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POWER SUPPLY FOR SOUND SYSTEMS 499
it into top half and bottom half. Each half serves the plate circuit of one rectifying" tube.
Let us trace the top half and its circuit. Since the current generated in the secondary is alternating, the center-tap will alternately be positive and negative with respect to the ends. Tracing from the center-tap, the circuit runs right and upward to the resistance unit. One of the plugs connected with this unit will be plugged into the plug board. The drawing shows the circuit open at this point merely to indicate that any plug may be used, according to the amount of resistance required. From the end of the plug board the circuit runs right, then down, to the d. c. output terminal near the lower righthand corner of the drawing and thence through the external load and in again at the other d. c. terminal. From there we trace it upward through the right-hand IS ampere plug fuse to the plate of bulb No. 1. From the filament of that bulb a wire returns to the upper end of the plate secondary of the transformer.
Under normal operating conditions the resistance unit is plugged to the plug board, as has been explained, and there is an external load across the two d. c. terminals; therefore this circuit is complete except for the gap in the tube itself. Now the plate of the tube is connected, through the circuit as traced, to the mid-tap of the secondary of the transformer, and the filament of the tube to the upper end of that secondary. Whenever the mid-cap is positive with respect to the upper end of the secondary coil, the plate of bulb No. 1 must be positive with respect to its filament. (17) Whenever that happens negatrons emitted by the filament will be attracted to the positive plate, closing the circuit across the vacuum.
When the mid-tap of the secondary becomes more negative than the upper end of the secondary, the plate of bulb No. 1 will be negative with respect to the filament of bulb No. 1, and the negatrons will be repelled by the plate, thus opening the circuit. The emitted negatrons will linger in the vacuum, thereby constituting a space charge, and eventually return to the filament, which at that moment is positive.