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REPRODUCTION IN THE THEATRE
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to the loud speakers where they appear as grating sounds. To prevent disturbances, the three vacuum tube sockets are mounted on a piece of sponge rubber which is fastened to a heavy steel plate and this plate is likewise suspended on a sponge rubber mounting. This suspension method is the mechanical analogue
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FIGURE 8
of the electric filters used to quiet the B supply on the amplifier systems, and represents a very efficient means of insulating the tubes from mechanical vibrations.
In order to control the gain or amplification of the system, a potentiometer is placed in the grid circuit of the first tube of the 41-A unit. This affords a gain control of 66 decibels in steps of three decibels each. A second means of controlling the gain of this amplifier is put in the grid circuit of the second tube. This gives an additional gain of 14 decibels. Gain controls could be put on the power amplfiers as well as on the 41-A unit, but this is not considered necessary. This gain control through the potentiometer should not be confused with the fader. The potentiometer is ordinarily set at the time of installation.
Each of the power amplifiers consists of a single stage circuit known as a "push pull" circuit. This is shown in Fig. 9. The power amplifiers operate entirely from alternating current. The 42-A and 43-A amplifiers shown in Fig. 7 consume approximately 80 watts and 300 watts respectively. The amplifiers each make