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THEORY AND WIRING OF AMPLIFIERS 577
generator of alternating current or "oscillator." The frequency of the a. c. thus created would be determined by the resonance point of the circuits involved, and might be low enough to sound like motor-boating or high enough to sound like a high-pitch squeal.
Now the plate supply for tubes V-3 and V-4 comes from the same rectifier that provides plate power to V-l. If the a. c. component of the plate circuit of V-3 and V-4 passed through that rectifier some of it would be "fed back" to the plate circuit of V-l, and the amplifier would "oscillate." One of the functions of the plate by-pass condenser (Figure 129) is to prevent this by keeping the a. c. component of any plate circuit out of the rectifier that provides the plate d. c. supply. Therefore an opencircuited by-pass condenser can cause feed-back and hence oscillation (motor:boating or howling) in an amplifier. The terminals of the by-pass condensers, and the continuity of the wires leading to them, should always be checked when the amplifier action seems at all unstable. There is also the possibility that the open circuit may be inside the condenser, a matter that can be checked as hereafter described.
Loss of Sound in Amplifier
(35) If sound should be lost and the amplifier be suspected, the first step is to determine whether the trouble is actually within the amplifier or not. Thus, in the case of Figure 140, headphones (high resistance phones only) might be connected across the primary of T-l, to be sure the amplifier is being supplied with sound current. If such is the case, the phones are next connected across the secondary of T-3, to determine whether sound current is leaving the amplifier. If sound is heard in the headphones during the first test, but not during the second, there is no question but that the amplifier is at fault.
If sound is not heard during the first test then the trouble is not in the amplifier, but between the amplifier and the photo-electric cell. If sound is heard on both tests then the trouble lies between the amplifier and the loud speakers.