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MANAGERS AND PROJECTIONISTS 1045
where except in the first stage of amplification, where the high degree of sensitivity required makes a noiseless tube essential.
If you suspect a noisy tube, set the fader at zero. If there then is noise,, you have good grounds for suspecting a tube, and should remove the first stage tube from the first amplifier, exchanging it for a tube from elsewhere, or inserting a spare tube. If this stops the noise, that of course settles the matter. If it does not, then you must look elsewhere for the seat of the trouble.
Batteries supplying current to the amplifiers must be kept in perfect condition. See page 1083. Loose connections or accumulations of dirt or acid on the battery tops may be the cause of fluctuations of current, and since this current is delivered to the amplifier, trouble will follow. Such fluctuations are amplified, and are reproduced by the horns or loud speakers, just as are the sound current fluctuations picked up from the projector pick-up system. They make themselves manifest by "frying" and crackling sounds.
WARNING. — As has been set forth under "Care of Batteries," page 1083, you must in no event use batteries for sound reproduction within from twenty minutes to half an hour after charging has been completed. During that period the electrolyte continues to give ofT gas, and noise at the horns will result if they are used.
INSPECT "C" BATTERIES.— The newest types of amplifiers have no "C" batteries, the "C" voltage being obtained from the potential drop across a resistance in the plate circuit. The amplifiers used previous to these, however, have "C" batteries and will not function properly unless the "C" battery voltage, which keeps the tube