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556 RICHARDSON'S BLUEBOOK OF PROJECTION
ter inch and down through L-5, the field winding of the stage speaker, which thus does double duty, serving as a filter choke also. Thence left and then upward about an inch, left an inch and a half, and down an inch to the center-tap of the high-voltage secondary, which is the negative terminal of this circuit.
We can now look at some of the branch lines that lead from this circuit to provide plate power for the photoelectric cells and the amplifying tubes, and screen-grid bias for the RCA-S7 tube.
Plate Power for RCA 45 Tubes
From the filaments of the rectifier tubes left a trifle and then upward about an inch. Thence, instead of turning left to filter choke L-l we now branch upward another half inch, right about two inches, and upward to the center-tap of T-2 output transformer. Thence to the plates of the 45 tubes, and (we are tracing from positive to negative) through these tubes to their filaments, and through the arrowhead connections to the 50-ohm resistor that bridges the 45 filament supply secondary of the power transformer, T-3. From the center-tap of that resistor left and upward to the lower, or positive, side of R-13. From the upper or negative side of this resistor (the voltage drop through which provides grid bias for the 45 tubes) right a trifle, upward, right and down to the center terminal of the speaker terminal strip, and thence through the speaker field coil to the center-tap of the plate secondary of T-3. The speaker field coil and condenser C-ll constitute the only filter in this branch of the circuit.
Plate Power for RCA 56 Tubes
A line branching upward from the left-hand side of choke coil L-l, through Resistor R-7 to the plate of one of the three 56 tubes. From the cathode of that tube downward through the grid bias resistor R-6, right about three and a half inches and downward through the speaker field coil to negative. This line is filtered by L-l and its associated condensers as well as by the field coil.