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1046 HANDBOOK OF PROJECTION FOR
grid negative and, in a way, is the key to sound amplification, be maintained at its proper value. This battery should be inspected once every thirty days, say on the first day of every month, and the voltage tested. If you have Movietone or Vitaphone, or both, then on the front of the battery box you will find a set of small metal contact studs with which connection may be made with a low reading voltometer.
Your operating instruction book will give you the proper voltages between each pair of these contact studs on each type of amplifier. Plate current greater than those specified in your operating instruction book are an indication that the grid voltage is not high enough, which, if the "C" battery is in good condition, may be due to their making poor connection with the grid circuit. This fault may be remedied by scraping the contact lightly with a knife blade, or rubbing with sand or emery paper or cloth. This refers to the battery springs and contact springs at the bottom of the battery box. It also may be advisable to bend the battery springs to insure a firmer contact pressure.
It also is possible that there will not be good contact at the base of the vacuum tube, which would, of course, result in lowering the grid voltage, or affect the plate or filament current. In such cases do not use an abrasive to clean the contact; also do not scrape the contact with a knife blade. Clean the contact with a rubber eraser. If you scrape them, or use an abrasive, the contact will become pitted, and eventually be worse than it was before you cleaned it.
WARNING. — Do not hold your voltometer in contact with the "C" battery longer than is necessary. It draws current and helps to exhaust the battery.