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OPERATING MANUAL 119
while they are warming up. Next, the plate current is put on the amplifier and is regulated. After the amplifiers have been tested by means of a monitor horn they are ready for use. It is quite important to adjust the filament currents carefully to the specified values, for the low filament current will have a bad effect upon the value and quality of the sound, and the higher currert will shorten the life of the tubes.
The original Electrical Research Products, Inc., equipments used amplifiers known as the 8-B, 9-A, and 10-A. The 8-B and 9-A amplifiers require a 360-volt storage battery for plate supply. The 10-A amplifier derives its plate supply from the 6000-A rectifier, which operates on no volts A.C.
The present types of amplifiers are known as the 41-A, 42-A, 43-A, and 46-B, and all derive their plate current from rectifying circuits contained in the 42-A, 43-A, and 46-B amplifiers. The 8-B, 9-A, 41-A, and 46-B amplifiers require 12-volt storage battery supply for their filaments. With the exception of the 46-B amplifier, which is mounted in a metal cabinet, all these amplifiers are panel type units, and may be combined as required to build up systems of varying capacities. The old type amplifiers are not, of course, used in combination with the present type.
Some of the foregoing amplifiers are provided with meters for reading the plate current or voltage, as well as the filament current. The readings of these meters must be carefully checked with the values given in the instruction book, as they are an important index to the condition of the tubes and amplifiers.
VII. Amplifier System
The amplifier system increases the weak currents from the reproducer, or the film amplifier, until they have