<electronics> (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.
Electronics Industry Association « electronic
whiteboarding « electron model « electron tube
» Electrostatic Discharge » elegant » element