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THEORY AND WIRING OF AMPLIFIERS 549
Class A amplification the a. c. component of the plate circuit is a comparatively small ripple upon the underlying (1. c, while in Class \\ the "plate swing" is much greater ; sometimes, as said above, the Class B plate current may vary from nearly nothing to the maximum during each alternation of the grid bias. Because of this difference small tubes can be used to provide sound for a moderately large theatre but the gain in economy is paid for in some sacrifice of quality.
(21) Class AB, sometimes called Class A Prime, represents a compromise between the two kinds of amplification above described. The grid bias is so adjusted that the grid charge will always be negative with low or medium volume, but at high volume the grid swings positive during part of the speech cycle. At low volume the tube will operate in Class A; at high volume in Class B.
All the foregoing represent essentially minor variations of the fundamental circuits shown in Figure 128, which are basic to every type of amplifier whatsoever.
It should be mentioned that these variations are combined in all possible ways. The>same amplifier may have one stage of Class A and another stage of Class B, just as it may have condenser coupling between tubes at one point, and transformer coupling between another two tubes. The combination in one amplifier of single-tube stages such as Figure 128 with push-pull stages such as Figure 134 is extremely common. But Figure 128 still remains the foundation of all amplifying circuits. No superficial variations alter that fundamental fact.
Amplifiers and Their Power Supplies
Figure 130 represents a complete amplifier, and there are still amplifiers in theatre use that resemble Figure 130 in needing power supplies from an external source. The modern practice, however, is to build the power supply equipment into the amplifier cabinet. Thus a modern amplifier includes not only the circuits explained above, but also a rectifier arrangement to convert 110volt a. c. to the d. c. of various voltages required by the