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614 RICHARDSON'S BLUEBOOK OF PROJECTION
the source of power being the third secondary of power transformer T-l. The plate is powered with 720-cycle a. c. provided by the secondary of transformer T-3. The primary of that transformer is excited by the 720-cycle output of the stationary armature. Current can flow in the secondary of T-3 only while the plate of V-4 is positive. Therefore the tube acts as a rectifier, and the rectified d. c. of its output completes its circuit through R-6 and R-7, where it creates a voltage drop that opposes and reduces the positive bias of the grids of V-l and V-2. The upper, or positive end of R-6 and R-7 connects to the filaments of V-l and V-2 as follows: upward, right, down through R-8 to the left-hand arrowhead, and thence down, left and down through the milliammeter. Condenser C-7 acts as a filter to steady the d. c. through R-6 and R-7.
The flow of space current in V-4 therefore controls the speed of our motor, and V-4 is a grid controlled halfwave rectifier.
The Grid Bias of V-4
The bias of V-4's grid can be traced backward from that grid as follows: left to switch D-2 (which we now assume is closed at "Regulate"), down and right through the secondary of the 242-B input transformer, and thence downward through R-3, R-4 and R-S to the left-hand or negative side of R-2. Right through R-2 down, right, and up to the arrowhead touching the lower or negative side of R-8. Up through R-8 and left to the filament of V-4.
We have now progressed to the point where we see that the speed of our projector motor is controlled by the grid voltage of V-4. Before going further we may review briefly, as follows: The grid voltage of V-4 controls the space current of that tube. The space current of V-4 determines the extent of the voltage drop through R-6 and R-7. The voltage drop through R-6 and R-7 is in series with, and modifies, the bias of V-l and V-2. (That bias is obtained from the plate circuit of V-3, through the voltage drop across R-8 and R-9.)