK in the 1940s and 50s by Cleaver Hume Press on all aspects of the design and application of vacuum tubes. They were originally published in Dutch in Holland. French and German editions were probably also published.
- RCA "Radiotron Designer's Handbook" 1953 Contains very useful chapters on the design and application of receiving tubes.
- RCA "Receiving Tube Manual" RC15, RC26 Issued every two years, contains details of the technical specs of the tubes that RCA sold at the time.
- Wireless World. "Radio Designer's Handbook". UK reprint of the above.
External links
- "" - John Harper's webpage about thermionic emission and vacuum tube theory, using introductory college-level mathematics.
- "". Fleming discovers the thermionic valve, or 'diode'.
- " : Is There An Audible Difference?" - 1972 AES paper on audible differences in sound quality between vacuum tubes and transistors.
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