A vacuum tube, vac tube, electron tube, thermionic valve (British usage), or tube (North America)John Algeo, "Types of English heteronyms", p. 23 in, Edgar Werner Schneider (ed), Englishes Around the World: General studies, British Isles, North America, John Benjamins Publishing, 1997 . is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. It takes the form of an evacuated tubular envelope of glass or sometimes metal containing electrodes connected to external connection pins.
The type known as a thermionic tube or thermionic valve utilizes thermionic emission of electrons from a hot cathode for fundamental electronic functions such as signal amplification and current rectification. Non-thermionic types such as vacuum phototubes achieve electron emission through the photoelectric effect, and are used for such purposes as the detection of light and measurement of its intensity. In both types the electrons are accelerated from the cathode to the anode by the electric field in the tube.
The first, and simplest, vacuum tube, the diode or Fleming valve, was invented in 1904 by John Ambrose Fleming. It contains only a heated electron-emitting cathode and an anode. Conventional current can flow in only one direction through the device: from the cathode to the anode (hence the name "valve", like a device permitting one-way flow of water). Adding one or more control grids within the tube, creating the triode, tetrode, etc., allows the current between the cathode and anode to be controlled by the voltage on the grids,Hoddeson L., Riordan M. (1997). Crystal Fire. New York: W. W. Norton & Co. Inc. p. 58. Retrieved Oct 2021 creating devices able to amplify as well as rectify electric signals. Multiple grids (e.g., a heptode) allow signals applied to different electrodes to be mixed.
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, television, radar, sound recording and reproduction, long-distance telephone networks, and analog and early digital computers. Although some applications had used earlier technologies such as the spark gap transmitter and crystal detector for radio or mechanical and electromechanical computers, the invention of the thermionic vacuum tube made these technologies widespread and practical, and created the discipline of electronics.
In the 1940s, the invention of semiconductor devices made it possible to produce solid-state electronic devices, which are smaller, safer, cooler, and more efficient, reliable, durable, and economical than thermionic tubes. Beginning in the mid-1960s, thermionic tubes were being replaced by the transistor and solid-state semiconductor technology. However, the cathode-ray tube (CRT), functionally an electron tube/valve though not usually so named, remained in use for electronic visual displays in television receivers, computer monitors, and oscilloscopes until the early 21st century.
Thermionic tubes are still employed in some applications, such as the magnetron used in microwave ovens, and some high-frequency amplifiers. Many audio enthusiasts prefer otherwise obsolete tube/valve amplifiers for the claimed "warmer" tube sound, including electric guitar players, with guitar amplifiers still relying heavily on tube/valve technology.
Not all electronic circuit valves or electron tubes are vacuum tubes. Gas-filled tubes 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 tetrodes, 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" pure tungsten filament or "dull-emitter" thoriated tungsten filament) by characteristic curves design (e.g., sharp- versus remote-cutoff in some pentodes) by application (receiving, transmitting, 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)
Vacuum tubes may have other components and functions than those described above, and are described elsewhere. These include as cathode-ray tubes, which create a beam of electrons for display purposes (such as the television picture tube, in electron microscopy, and in electron beam lithography); X-ray tubes; phototubes and photomultipliers (which rely on electron flow through a vacuum where electron emission from the cathode depends on energy from photons rather than thermionic emission).
Description
A vacuum tube consists of two or more electrodes in a vacuum inside an airtight envelope, with pins accessible from outside the envelope to make electrical connections to the electrodes. Most tubes have glass envelopes with a glass-to-metal seal based on kovar sealable borosilicate glasses, although 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 mainly to the filament burning out or the cathode coating becoming depleted, gradually reducing performance, with 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 making tubes, a frequent cause of failure in electronic equipment, easy to remove and replace.
In addition to the base terminals, some tubes had an electrode terminating at a connection at the top of the tube, a top cap. This avoids leakage resistance, particularly for the high impedance grid input, through the tube base (sometimes made with phenolic insulation which performs poorly as an insulator in humid conditions) and improves stability and high-frequency performance by reducing capacitance between the grid and other electrodes., p. 571 Some tubes requiring a very high anode voltage used a top cap for the anode connection, helping to isolate it from the lower potential of other electrodes. In tubes with a base accommodating a limited number of electrodes, a top cap provided an extra connection. A few tubes 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 electrons into the vacuum, a process called thermionic emission. An added second electrode at a positive voltage relative to the filament, the anode or plate, will attract those electrons. The result is a net unidirectional flow of electrons from the filament to plate; there is no flow in the reverse direction because the plate does not emit positive ions. The filament 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. 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. This allow diodes to be used to convert AC to DC in a power supply, as a demodulator of amplitude modulated (AM) radio signals, and for similar functions.
Early tubes used the filament as the negative electrode, the cathode; this is called a "directly heated" tube. Most later tubes are "indirectly heated" by a filament or heater element inside a metal tube functioning as the cathode, with an oxide coating that much improves electron emission. The heater is electrically isolated from the surrounding cathode and simply serves to heat the cathode sufficiently for thermionic emission of electrons. H. J. Round invented the indirectly heated tube around 1913. The electrical isolation allows all the tubes' heaters to be supplied from a common circuit (which can be AC, although the cathodes must be at a steady potential) separate from operating voltages; in particular the cathodes in different tubes can operate at different voltages. The filaments require constant and often considerable power, even when amplifying signals at the microwatt level.
Power is dissipated by the filaments, and by electrons from the cathode impacting and heating the anode (plate); this occurs 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 is evacuated, convective electrode cooling is not possible; the anodes in most small and medium power tubes are cooled by radiation through the glass envelope. In some very high power applications, cooling is drastically improved by making the anode part of the (metal) tube envelope, in thermal contact with a heat sink cooled by forced air or water (the anode would normally be at ground potential, with the cathode and grids at a high negative potential). Klystrons and magnetrons are tubes that often operate their anodes (called collectors in klystrons) at ground potential to facilitate cooling, particularly with water, without high-voltage insulation.
Except for diodes, additional electrodes are positioned between the cathode and the plate (anode). These electrodes are not solid sheets but sparse elements through which electrons can pass, referred to as grids, influencing but not obstructing the flow of electrons to the anode. The vacuum tube is then known as a triode, tetrode, 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, although usually using much lower voltages, is the junction field-effect transistor (JFET). Vacuum tube characteristics
Vacuum tubes are commonly described using characteristic curves published in tube manuals and engineering texts. These curves show relations between electrode voltages and currents, such as plate current versus plate voltage for different control-grid voltages. From these curves designers determine operating points, voltage gain, load-line behavior, and power output.
History and development