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558 RICHARDSON'S BLUEBOOK OF PROJECTION
from both projectors. From the cathode of whichever photo-cell is illuminated (the other, being dark, is the same as an open switch) through the shielded cable to the upper end of the coupling resistor R-l. Through R-l, then right about seven inches, and down to negative through the speaker field. The supply to this circuit is still further filtered by the bridging condensers, C-8.
Grid Bias Resistors in Figure 139
Every amplifying tube of Figure 139 receives control grid bias by virtue of the voltage drop across a resistor that is connected in series with its plate circuit. (Page 553.) Having traced all the plate circuits we have encountered all those resistors. They may be listed here for convenience, as follows: for the 57 tube, R-3; for the single 56 tube, R-6; for the push-pull 56 tubes, R-10. These resistors are shown close to their respective cathodes. The bias resistor for the 45's is shown just left of the lower rectifying tube. It is R-l 3.
Speech Circuits and Frequency^ Control
The speech voltage of the photo-cell circuits is condenser-coupled to the 57 tube. The greater part of the a. c. component created by the photo-cell action completes its circuit through C-l and R-2, only a relatively small portion flowing through the high resistance of R-l. The a. c. potential difference thus developed across R-2 is impressed upon the grid and cathode of the 57.
The a. c. (speech) return through R-3 is by-passed by condensers C-2 and C-7. These condensers favor the high-frequency response of the amplifier, inasmuch as they provide an easy path for the high frequencies while low frequencies must suffer loss of power in passing through 1,300 ohms of R-3. Open-circuiting either or both of these condensers destroys that discrimination and reduces the percentage of high frequencies (which is the same thing as increasing the percentage of lows).
The 57 is condenser-coupled to the 56, through C-3 and Resistors R-4 and R-8. R-8 is the volume control of this amplifier. The sliding contact shown in the