Generic Tube types

DIODES

The simplest form of radio vacuum tube is the two-electrode type consisting of a cathode and an anode, or plate. This type, called a diode, is used in transmitting service mainly as a rectifier to convert low-frequency a-c voltages from the power line to d-c voltages for plate, screen, and grid-bias supplies. Simple diodes, such as the 866, are called half-wave rectifiers because they rectify but one-half of each alternating voltage cycle. When two diodes are enclosed in a single envelope, the tube is called a full-wave rectifier because it rectifies both halves of each a-c cycle. The receiving types 5Z3 and 83 (described in the RCA Receiving-Tube Manual) are typical examples.

Both half- and full-wave rectifiers are of two general types -high-vacuum and mercury-vapor. The latter type, represented by the 866 and the 83, is characterized by a very low and approximately constant internal voltage drop, amounting to about 15 volts. This drop is practically independent of d-c load current, but depends to some extent upon the temperature of the mercury vapor within the bulb. Mercury-vapor rectifiers, in operation, have a characteristic bluish glow which fills a considerable portion of the bulb. The extent of the glow depends on the value of the d-c load current.

Due to their low and relatively constant internal voltage drop, mercury-vapor rectifiers are very useful in applications where excellent voltage regulation of the d-c power supply is desired. Class B modulators represent one such application.

High-vacuum rectifiers have an internal voltage drop which is proportional to the d-c load current being drawn. With varying d-c load currents they do not, in general, provide the good voltage regulation obtained from mercury-vapor rectifiers Some high-vacuum rectifiers such as the 836, are designed with close spaced electrodes, so that a voltage regulation almost as good as that of a mercury-vapor type is obtained.