electron tube

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.

(1996-02-05)