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THEORY AND WIRING OF AMPLIFIERS 509
filament or heater wires are omitted in favor of arrowheads. The plate power input terminal is shown at right, center. One branch runs up through R-8 to VT-2. A second branch runs left, up through R-4 to the plate of VT-1. Another branch runs left through R-5, and then up to the screen grid of VT-1. In each case the return is through the cathodes of the tubes to the ground bus. The amplifier also has a voltage divider, consisting of R-9, R-10 and R-ll, from which 85 volts are tapped off for the photo-cell. C-6 serves as a filter condenser for that supply, while C-7 helps filter the entire B circuit so far as this amplifier is concerned.
The grid bias for the two tubes is obtained by the method of Fig. 199 with, however, the modification of a variable resistor, R-3, by which the bias of VT-1 can be altered to a limited extent. R-3 can be set to compensate for unequal volume from the two projectors. A different modification in VT-2 grid bias circuit is discussed below.
The speech input voltage across R-l is connected directly across the control grid and cathode of VT-1. That tube is a pentode, with its suppressor grid connected to cathode externally.
Since the action of the tube is to superimpose an amplified speech ripple on the plate d.c. flowing across the vacuum, plate and cathode may be regarded as the poles of a generator of speech-frequency a.c. This speech output may be traced from the plate down through R-4, right, down through C-7 and back to cathode through R-3 and R-2. A parallel path runs right through C-2, down through P-l and C-4, and then to cathode.
Still another parallel path can be traced from the slider of P-l to the grid of VT-2, and from the cathode of VT-2 left through C-5. A portion of the speech voltage drop across P-l is therefore applied to the grid of VT-2. exactly as in the case of Fig. 194. P-l is therefore a volume control; the large dial or knob of which is illustrated in Fig. 186.
VT-2, although a pentode, and actually the same tube as VT-1, is in this case connected as a triode, the plate, screen grid and suppressor grid terminals being wired