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THEORY AND WIRING OF AMPLIFIERS 557
Another branch of the same line, instead of going upward to R-7, turns right, and then upward through resistors R-ll to the plates of the push-pull 56 stage. From the cathodes of that stage through the bias resistor R-10 and left still another half inch, then down about an inch and right and down through the stage speaker field to negative.
Plate Power for the 57 Tube
From the left-hand side of L-l left through R-20, then up, left and up through R-4 to the plate of the RCA-57. From the cathode of that tube through the bias resistor R-3, and thence right about six inches and back to negative through the speaker field.
Screen Grid Bias for the 57 Tube
From the left-hand side of R-20 left through R-19 and then down through R-18. From the lower end of R-18 left, up, left and up to the screen grid of the 57. The return to negative is through the cathode of that tube as before. The screen positive voltage is lower than that of the plate to the extent of the drop through the "voltage divider" resistors R-19 and R-18. (The screen voltage is 40; the plate voltage 121.) The screen grid circuit is not intended to carry appreciable current. The majority of the emitted negatrons in the tube go to the more positive plate. The greater part of the current through R-18 does not complete its circuit through the screen grid, but returns to negative through R-17 and R-16.
Plate Power for Photo-Cells
From the left-hand side of R-19 left and then downward to the two resistors R-15. Through these resistors and then down, right, through R-16, right, up, right and down through the speaker field. Through the arrowhead variable contacts to resistors R-15 to the anodes of both photo-cells. The purpose of these variable contacts is to compensate for small inequalities in the photo-cells or in their exciting light, and thus produce equal volume