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From The Free On-line Dictionary of Computing (05 January 2017) [foldoc]:
electron tube
firebottle
vacuum tube
(Or tube, vacuum tube, UK: valve, electron
valve, thermionic valve, firebottle, glassfet) An electronic
component consisting of a space exhausted of gas to such an
extent that electrons may move about freely, and two or more
electrodes with external connections. Nearly all tubes are of
the thermionic type where one electrode, called the cathode,
is heated, and electrons are emitted from its surface with a
small energy (typically a Volt or less). A second electrode,
called the anode (plate) will attract the electrons when it is
positive with respect to the cathode, allowing current in one
direction but not the other.
In types which are used for amplification of signals,
additional electrodes, called grids, beam-forming electrodes,
focussing electrodes and so on according to their purpose, are
introduced between cathode and plate and modify the flow of
electrons by electrostatic attraction or (usually) repulsion.
A voltage change on a grid can control a substantially greater
change in that between cathode and anode.
Unlike semiconductors, except perhaps for FETs, the
movement of electrons is simply a function of electrostatic
field within the active region of the tube, and as a
consequence of the very low mass of the electron, the currents
can be changed quickly. Moreover, there is no limit to the
current density in the space, and the electrodes which do
dissapate power are usually metal and can be cooled with
forced air, water, or other refrigerants. Today these
features cause tubes to be the active device of choice when
the signals to be amplified are a power levels of more than
about 500 watts.
The first electronic digital computers used hundreds of vacuum
tubes as their active components which, given the reliability
of these devices, meant the computers needed frequent repairs
to keep them operating. The chief causes of unreliability are
the heater used to heat the cathode and the connector into
which the tube was plugged.
Vacuum tube manufacturers in the US are nearly a thing of the
past, with the exception of the special purpose types used in
broadcast and image sensing and displays. Eimac, GE, RCA, and
the like would probably refer to specific types such as "Beam
Power Tetrode" and the like, and rarely use the generic terms.
The cathode ray tube is a special purpose type based on
these principles which is used for the visual display in
television and computers. X-ray tubes are diodes (two element
tubes) used at high voltage; a tungsten anode emits the
energetic photons when the energetic electrons hit it.
Magnetrons use magnetic fields to constrain the electrons;
they provide very simple, high power, ultra-high frequency
signals for radar, microwave ovens, and the like. Klystrons
amplify signals at high power and microwave frequencies.
(1996-02-05)