A vacuum tube, vac tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference 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, 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).
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