The presence of gas within a tube has several primary effects. First, of course, positively charged gas ions tend to reduce the tube’s effective plate-to-cathode resistance, thus reducing its internal voltage drop. For this reason, gas-filled (generally, mercury-vapor) rectifiers are extremely popular where large currents are handled. Second, an ionized gas gives the tube an “all or nothing” characteristic. Until ionization occurs, relatively little current can flow. Once the gas is ionized, however, current reaches a maximum very quickly, and stays at that maximum value (determined by load resistances and supply voltages) until the plate voltage is reduced to a very low value or cut off altogether.
Gas-filled triodes, or thyratrons, like gas-filled diodes, have an “all-or-nothing” characteristic, but with one difference. The difference is that the control grid in the thyratron, being relatively close to the cathode compared to the plate, can be used to “trigger” ionization even though the plate voltage is below the value normally required to ionize the gas. Thus, a small signal (or trigger) voltage applied to the grid can switch the tube from a non-conducting state very quickly. Thyratrons are used extensively as relaxation oscillators and for control and switching applications.
A number of gas-filled cold-cathode triodes have been manufactured for special applications. Their operation is somewhat similar to that of the conventional thyratron, except that no filament is used. The most familiar example of this type of tube is the flash tube used in photographic electronic flash lamps.
Receiving Tubes. By far the most popular general class of tubes are low- to medium-power types primarily designed for use in radio and television receivers. This general class encompasses all the basic tube types-diodes, triodes, tetrodes, pentodes, beam power, and multi-purpose tubes.
In the early days, there was little need to identify tubes except by their manufacturer’s name, for most units were essentially the same. Later, as more types were developed, identifying type numbers were introduced. These served to identify a particular type of tube in terms of its characteristics, permitting tubes produced by different firms but having the same type number to be used interchangeably. The first type numbers were simple numerical designations, such as 01A, 15, 19, 20, 42, 45, 76, and 80.
As more and more types were developed, a different numbering system became necessary. The tube manufacturers decided to adopt a system of numbers and letters such that the type number itself would give an indication of the tube’s basic application. With this system, the first number would indicate the tube’s nominal filament voltage, a middle letter the intended application (amplifier or rectifier, for example), and the last number the number of active elements. Amplifier type tubes were to receive letter designations from the first part of the alphabet, rectifiers from the end of the alphabet.