experimental bulbs The 19th century saw increasing research with evacuated tubes, such as the
Geissler and
Crookes tubes. 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 was originally reported in 1873 by
Frederick Guthrie, it was Thomas Edison's apparently independent discovery of the phenomenon in 1883, referred to as the
Edison effect, 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 of radio signals, as an improvement over the
magnetic detector. Amplification by vacuum tube became practical only with
Lee de Forest's 1907 invention of the three-terminal "
audion" 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 systems, and introduced a far superior and versatile technology for use in radio transmitters and receivers.
Diodes first 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 device that extracts information from a modulated radio frequency. Marconi had developed a
magnetic detector, 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, 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 (
anode) when the plate was at a positive voltage with respect to the cathode. Fleming patented these tubes for the Marconi company in the UK, filing in November 1904, with patent 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 not more sensitive than a properly working
crystal detector, the Fleming valve did not need the fiddly adjustment the whisker of a crystal detector required, and was not susceptible to becoming bumped out of the optimum position by vibration or movement, which was particularly advantageous for use on a moving ship.
Triodes Audion, invented in 1906 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 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. By placing an additional electrode between the filament (
cathode) and
plate (anode), he discovered the ability of the resulting device to amplify signals. As the voltage applied to the
control grid (or simply "grid") was lowered below the cathode voltage, the current flowing from the filament to the plate decreased. The negative electrostatic field created by the grid in the vicinity of the cathode inhibited the passage of emitted electrons and reduced the current to the plate. With the grid negative relative to the cathode, no direct current could pass from the cathode to the grid. Consequently, a change of voltage applied to the grid, requiring no power input as no current flowed, 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. Pliotron, at the
Science History Institute De Forest's original device was made with conventional vacuum technology. The vacuum was not a "hard vacuum" but rather left a very small amount of residual gas. The physics behind the device's operation was not fully understood. The residual gas would cause a blue glow due to visible ionization when the plate voltage exceeded about 60 volts. In 1912, de Forest and
John Stone Stone brought the Audion for demonstration to AT&T's engineering department, where
Harold D. Arnold realized that the blue glow was caused by ionized gas. He recommended that AT&T purchase the patent. Arnold developed high-vacuum tubes which operated at high plate voltages without a blue glow; they were tested in the summer of 1913 on AT&T's long-distance network. Finnish inventor
Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his
sound-on-film process in Berlin, Germany. Tigerstedt's innovation was to make the electrodes concentric cylinders with the cathode at the centre, thus greatly increasing the collection of emitted electrons at the anode.
Irving Langmuir at the
General Electric research laboratory (
Schenectady, New York) had improved
Wolfgang Gaede's
high-vacuum diffusion pump and used it to settle the question of thermionic emission and conduction in a vacuum. Consequently, General Electric started producing hard vacuum triodes (which were branded Pliotrons) in 1915. 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 British 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. By comparison the later
bipolar junction transistor uses a small current to control a larger 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 main parameters of a vacuum tube, the other two being its gain μ and plate resistance or ; these parameters are related by the Van der Bijl equation: g_m = {\mu \over R_p} The plate current of the triode was not accurately proportional to the grid voltage, i.e. the operating characteristic was non-linear, causing 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 curve was 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 nearly linear variation of plate current in response to positive and negative variation of the input voltage around that point. This concept is called
grid bias. Many early radio sets had a third battery called the "C battery" (unrelated to the present-day
C cell, a format). The C battery's positive terminal was connected to the cathode of the tubes ("ground" in most circuits) and the negative terminal supplied 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. Current flow through the resistor 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 with
tuned rather than resistive loads in radio-frequency transmitters and receivers, it was found that tuned amplification stages had a tendency to oscillate unless their gain was very limited, due to the parasitic capacitance, termed
Miller capacitance, between the plate (the amplifier's output) and the control grid (the amplifier's input). Eventually the technique of
neutralization was developed whereby the RF transformer connected to the plate (anode) included an additional winding in the opposite phase, connected to the grid through a small capacitor. When properly adjusted this cancelled the Miller capacitance. This technique was successfully employed in the
Neutrodyne radio during the 1920s. Neutralization was dependent upon the frequency; it required careful adjustment and did not work over a wide range of frequencies.
Tetrodes and pentodes symbol. From top to bottom: plate (anode), screen grid, control grid, cathode, heater (filament). 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 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. This arrangement substantially decouples the plate and the
control grid, 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 marketed by late 1927. 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 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 or
beam power tube, discussed below.
Multifunction and multisection tubes contains five grids between the cathode and the plate (anode).
Superheterodyne receivers require a
local oscillator and
mixer, combined in the function of a single
pentagrid converter tube. Various alternatives such as using a combination of a
triode with a
hexode and even an
octode have been used for this purpose. The additional grids include
control grids (at a low potential) and
screen grids (at a high voltage). Many designs use such a screen grid as an additional anode to provide feedback for the oscillator function, whose current adds to that of the incoming radio frequency signal. The pentagrid converter thus became widely used in AM receivers, including the miniature tube version of the "
All American Five". Octodes, such as the 7A8, were rarely used in the United States, but much more common in Europe, particularly in battery operated radios where the lower power consumption was an advantage. To further reduce the cost and complexity of radio equipment, two separate structures (triode and pentode for instance) can be combined in the bulb of a single
multisection tube. An early example is the
Loewe 3NF. This 1920s device has three triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver. As the Loewe set had only one tube socket, it was able to substantially undercut the competition, since, in Germany, state tax was levied by the number of sockets. However, reliability was compromised, and production costs for the tube were much greater. In a sense, these were akin to integrated circuits. In the United States, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also has a single tube socket, but because it uses a four-pin base, the additional element connections are made on a "mezzanine" platform at the top of the tube base. By 1940 multisection tubes had become commonplace. There were constraints, however, due to patents and other licensing considerations (see
British Radio Valve Manufacturers' Association). Constraints due to the number of external pins (leads) often forced the functions to share some of those external connections such as their cathode connections (in addition to the heater connection). The RCA Type 55 is a
double diode triode used as a detector,
automatic gain control rectifier and audio
preamplifier in early AC powered radios. These sets often include the 53 Dual Triode Audio Output. Another early type of multi-section tube, the
6SN7, is a "dual triode" which performs the functions of two triode tubes while taking up half as much space and costing less. The
12AX7 is a dual "high mu" (high voltage gain) triode in a miniature enclosure, and became widely used in audio signal amplifiers, instruments, and
guitar amplifiers. The introduction of the miniature tube base (see below) which can have 9 pins, more than previously available, allowed other multi-section tubes to be introduced, such as the
6GH8/ECF82 triode-pentode, quite popular in television receivers. The desire to include even more functions in one envelope resulted in the General Electric
Compactron which has 12 pins. A typical example, the 6AG11, contains two triodes and two diodes. Some otherwise conventional tubes do not fall into standard categories; the 6AR8, 6JH8 and 6ME8 have several common grids, followed by a pair of
beam deflection electrodes which deflected the current towards either of two anodes. They were sometimes known as the 'sheet beam' tubes and used in some color TV sets for
color demodulation. The similar 7360 was popular as a balanced
SSB (de)modulator.
Beam power tubes designed for radio frequency use. The tube plugs in to a socket that creates an air-tight seal around the outer periphery. A blower and duct work in the chassis force air through the tube's fins to carry away heat. This type of tube is sometimes referred to as a "doorknob" tube, owing to its shape and size. A
beam tetrode (or "beam power tube") forms the electron stream from the cathode into multiple
partially collimated beams to produce a low potential
space charge 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 tubes such as
discharge tubes and
cold cathode tubes are not
hard vacuum tubes, though are always filled with gas at less than sea-level atmospheric pressure. Types such as the
voltage-regulator tube and
thyratron resemble hard vacuum tubes and fit in sockets designed for vacuum tubes. Their distinctive orange, red, or purple glow during operation indicates the presence of gas; electrons flowing in a vacuum do not produce light within that region. These types may still be referred to as "electron tubes" as they do perform electronic functions. High-power rectifiers use
mercury vapor to achieve a lower forward voltage drop than high-vacuum tubes.
Miniature tubes , is high and in diameter. 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 allowed the design of a much smaller tube (typically about 20mm in diameter), known as the miniature tube; tubes with standard seven and nine-pin (noval) bases were made. Tubes with different
bases of about the same size were also introduced. Making tubes smaller reduced the voltage where they could safely operate, and also reduced the filament power required. Miniature tubes became predominant in consumer applications such as radio receivers and hi-fi amplifiers. However, the larger styles continued to be required for tubes dissipating more power, such as higher-power
rectifiers, in higher-power audio output stages and as RF power transmitting tubes.
Sub-miniature tubes " triode, c. by Tubes requiring little power, such as hearing-aid amplifiers, could be much smaller than those dissipating significant heat; sub-miniature tubes with a size roughly that of half a cigarette were used. These tubes tended to be long-lived due to their low power; they did not have pins plugging into a socket for easy replacement, but were soldered in place. For higher radio frequencies larger tubes introduced frequency-limiting
stray capacitance that could be reduced by reducing the size. Small tubes for high frequencies included the "
acorn tube" type (named for its shape and size), and the metal-cased RCA
nuvistor type from 1959, about the size of a
thimble. The nuvistor was developed to compete with transistors, and could operate at higher frequencies than early transistors, and also bigger tubes, due to the nuvistor's small size (compared to signal wavelength). Nuvistors were used in aircraft radio transceivers,
UHF television tuners, and some HiFi FM radio tuners (Sansui 500A) until replaced by newer 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'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 roughlye.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 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 computer used 17,468 vacuum tubes and consumed of power. Vacuum tubes used as switches made electronic computing possible for the first time, but the cost and relatively short
mean time to failure of tubes were limiting factors. "The common wisdom was that valveswhich, like light bulbs, contained a hot glowing filamentcould 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". In 1934 Flowers built a successful experimental installation using over 3,000 tubes in small independent modules; when a tube failed, it was possible to switch off one module and keep the others going, thereby reducing the risk of another tube failure being caused; this installation was accepted by the
Post Office (who operated telephone exchanges). Flowers was also a pioneer of using tubes as very fast (compared to electromechanical devices)
electronic switches. Later work confirmed that tube unreliability was not as serious an issue as generally believed; the 1946
ENIAC, with over 17,000 tubes, had a tube failure (which took 15 minutes to locate) on average every two days. The quality of the tubes was a factor, and the diversion of skilled people during the Second World War lowered the general quality of tubes. During the war Colossus was instrumental in breaking German codes. After the war, development continued with tube-based computers including, military computers
ENIAC and
Whirlwind, the
Ferranti Mark 1 (one of the first commercially available electronic computers), and
UNIVAC I, also available commercially. Advances using subminiature tubes included the Jaincomp series of machines produced by the Jacobs Instrument Company of Bethesda, Maryland. Models such as its Jaincomp-B employed just 300 such tubes in a desktop-sized unit that offered performance to rival many of the then room-sized machines.
Colossus at
Bletchley Park, England Colossus I and its successor Colossus II (Mk2) were designed by
Tommy Flowers and built by the
General Post Office for
Bletchley Park (BP) during World War II to substantially speed up the task of breaking the German high level
Lorenz encryption. 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. 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. and
stress testing the tubes during offline maintenance periods to bring on early failure of weak units. Another commonly used computer tube was the
5965 double triode. This, according to a memorandom from
MIT for
Project Whirwind, was developed for
IBM by
General Electric, primarily for use in the
IBM 701 calculator, and was designated as a general-purpose triode tube. Tubes using a
European designation standard used letters to indicate heater voltage and construction, followed by an indicator of base type and series number; e.g. the
ECC82 was a 6.3V (E) double triode (CC) with a noval base (8x), as was the
ECC83. Special quality tubes placed the number immediately after the voltage letter; e.g. the European version of the 5965 was labeled E180CC. 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 and also made mid-cable amplifiers in
submarine cables possible. == Heat generation and cooling ==