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548 RICHARDSON'S BLUEBOOK OF PROJECTION
However, in the 57-type pentode tube shown in Figure 139, it is made at the socket terminals.
(19) The three-electrode tube is often called the "triode," the four-electrode or screen grid tube is the "tetrode," and the five-element tube is a "pentode."
"Class A" and "Class B" Circuits
(20) Amplifier circuits may be either Class A or Class B. The difference is a matter of grid bias, but the constants of coupling transformers and other apparatus must be adapted to the type of amplification used. Class A amplification is the kind we have been considering up to this point, in which the grid is always negative, and made a little more or a little less negative by the speech a. c, and the milliamperage of the plate current varies or "swings" around a medium level. Circuits designed to operate in Class B commonly have a low grid bias, and when speech a. c. is added the grid "swings" positive during part of the cycle. In some Class B circuits there is no grid bias at all, and the grid is positive over full one-half of the speech current cycle. The plate circuit constants are so adjusted that there is a little flow of plate current while the grid remains at all negative, and a sudden rush of plate current results when it swings neutral or positive. The circuit of Figure 134 is one used for audio-frequency Class B.
Tubes that were originally designed for Class A work can be operated as Class B by providing a very high grid bias ; one so high that there is practically no plate current except during such time as the speech input is positive with respect to grid and the extremely negative bias is reduced accordingly.
The projectionist cannot expect to distinguish between Class A and Class B amplifiers by looking at them, or in most cases, even by studying their wiring diagrams. Manufacturers, however, grade their amplifiers as Class A or Class B. The large majority of theatre amplifiers are Class A.
Class B gives greater power per tube than Class A, which can be understood when it is remembered that in