F. H. Richardson's bluebook of projection (1935)

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THEORY AND WIRING OF AMPLIFIERS 543 electric cells to their amplifiers (see Figures 120 and 121), and there is no need of rehearsing the electrical Fig. 131. — Condenser-resistor-coupled amplifier. (Compare Fig. 121). This circuit, as well as Figures t28, 129 and 130, has series plate feed. Compare Fig. 132. action in this place. Both methods of coupling are commonly used in theatre amplifiers, usually with some modification. sound r C > U < •) SOUMP /HPUT r > °1 OUTPUT Fig. 132. — One-stage amplifier with parallel plate feed. Transformer coupling is often accompanied by "parallel plate feed/' an arrangement shown in Figure 132. This differs from Figure 129 only in that the d. c. component is kept out of the transformer, permitting the use of a transformer that is smaller, less expensive and more efficient. Figure 133 duplicates Figure 131 except that the grid resistor of the second tube has been made variable, and hence serves as a volume control. When the variable contact in Figure 133 is placed at the top of the resistor the arrangement duplicates Figure 131 exactly. When the variable contact is moved downward less than the full a. c. voltage drop developed across the resistor is applied across the grid and cathode of the second tube. Hence changes in grid voltage are smaller, the plate current across that tube varies less, and the volume drawn from the amplifier is lowered. This