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THEORY AND WIRING OF AMPLIFIERS 559
drawing is represented in physical fact by a knob on the front of the amplifier panel. C-4 and R-5 are frequency regulators. By bridging the condenser C-4 a path of I6wer resistance is offered to the high frequencies which is equivalent to decreasing the percentage of low frequency response.
The 56 tube is transformer-coupled to the first pushpull stage, but the plate d. c. supply is kept out of the transformer windings by use of Resistor R-7 and Condenser C-6, the a. c. component flowing through the capacitor C-6, the reactance L-3 and the primary of T-l. The transformer has a voltage step-up, and its use increases the grid swing of tubes 3 and 4. L-3 is added to adjust the frequency response.
The push-pull 56 tubes are coupled to the push-pull 45 tubes through the condensers C-15 and reactors L-4, the constants of which are chosen to give the amplifier the desired frequency response. The primary of T-2 output transformer carries the plate d. c.
The secondary circuit of T-2 is easily traced through the speech windings of the stage and monitor speakers. The latter is of the "magnetic" type. (Page 586.) Rheostat R-22, shown left of and somewhat above the stage speaker field, is used to control the volume of the monitor speaker.
Condenser C-ll, just left of the speaker terminal strip, serves as part of the ripple filter. The two condensers C-8 keep the photo-cell a. c. component out of the rectifier. *
The extreme left-hand portion of Figure 139 does not, properly, deal with the amplifier at all, but represents the exciter lamp power supply. The lamps are lit with a. c, the frequency characteristics of the amplifier being such that the 120-cycle hum created is filtered out in the coupling circuits and does not reach the speaker. T-4 are step-down transformers developing 10 volts for the lamp filaments. Fader "A" is a double-pole double-throw switch and Fader "B" is a single-pole double-throw switch. One is mounted to the right of each projector in convenient operating position. (See Figure 141.) No