International projectionist (Jan-Dec 1936)

24 INTERNATIONAL PROJECTIONIST December 1936 or converter, tube of a super-heterodyne radio receiver, in which two frequencies are made to "beat together" to produce other frequencies. Thus, 500 cycles and 1000 cycles "beat" to produce a difference frequency of 500 cycles (difference frequencies are the ones used in radio receivers) and a sum frequency of 1500 cycles. The sum frequency is used in Fig. 1 to cancel the 1500-cycle, thirdharmonic distortion which is already present. The critical factor in this circuit is the value of the lower half of R-5, 60 ohms. At any other value the third harmonics generated in the output tubes would either be too weak to cancel the original distortion entirely, or so strong as to more than balance it and thus introduce distortion themselves. It is imperative, therefore, that this value, as well as the 16 microfarads of C-10-C-13, lie maintained undisturbed in any work that may be done on the amplifier, it the quality of its output is to be preserved. THE W. E. 87-TYPE AMPLIFIER THE a.c. power input enters this 87type amplifier (Fig. 2) at the bottom of the drawing, completing its circuit through Switch D-3 and Fuse F-l. D-3 is a familiar type of switch, having three positions. In the "OFF" position, Terminals P, F and L all are open. In the F position, Terminal L is paralleled to Terminal F, closing the circuit to the left-hand power transformer, T-3. This transformer, as will be seen, lights all the filaments of the amplifier. When the tubes are warm, D-3 is thrown to the next position, Terminal P is added to the line, and the circuit is closed to the plate power transformer, T-4, through the safety-door switch, D-4. T-3, the filament power transformer, has three secondary windings. The lefthand secondary heats the filament of amplifier tube V-l. The center second ary heats the filament of V-2. From the end terminals of these windings trace directly upward to the filaments of the amplifying tubes. These separate filament windings provide a simple arrangement, to be traced in detail, for testing the plate currents of V-l and V-2 individually. The right-hand secondary of T-3 heats the filaments of the two rectifier tubes, V-3 and V-4. Plate Power Circuits Plate power is derived from the secondary winding of transformer T-4, and that winding is, of course, energized only when switch D-3 has been thrown to "P" position, and when the safety switch, D-4, is closed. From whichever end of the secondary of T-4 is positive at the moment, trace to the corresponding rectifier tube plate, V, then to the filament of the same tube, and then to the center-tap of the righthand secondary of T-3. From there straight up to L-l, through L-l, right and up through the upper portion of L-2, and right to the center-tap of the primary of the output transformer, T-2. From the two ends of that winding to the plates of V-l and V-2, and then to the filaments of those, tubes, and to their respective filament transformer secondaries, the left-hand and center secondaries of T-3. From the center taps of those secondaries, left, and to the blades of D-l and D-2. From the blades of those switches trace upward through R-9 and R-10, both of which are 1.44 ohms. [Some drawings of this amplifier show R-9 as 144 ohms, which is incorrect.] From the upper ends of R-9 and R-10 the plate currents of V-l and V-2 continue up and join just below R-4, the grid bias resistor. Trace through R-4 and then right to the fifth junction; then down to the center tap of the secondary of Transformer T-4, completing this circuit. The Safety Resistor C-3 and C-4, seen above and just left of choke coil L-l, are the filter condensers of this circuit and constitute six microfarads bridging across the rectifier output line between L-l and L-2. The lower portion of Coil L-2 is never used in this particular amplier. Resistor R-5, 1 megohm, shown be GROLIND SIDE OF LINE /OSUS VOLTS AC. S0-6S CVCLES o -»/\< o o OFF