Gas filled tubes are arrangements of electrodes in a gas within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramics, and military tubes often use glass-lined metal.
Gas filled tubes operate by ionizing the gas to get conduction. Both hot and cold cathode type devices are encountered. Depending on application, either glow discharge or arc discharge may be emphasised.
The result of this is a tube that will begin to conduct under certain conditions, then tend to stay in a state of conduction until the applied voltage across the gas and/or the heat-generating current drops to a minimum level.
The most common type of discharge tube is the neon light, used popularly as a source of colorful illumination, the color of the light emitted being dependent on the type of gasfilling the tube.
If a regular triode was filled with gas instead of a hard vacuum, it would manifest all the hysteresis and nonlinearity of other gastubes with one major advantage: the amount of voltage applied between grid and cathode would determine the minimum plate-to cathode voltage necessary to initiate conduction.
Tubes with 5 grids, called pentagrid converters, were generally used although alternative such as using a combination of a triode with a hexode were also used, even octodes have been used for frequency conversion The additional grids were all control grids, with different signals applied to each one.
Once the tube envelope is evacuated and sealed, the getter is heated to a high temperature (usually by means of RF induction heating) causing the material to evaporate, adsorbing/reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside of the envelope of the tube.
Tubes were ubiquitous in the early generations of electronic devices, such as radios, televisions, and early computers such as the Colossus which used 2000 tubes, the ENIAC which used nearly 18,000 tubes, and the IBM 700 series.
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