The Operation of Vacuum Tubes as Class B and Class C Amplifiers

A simple theoretical development of the action of a vacuum tube and its associated circuit when used as a class B or class C amplifier is given. An expression for the power output is obtained and the conditions for maximum output are indicated. The way in which the tuned plate circuit filters out the harmonics in the pulsating plate current wave is illustrated by a hypothetical example. A set of dynamic output current characteristics is developed graphically from a set of static characteristics. The class B dynamic curves are found to give a better approximation to a straight line than the class C curves because of a reversed curvature which appears at the lower ends. It is pointed out that the screen-grid tube should function similarly to a high μ three-element tube in this type of operation. Experimental dynamic characteristics of a three element tube, Western Electric 251-A, and of a screen-grid tube, Western Electric 278-A, of identical dimensions are shown which verify the theoretical results. The screen-grid tube gives about the same output and efficiency as the three-element tube, but its dynamic characteristic tends to bend more rapidly at the upper end.