International projectionist (July-Dec 1934)

STEP-BY-STEP ANALYSIS OF COMMON AMPLIFIER TYPES Aaron Nadell V. RCA Photophone Type PA 83 FIGURE 1 shows a complete sound system from photo-electric cells to loud-speakers. Modern sound equipment is more compactly made, more closely knit than were earlier models, and it is not always easy to isolate the new amplifiers and analyze them separately. In Figure 1 both the power and the speech circuits of the amplifier are intimately linked with the loud-speaker on one side and the photo-cells on the other. Among those portions of Figure I that have no direct connection with the amplifier are the two exciter lamp supply rectifiers in the lower left-hand corner of the drawing. Since the two are identical, we need trace only one of them. The function of these rectifiers is to convert line a. c. to filtered d. c. for the use of the exciter lamps. At the upper right of this circuit is rheostat R-19, through which the line voltage is reduced and the exciter supply regulated. Next to the left is the diamond-shaped "stack" rectifier, the action of which is by now familiar to readers who have followed previous articles of this series. To the left of the rectifier are the two reactance coils, L-5 and L-6, and the condenser, C-ll, making up the filter that removes the ripple from the rectified d. c. To the left of L-5 is the a. c. line switch, and to the left of this is the exciting lamp. We may trace this circuit briefly, assuming the upper input wire to be positive. Then, from positive to negative, the circuit is through the a. c. switch, through R-19, down left through the upper left-hand stack of the rectifier, up and left through the filter coils, left through the exciter, down right to the right-hand corner of the diamond, down left through the lower right-hand stack, and back to the a. c. line. Monitor Speaker Amplifier When the lower a. c. wire becomes positive, the circuit runs up left through the lower left stack ; then around through the d. c. circuit precisely as before, up left through the upper left-hand stack, and back to the power line through R-19 and the a. c. switch. This disposes of the two circuits in the lower left-hand corner of Fig. 1. The lower right-hand corner of Fig. 1 contains a separate one-stage amplifier, complete with its own power supply. It will be very helpful to trace this circuit before considering the larger system amplifier. One reason is that, while most of the circuits of the diagram are essentially similar to others considered earlier in this series, the method of drawing them is to some degree new. For example, consider the filaments in the two 245 tubes in the monitor amplifier. They are equipped with arrowheads instead of connecting wires carrying heating current. About an inch from the lower of the 245 tubes, diagonally downward and right, is shown the centertapped secondary winding of a power transformer, with its outer leads terminating in arrowheads that are labelled "To UX 245 FIL". This, plainly, is the source of the filament power. The connecting wires are omitted to make the drawing simpler. That is only one of the several novelties in this drawing. The a. c. supply to this one-stage, pushpull amplifier is shown at the extreme lower right. To the left of this is a power transformer, T-8, with three secondaries. The upmost secondary, mentioned in the preceding paragraph, is the one that supplies the filaments of the 245 amplifying tubes. The lowest secondary lights the filament of the 280 full-wave rectifying tube. The center secondary supplies plate power to the rectifier, and through it to the 245's. The circuit of the 280 and the centertapped plate secondary of the transformer is similar to the full-wave rectifiers we have studied in other amplifier drawings. Most of those used two tubes for fullwave rectification; in this case the same purpose is served equally well by a single tube equipped with two separate and insulated plates. Only one rectifier plate is active at a time — that plate, of course, which happens to be positive at the moment. With respect to that plate (whichever it is), the center-tap of the transformer secondary is always negative. The wire that leads outward from the center-tap of the secondary is always the negative wire of the d. c. output of the rectifier. The filament of the rectifying tube is also negative with respect to the positive plate, but not so much so; and the wire leading outward from the filament is always the d. c. positive. Let us trace this circuit, beginning at the negative end, or center-tap, of the plate secondary. From that tap upward, right, and up again to the 1820-ohm coil, L-3. Through this coil and down through the biasing resistor R-26 to the mid-tap of the filament supply secondary. Thence through the arrowheads to the filaments of the amplifying tubes. Through the vacuum to the plates of those tubes and through the plate transformer primary to its center-tap ; thence downward to the filament of the 280 rectifier tube. Through this filament and the vacuum to the positive plate, and thus to the positive end of the power transformer secondary. In this circuit L-3, which is also the field winding of the monitor speaker, performs the work of a filter choke-coil to smooth the output of the rectifier. This particular choke happens to be in the negative d. c. lead, instead of in the positive lead, as is more usual. The condensers C-15 and C-16 bridge across to the positive line from either side of this choke-coil and complete the filter arrangement. The reader who remembers the chokeand-condenser filter combinations of earlier drawings of this series, will note that the draftsman has used a different pattern in this instance; but tracing the wires will show him at once that the electrical arrangements are the same. The grid bias of this amplifier is secured through the voltage-drop across the 810-ohm resistor, R-26. The positive side of this resistor connects to the filaments through the center-tap of the filament supply transformer. The negative side may be traced to the grid upward, left, and again upward to the junction of the two grid resistors. At the extreme left of the monitor amplifier is a 500-ohm potentiometer, R-23. Two leads from this run upward to the secondary of the output transformer of the system amplifier. The sliding contact can be set to tap any desired proportion of the voltage drop developed across the 500-ohm resistor, and therefore serves as a monitor volume control. A portion of the voltage-drop across R-23 is thus connected in parallel to the prim [16]