International projectionist (July-Dec 1934)

STEP-BY-STEP-ANALYSIS OF COMMON AMPLIFIER TYPES Aaron Nadell II. The Western Electric 42-A LAST month a common and familiar type of theatre amplifier was subjected to detailed circuit analysis. The amplifier shown in Figure 1 of the present article is a companion piece to the one described last month. Neither is used without the other. The reader will note along the righthand edge of the present drawing a row of terminals for external connections. The fifth and sixth from the top are marked +390 V. Here is the source of the 390-volt plate power used in the amplifier described last month. To analyze Figure 1 systematically let us first identify the tubes. Along the top of the drawing, near the center, we see a little zig-zag line representing a grid, to the right of it a small rectangle symbolizing the plate, and between the two the inverted "V" that stands for a filament. This arrangement is marked 205-D, V-2, or vacuum tube 2 of this amplifier. Directly below we see the same arrangement of symbols, drawn upside down for the convenience of the draftsman, and marked V-l, 205-D. Below and somewhat to the right of this pair of tubes are two others, both drawn right side up. marked V-3 and V-4. These also are 205-D tubes. There are no other tubes shown in this drawing. We can see at once that V-3 and V-4 are not used as amplifying tubes, because the drawing shows that their grids and plates are joined, therefore, these two tubes must be used as rectifiers to provide plate power for the amplifying tubes, V-l and V-2. Rectifiers mean an alternating current power supply. As in the case of last month's amplifier, it will be advisable, once the tubes have been identified, to trace the power circuits first and the speech circuits afterward. At the extreme bottom of the drawing there are two terminals, about an inch apart, marked 110V. A.C. The power line is connected to these. Directly above the right-hand power terminal is the door switch, D-l. This is a spring switch, closed by the back cover of the amplifier. When this cover is opened the switch is released and breaks the power circuit as a matter of safety. However, this spring switch cannot provide complete protection, because a powerful charge is stored in the filter condensers, described hereafter. Anyone working at this amplifier must discharge the filter condensers immediately after opening the back cover, in order to be safe against shock. Above and to the left of the door switch is the rotary control switch. This has two segments and six contact points. It has three positions. One (not in this drawing, but on the switch itself) is marked "OFF". The second position is marked "FIL.", and the third "PLATE". As drawn in Figure 1, the switch is in the third position. When the switch is in the OFF position, contact terminals A and D, connecting to the power supply, are open, and no power reaches any part of the amplifier. In FIL position contacts A and D are connected with B and E, which are wired, as the drawing shows, to the lower or primary winding of the 303-C transformer, T-3. Let us assume that the switch is set at FIL. and trace the power circuits active at that setting. The 110-volt A.C. flowing through the primary of T-3 induces a 4.5 volt alternating current in each of the two secondary windings of that transformer. The secondaries are seen just above the grounded transformer core, v/hich in turn is drawn just above the primary. Each of these secondary windings has a center tap which does not concern us just now. From the outside terminals of the lefthand secondary a pair of 4.5 volt lines run upward to the parallel filaments of V-l and V-2. The parallel filaments of V-3 and V-4 are supplied by the righthand winding of this transformer. Transformer T-3, therefore, is the filament power transformer of this amplifier, having two secondaries, each of which lights the filaments of two tubes. These tubes are lighted the moment the rotary control switch is set at FIL. position, in which it supplies current to the T-3 primary. After the filaments of these tubes have been heated for a short while the rotary control, or "FIL.-PLATE SWITCH" is thrown to the PLATE position, as shown in the drawing. In this position terminals C and F are connected in parallel to the 110-volt line, and the line power is also supplied to the primary winding of the right-hand transformer, 303-B, T-4. The secondary winding of this transformer— only one winding, in this case, equipped with a center tap — has more turns than the primary, and delivers a higher voltage than 110; probably in the vicinity of 600 volts. The left-hand terminal of this secondary connects to the plate of V-3; the right-hand terminal to the plate of V-4. With respect to these two tubes, the grids may be thought of as merely physical extensions of the plates, since grids and plates are wired together. Since the current developed in the secondary of T-4 is alternating, the polarity of that winding will reverse its direction periodically. The two outside terminals will be positive and negative alternately. Therefore, the plate of V-3 will be positive while the plate of V-4 is negative, and vice-versa. Current flows across the vacuum of a tube only when the plate of that tube is positive and at no other time. Therefore, these tubes do not both operate at the same time but alternately. This high voltage circuit we are tracing, which originated in the secondary of T-4 transformer, runs through the vacuum of either V-3 or V-4, thence from the filament of either tube, through the right-hand secondary of transformer T-3, and out and upward from the center tap of that secondary. In the wire leading upward from the center tap of the right-hand secondary of T-3 the high-voltage current is direct current, flowing always in the same direction, regardless of whether it comes from V-3 or from V-4. Those two tubes have to perform the function of rectifiers, converting the A.C. created in the secondary of T-4 into D.C. The circuit of V-3 and V-4 is that of a "full-wave" rectifier, because it operates upon both halves of the alternating cycle. Remove either tube from its socket and the circuit would be that of a half-wave rectifier. In the wire leading upward from the center tap of the righthand secondary of T-3 the current would be D.C, but flowing only half the time. The direct current drawn from this [12]