International projectionist (July-Dec 1934)

July 1934 INTERNATIONAL PROJECTIONIST 13 full-wave rectifier is not smooth and constant. This is easily understood. Consider a moment when the alternating current supplied to this rectifier is changing its direction of flow. At that moment the plates of both V-3 and V-4 are at zero potential, they have no charge at all, and no current flows across the vacuum of either tube. Therefore, the direct current drawn from this rectifier is pulsating, varying periodically between zero and maximum. Those pulsations must be removed and the current "smoothed out" before it can be used in the amplifying tubes V-l and V-2. Beginning at the center tap of the righthand secondary of T-3, the positive line of this high voltage circuit — now converted to a D.C. circuit — runs upward, right and then upward to the lower end of the 134-A retard coil, L-2, which is part of the filter system by means of which the pulsations or irregularities are removed from the direct current of this circuit. From the upper end of L-2 upward, left and again upward to retard coil L-l. Thence upward to the milliammeter, M-l, which records the amperage flowing. Thence right to the center tap of the output transformer, T-2. Here the circuit branches into two parallel lines, one of which flows through each half of the 233-G INPUT TRANS. T, transformer and from there to the plates of V-l and V-2. From the filaments of those tubes the circuit continues downward through the two wires leading to the left-hand secondary of T-3. From the center tap of this secondary upward and left. Two wires lead upward. One runs to the .5 microfarad by-pass condenser C-9. Since only A.C. can pass through a condenser, the d.c. circuit we are tracing cannot run that way. Therefore, it has no other path except upward through the 575-ohm resistor, R-2. From the upper end of this resistor it runs upward about an inch, right about three inches, and downward about two inches to the center point of the secondary winding of T-4, which constitutes the negative terminal of this circuit. What we have been tracing is the plate power supply for the two tubes, V-l and V-2. It begins as alternating current in the secondary winding of T-4. Only one-half of this winding operates during any given moment — the positive half. Therefore, the negative source of this circuit is always the center point of the secondary winding of T-4, and its positive source is whichever of the two outside terminals of that winding happens to be positive at the moment. The arrangement of the two tubes, V-3 and V-4. constitutes a full-wave rectifier, converting this high-voltage a.c. to high 127-A OUTPUT TRANS T2 L. _! SOO*OUT 03 Schematic of The W. E. 42— A Amplifier voltage d.c. Leaving the rectifier system, the circuit continues to a filter, composed of the coil L-2 and two banks of filter condensers — C-l, C-2, C-3, C-4, and C-5, C-6, C-7, C-8. The coil L-2 is in series with the line and the two banks of condensers bridge across from the positive to the negative side of the line. This arrangement constitutes a "brute force" filter for smoothing the irregularities of direct current drawn from a rectifier. These are the two banks of condensers that hold the dangerous charge previously spoken of in connection with the door switch, D-l. Look at the points where these condensers connect to the negative line. Halfway between those two points a wire will be seen running to the right to a terminal marked "GND.PLATE." The negative side of this high voltage line is grounded. Whoever attempts to work on this amplifier should discharge the filter condensers as soon as he opens the back cover, by grounding both terminals of the coil L-2. This can be done with a screw driver that has a well-insulated handle. About a half inch above the upper terminal of the filter choke coil, L-2, a branch circuit runs off to the right, and then branches again, running through two 17.000-ohm resistors to two terminals marked + 390V. Thus the plate rectifier and plate filter of this amplifier can be used to provide plate power for two amplifiers of the type examined last month. Commonly, however, only one of them is used with the amplifier shown here. We have now traced the filament and plate power circuits; the grid bias and speech circuits remain to be considered. Grid bias is obtained by means of the voltage drop through the 575-ohm resistor, R-2. The plate current flowing through this resistor is approximately 60/1.000ths of an ampere, or 60 mils., and this current, multiplied by 575 ohms, gives a voltage drop across R-2 of 34% volts. The lower, or positive end of R-2 connects with the filaments of V-l and V-2 through the left-hand secondary of transformer T-3. The upper or negative end of R-2 connects with the grids of V-l and V-2 through the grid resistor R-l and the secondary windings of transformer T-l. Since the grid bias is voltage only, and not current, there is no drop in resistor R-l. There is very little drop in the windings of the left-hand secondary of T-3, which are heavy enough to carry the 3.2 amperes needed for lighting the amplifier tubes; hence the grid bias of this amplifier is substantially the same as the drop across R-2, or 34% volts, more or less, depending upon the plate current flowing, which in turn depends upon the 110-volt line voltage