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Dubulttriode darbiibaa.jpg
ECC83, a dual triode used in 1960-era audio equipment
The 3CX1500A7, a modern 1.5 kW power triode used in radio transmitters. The cylindrical structure is a heat sink attached to the plate, through which air is blown during operation.
Examples of low power triodes from 1918 (left) to miniature tubes of the 1960s (right) Triody var.jpg
Examples of low power triodes from 1918 (left) to miniature tubes of the 1960s (right)

A triode is an electronic amplifying vacuum tube (or valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode (Fleming valve), the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer[ vague ] the pleasantly (warm) distorted sound of tube-based electronics.[ citation needed ]


The name "triode" was coined by British physicist William Eccles [1] [2] some time around 1920, derived from the Greek τρίοδος, tríodos, from tri- (three) and hodós (road, way), originally meaning the place where three roads meet.


Precursor devices

De Forest Audion tube from 1908, the first triode. The flat plate is visible on the top, with the zigzag wire grid under it. The filament was originally present under the grid but was burnt out. Triode tube 1906.jpg
De Forest Audion tube from 1908, the first triode. The flat plate is visible on the top, with the zigzag wire grid under it. The filament was originally present under the grid but was burnt out.
Lieben-Reisz tube, another primitive triode developed the same time as the Audion by Robert von Lieben Lieben-Reisz vacuum tube.jpg
Lieben-Reisz tube, another primitive triode developed the same time as the Audion by Robert von Lieben

Before thermionic valves were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. [3]

The first vacuum tube used in radio [4] [5] was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for radio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament (cathode) and a plate (anode).


Triodes came about in 1906 when American engineer Lee De Forest [6] and Austrian physicist Robert von Lieben [7] independently patented tubes that added a third electrode, a control grid, between the filament and plate to control current. [8] [9] Von Lieben's partially-evacuated three-element tube, patented in March 1906, contained a trace of mercury vapor and was intended to amplify weak telephone signals. [10] [11] [12] [7] Starting in October 1906 [8] De Forest patented a number of three-element tube designs by adding an electrode to the diode, which he called Audions, intended to be used as radio detectors. [13] [6] The one which became the design of the triode, in which the grid was located between the filament and plate, was patented January 29, 1907. [14] [6] [15] Like the von Lieben vacuum tube, De Forest's Audions were incompletely evacuated and contained some gas at low pressure. [16] [17] von Lieben's vacuum tube did not see much development due to his death seven years after its invention, shortly before the outbreak of the First World War. [18]

De Forest's Audion did not see much use until its ability to amplify was recognized around 1912 by several researchers, [17] [19] who used it to build the first successful amplifying radio receivers and electronic oscillators. [20] [21] The many uses for amplification motivated its rapid development. By 1913 improved versions with higher vacuum were developed by Harold Arnold at American Telephone and Telegraph Company, which had purchased the rights to the Audion from De Forest, and Irving Langmuir at General Electric, who named his tube the "Pliotron", [17] [19] These were the first vacuum tube triodes. [16] The name "triode" appeared later, when it became necessary to distinguish it from other kinds of vacuum tubes with more or fewer elements (e.g. diodes, tetrodes, pentodes, etc.). There were lengthy lawsuits between De Forest and von Lieben, and De Forest and the Marconi Company, who represented John Ambrose Fleming, the inventor of the diode. [22] [ citation needed ].

Wider adoption

The discovery of the triode's amplifying ability in 1912 revolutionized electrical technology, creating the new field of electronics , the technology of active (amplifying) electrical devices. The triode was immediately applied to many areas of communication. Triode "continuous wave" radio transmitters replaced the cumbersome inefficient "damped wave" spark gap transmitters, allowing the transmission of sound by amplitude modulation (AM). Amplifying triode radio receivers, which had the power to drive loudspeakers, replaced weak crystal radios, which had to be listened to with earphones, allowing families to listen together. This resulted in the evolution of radio from a commercial message service to the first mass communication medium, with the beginning of radio broadcasting around 1920. Triodes made transcontinental telephone service possible. Vacuum tube triode repeaters, invented at Bell Telephone after its purchase of the Audion rights, allowed telephone calls to travel beyond the unamplified limit of about 800 miles. The opening by Bell of the first transcontinental telephone line was celebrated 3 years later, on January 25, 1915. Other inventions made possible by the triode were television, public address systems, electric phonographs, and talking motion pictures.

The triode served as the technological base from which later vacuum tubes developed, such as the tetrode (Walter Schottky, 1916) and pentode (Gilles Holst and Bernardus Dominicus Hubertus Tellegen, 1926), which remedied some of the shortcomings of the triode detailed below.

The triode was very widely used in consumer electronics such as radios, televisions, and audio systems until it was replaced in the 1960s by the transistor, invented in 1947, which brought the "vacuum tube era" introduced by the triode to a close. Today triodes are mostly used in high-power applications for which solid state semiconductor devices are unsuitable, such as radio transmitters and industrial heating equipment. However, more recently the triode and other vacuum tube devices have been experiencing a resurgence and comeback in high fidelity audio and musical equipment. They also remain in use as vacuum fluorescent displays (VFDs), which come in a variety of implementations but all are essentially triode devices.


Structure of a modern low-power triode vacuum tube. The glass and outer electrodes are shown partly cut away to reveal the construction. Triode-english-text.svg
Structure of a modern low-power triode vacuum tube. The glass and outer electrodes are shown partly cut away to reveal the construction.
Schematic symbol used in circuit diagrams for a triode, showing symbols for electrodes. Triode schematic labeled.svg
Schematic symbol used in circuit diagrams for a triode, showing symbols for electrodes.

All triodes have a hot cathode electrode heated by a filament, which releases electrons, and a flat metal plate electrode to which the electrons are attracted, with a grid consisting of a screen of wires between them to control the current. These are sealed inside a glass container from which the air has been removed to a high vacuum, about 10−9 atm. Since the filament eventually burns out, the tube has a limited lifetime and is made as a replaceable unit; the electrodes are attached to terminal pins which plug into a socket. The operating lifetime of a triode is about 2000 hours for small tubes and 10,000 hours for power tubes.

Low power triodes

Low power triodes have a concentric construction (see drawing right), with the grid and anode as circular or oval cylinders surrounding the cathode. The cathode is a narrow metal tube down the center. Inside the cathode is a filament called the "heater" consisting of a narrow strip of high resistance tungsten wire, which heats the cathode red-hot (800 - 1000 °C). This type is called an "indirectly heated cathode". The cathode is coated with a mixture of alkaline earth oxides such as calcium and thorium oxide which reduces its work function so it produces more electrons. The grid is constructed of a helix or screen of thin wires surrounding the cathode. The anode is a cylinder or rectangular box of sheet metal surrounding the grid. It is blackened to radiate heat and is often equipped with heat-radiating fins. The electrons travel in a radial direction, from cathode through the grid to the anode. The elements are held in position by mica or ceramic insulators and are supported by stiff wires attached to the base, where the electrodes are brought out to connecting pins. A "getter", a small amount of shiny barium metal evaporated onto the inside of the glass, helps maintain the vacuum by absorbing gas released in the tube over time.

High-power triodes

High-power triodes generally use a filament which serves as the cathode (a directly heated cathode) because the emission coating on indirectly heated cathodes is destroyed by the higher ion bombardment in power tubes. A thoriated tungsten filament is most often used, in which thorium in the tungsten forms a monolayer on the surface which increases electron emission. These generally run at higher temperatures than indirectly heated cathodes. The envelope of the tube is often made of more durable ceramic rather than glass, and all the materials have higher melting points to withstand higher heat levels produced. Tubes with anode power dissipation over several hundred watts are usually actively cooled; the anode, made of heavy copper, projects through the wall of the tube and is attached to a large external finned metal heat sink which is cooled by forced air or water.

Lighthouse tubes

Soviet lighthouse tube 6S5D (6S5D) Scheib3.jpg
Soviet lighthouse tube 6С5Д (6S5D)

A type of low power triode for use at ultrahigh frequencies (UHF), the "lighthouse" tube, has a planar construction to reduce interelectrode capacitance and lead inductance, which gives it the appearance of a "lighthouse". The disk-shaped cathode, grid and plate form planes up the center of the tube - a little like a sandwich with spaces between the layers. The cathode at the bottom is attached to the tube's pins, but the grid and plate are brought out to low inductance terminals on the upper level of the tube: the grid to a metal ring halfway up, and the plate to a metal button at the top. These are one example of "disk seal" design. Smaller examples dispense with the octal pin base shown in the illustration and rely on contact rings for all connections, including heater and D.C. cathode.

As well, high-frequency performance is limited by transit time: the time required for electrons to travel from cathode to anode. Transit time effects are complicated, but one simple effect is input conductance, also known as grid loading. At extreme high frequencies, electrons arriving at the grid may become out of phase with those departing towards the anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic.

Transit time effects are reduced by reduced spacings in the tube. Tubes such as the 416B (a Lighthouse design) and the 7768 (an all-ceramic miniaturised design) are specified for operation to 4 GHz. They feature greatly reduced grid-cathode spacings in the order of 0.1 mm.

These greatly reduced grid spacings also give a much higher amplification factor than conventional axial designs. The 7768 has an amplification factor of 225, compared with 100 for the 6AV6 used in domestic radios and about the maximum possible for an axial design.

Anode-grid capacitance is not especially low in these designs. The 6AV6 anode-grid capacitance is 2 picofarads (pF), the 7768 has a value of 1.7 pF. The close electrode spacing used in microwave tubes increases capacitances, but this increase is offset by their overall reduced dimensions compared to lower-frequency tubes.


Triode with filament and cathode labeled.svg
Triode with separate cathode and filament.
Triode with filament labeled.svg
Triode in which filament serves as cathode.
Triode with cathode labeled.svg
Filament omitted from diagram.
Schematic circuit symbols for triodes. (F) filament, (C) cathode, (G) grid, (P) plate

In the triode, electrons are released into the tube from the metal cathode by heating it, a process called thermionic emission. The cathode is heated red hot by a separate current flowing through a thin metal filament. In some tubes the filament itself is the cathode, while in most tubes there is a separate filament which heats the cathode but is electrically isolated from it. The interior of the tube is well evacuated so that electrons can travel between the cathode and the anode without losing energy in collisions with gas molecules. A positive DC voltage, which can be as low as 20V or up to thousands of volts in some transmitting tubes, is present on the anode. The negative electrons are attracted to the positively charged anode (or "plate"), and flow through the spaces between the grid wires to it, creating a flow of electrons through the tube from cathode to anode.

The magnitude of this current can be controlled by a voltage applied on the grid (relative to the cathode). The grid acts like a gate for the electrons. A more negative voltage on the grid will repel more of the electrons, so fewer get through to the anode, reducing the anode current. A less negative voltage on the grid will allow more electrons from the cathode to reach the anode, increasing the anode current. Therefore, an input AC signal on the grid of a few volts (or less), even at a very high impedance (since essentially no current flows through the grid) can control a much more powerful anode current, resulting in amplification. When used in its linear region, variation in the grid voltage will cause an approximately proportional variation in the anode current; this ratio is called the transconductance. If a suitable load resistance is inserted in the anode circuit, although the transconductance is somewhat lowered, the varying anode current will cause a varying voltage across that resistance which can be much larger than the input voltage variations, resulting in voltage gain.

The triode is a normally "on" device; and current flows to the anode with zero voltage on the grid. The anode current is progressively reduced as the grid is made more negative relative to the cathode. Usually a constant DC voltage ("bias") is applied to the grid along with the varying signal voltage superimposed on it. That bias is required so that the positive peaks of the signal never drive the grid positive with respect to the cathode which would result in grid current and non-linear behaviour. A sufficiently negative voltage on the grid (usually around 3-5 volts in small tubes such as the 6AV6, but as much as –130 volts in early audio power devices such as the '45), will prevent any electrons from getting through to the anode, turning off the anode current. This is called the "cutoff voltage". Since beyond cutoff the anode current ceases to respond to the grid voltage, the voltage on the grid must remain above the cutoff voltage for faithful (linear) amplification as well as not exceeding the cathode voltage.

The triode is somewhat similar in operation to the n-channel JFET; it is normally on, and exhibits progressively lower and lower plate/drain current as the grid/gate is pulled increasingly negative relative to the source/cathode. Cutoff voltage corresponds to the JFET's pinch-off voltage (Vp) or VGS(off); i.e., the voltage point at which output current essentially reaches zero. This similarity is limited, however. The triode's anode current is highly dependent on anode voltage as well as grid voltage, thus limiting the voltage gain. On the other hand the JFET's drain current is virtually unaffected by drain voltage, thus it appears as a constant-current device, similar in action to a tetrode or pentode tube (high dynamic output impedance). Both the JFET and tetrode/pentode valves are thereby capable of much higher voltage gains than the triode which seldom exceeds 100. However the power gain, or the output power obtained from a certain AC input voltage is often of greater interest. When these devices are used as cathode followers (or source followers), they all have a voltage "gain" of just under 1, but with a large current gain.


Although S.G. Brown's Type G Telephone Relay (using a magnetic "earphone" mechanism driving a carbon microphone element) was able to give power amplification and had been in use as early as 1914, it was a purely mechanical device with limited frequency range and fidelity. It was suited only to a limited range of audio frequencies - essentially voice frequencies. [23]

The triode was the first non-mechanical device to provide power gain at audio and radio frequencies, and made radio practical. Triodes are used for amplifier s and oscillator s. Many types are used only at low to moderate frequency and power levels. Large water-cooled triodes may be used as the final amplifier in radio transmitters, with ratings of thousands of watts. Specialized types of triode ("lighthouse" tubes, with low capacitance between elements) provide useful gain at microwave frequencies.

Vacuum tubes are obsolete in mass-marketed consumer electronics, having been overtaken by less expensive transistor-based solid-state devices. However, more recently, vacuum tubes have been making somewhat of a comeback. Triodes continue to be used in certain high-power RF amplifiers and transmitters. While proponents of vacuum tubes claim their superiority in areas such as high-end and professional audio applications, the solid-state MOSFET has similar performance characteristics. [24]


ECC83 triode operating characteristic. TriodeECC83Characteristic1.png
ECC83 triode operating characteristic.

In triode datasheets, characteristics linking the anode current (Ia) to anode voltage (Va) and grid voltage (Vg) are usually given. From here, a circuit designer can choose the operating point of the particular triode. Then the output voltage and amplification of the triode can be evaluated graphically by drawing a load line on the graph.

In the example characteristic shown on the image, suppose we wish to operate it at a quiescent anode voltage Va of 200 V and a grid voltage bias of −1 volt. This implies a quiescent plate (anode) current of 2.2 mA (using the yellow curve on the graph). In a class-A triode amplifier, one might place an anode resistor (connected between the anode and the positive power supply). If we choose Ra = 10000 ohms, the voltage drop on it would be V+ - Va = Ia × Ra = 22. V for the chosen anode current of Ia = 2.2 mA. Thus we require a power supply voltage V+ = 222V in order to obtain Va = 200V on the anode.

Now suppose we impress on the -1V bias voltage a signal of 1V peak-peak, so that the grid voltage varies between -.5V and -1.5V. When Vg=-.5V, the anode current will increase to 3.1mA, lowering the anode voltage to Va = V+ - 10000Ω × 3.1mA = 191V (orange curve). When Vg=-1.5V, the anode current will decrease to 1.4mA, raising the anode voltage to Va = V+ - 10000Ω × 1.4mA = 208V (green curve). Therefore a 1V peak-peak signal on the input (grid) causes an output voltage change of about 17V.

Thus voltage amplification of the signal is obtained. The ratio of these two changes, the voltage amplification factor (or mu) is 17 in this case. It is also possible to use triodes as cathode followers in which there is no voltage amplification but a huge reduction in dynamic impedance; in other words, the current is greatly amplified (as it also is in the common-cathode configuration described above). Amplifying either the voltage or current results in power amplification, the general purpose of an amplifying tube (after all, either the current or voltage alone could be increased just using a transformer, a passive device).

See also

Related Research Articles

Cathode ray Stream of electrons observed in vacuum tubes

Cathode rays are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to electrons emitted from the cathode. They were first observed in 1869 by German physicist Julius Plücker and Johann Wilhelm Hittorf, and were named in 1876 by Eugen Goldstein Kathodenstrahlen, or cathode rays. In 1897, British physicist J. J. Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, which was later named the electron. Cathode-ray tubes (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to render an image on a screen.

Cathode An electrode where reduction take place

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

A vacuum tube, electron tube, valve, or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.

Valve amplifier Type of electronic amplifier

A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers in the 1960s and 1970s. Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications and very high power radio and UHF television transmitters.

Control grid

The control grid is an electrode used in amplifying thermionic valves 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 consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest, who in 1906 added a grid to the Fleming valve to create the first amplifying vacuum tube, the Audion (triode).

A suppressor grid is a wire screen (grid) used in a thermionic valve to suppress secondary emission. It is also called the antidynatron grid, as it reduces or prevents dynatron oscillations. It is located between the screen grid and the plate electrode (anode). The suppressor grid is used in the pentode vacuum tube, so called because it has five concentric electrodes: cathode, control grid, screen grid, suppressor grid, and plate, and also in other tubes with more grids, such as the hexode. The suppressor grid and pentode tube were invented in 1926 by Gilles Holst and Bernard D. H. Tellegen at Phillips Electronics.

Dynatron oscillator Vacuum tube electronic oscillator circuit

In electronics, the dynatron oscillator, invented in 1918 by Albert Hull at General Electric, is an obsolete vacuum tube electronic oscillator circuit which uses a negative resistance characteristic in early tetrode vacuum tubes, caused by a process called secondary emission. It was the first negative resistance vacuum tube oscillator. The dynatron oscillator circuit was used to a limited extent as beat frequency oscillators (BFOs), and local oscillators in vacuum tube radio receivers as well as in scientific and test equipment from the 1920s to the 1940s but became obsolete around World War 2 due to the variability of secondary emission in tubes.

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 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. Beam tetrodes are usually used for power amplification, from audio frequency to radio frequency. The beam tetrode produces greater output power than a triode or pentode with the same anode supply voltage. The first beam tetrode marketed was the Marconi N40, introduced in 1935. Beam tetrodes manufactured and used in the 21st century include the 4CX250B, KT66 and variants of the 6L6.


A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube, which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode consists of an evacuated glass envelope containing five electrodes in this order: a filament for indirectly heating a cathode, a control grid, a screen grid, a suppressor grid, and a plate (anode). The pentode was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted by the plate from reaching the screen grid, which caused instability and parasitic oscillations in the tetrode. The pentode is closely related to the beam tetrode. Pentodes were widely used in industrial and consumer electronic equipment such as radios and televisions until the 1960s, when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. The obsolete consumer tubes are still used in a few legacy and specialty vacuum tube audio devices.

Grid-leak detector

A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube. Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.

Single-ended triode Vacuum tube electronic amplifier that uses a single triode to produce an output

A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted signals added and the distortion components subtracted. Single-ended amplifiers normally operate in Class A; push-pull amplifiers can also operate in Classes AB or B without excessive net distortion, due to cancellation.

In Europe, the principal method of numbering vacuum tubes was the nomenclature used by the Philips company and its subsidiaries Mullard in the UK, Valvo(deit) in Germany, Radiotechnique (Miniwatt-Dario brand) in France, and Amperex in the United States, from 1934 on. Adhering manufacturers include AEG (de), CdL (1921, French Mazda brand), CIFTE (fr, Mazda-Belvu brand), EdiSwan (British Mazda brand), Lorenz (de), MBLE(frnl), RCA (us), RFT(desv) (de), Siemens (de), Telefunken (de), Tesla (cz), Toshiba (ja), Tungsram (hu), and Unitra. This system allocated meaningful codes to tubes based on their function and became the starting point for the Pro Electron naming scheme for active devices.

In electronics, cut-off is a state of negligible conduction that is a property of several types of electronic components when a control parameter, is lowered or increased past a value. The transition from normal conduction to cut-off can be more or less sharp, depending on the type of device considered, and also the speed of this transition varies considerably.

Valve RF amplifier Device for electrically amplifying the power of an electrical radio frequency signal

A valve RF amplifier or tube amplifier (U.S.) is a device for electrically amplifying the power of an electrical radio frequency signal.

Fleming valve

The Fleming valve, also called the Fleming oscillation valve, was a thermionic valve or vacuum tube invented in 1904 by English physicist John Ambrose Fleming as a detector for early radio receivers used in electromagnetic wireless telegraphy. It was the first practical vacuum tube and the first thermionic diode, a vacuum tube whose purpose is to conduct current in one direction and block current flowing in the opposite direction. The thermionic diode was later widely used as a rectifier — a device which converts alternating current (AC) into direct current (DC) — in the power supplies of a wide range of electronic devices, until beginning to be replaced by the selenium rectifier in the early 1930s and almost completely replaced by the semiconductor diode in the 1960s. The Fleming valve was the forerunner of all vacuum tubes, which dominated electronics for 50 years. The IEEE has described it as "one of the most important developments in the history of electronics", and it is on the List of IEEE Milestones for electrical engineering.

In the years 1942-1944, the Radio Manufacturers Association used a descriptive nomenclature system for industrial, transmitting, and special-purpose vacuum tubes. The numbering scheme was distinct from both the numbering schemes used for standard receiving tubes, and the existing transmitting tube numbering systems used previously, such as the "800 series" numbers originated by RCA and adopted by many others.

955 acorn triode Thermionic valve for VHF operation

The type 955 triode "acorn tube" is a small triode thermionic valve designed primarily to operate at high frequency. Although data books specify an upper limit of 400–600 MHz, some circuits may obtain gain up to about 900 MHz. Interelectrode capacitances and Miller capacitances are minimized by the small dimensions of the device and the widely separated pins. The connecting pins are placed around the periphery of the bulb and project radially outward: this maintains short internal leads with low inductance, an important property allowing operation at high frequency. The pins fit a special socket fabricated as a ceramic ring in which the valve itself occupies the central space. The 955 was developed by RCA and was commercially available in 1935.

Barkhausen–Kurz tube

The Barkhausen–Kurz tube, also called the retarding-field tube, reflex triode, B–K oscillator, and Barkhausen oscillator was a high frequency vacuum tube electronic oscillator invented in 1920 by German physicists Heinrich Georg Barkhausen and Karl Kurz. It was the first oscillator that could produce radio power in the ultra-high frequency (UHF) portion of the radio spectrum, above 300 MHz. It was also the first oscillator to exploit electron transit time effects. It was used as a source of high frequency radio waves in research laboratories, and in a few UHF radio transmitters through World War 2. Its output power was low which limited its applications. However it inspired research that led to other more successful transit time tubes such as the klystron, which made the low power Barkhausen-Kurz tube obsolete.


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