International projectionist (Jan 1943-Dec 1944)

voltage with its own positive and negative terminals — the positive lines then running through series resistors and coaxial cable to the anodes of the photocells while the negative legs, of course, trace from cathode through the coaxial shells to chassis. Additional filtering is connected across the voltage dividers in the form of condenser C-17, drawn just to the right of R-23. The power filter of the entire plate circuit consists of two choke coils, 1505 and 0117-A. together with the 4 mfd and 6 mfd condensers drawn just below them. The microphone supply is filtered by the field winding of the loudspeaker or speakers, and additionally, as already noted, by C-13. C-20, under coil 0117-A, would appear to be a stabilizing condenser introduced as further precaution against possible undesired feedback. Grid Bias Circuits As already found in tracing the plate circuits, the plate current of each tube completes its path through resistors wired in series with the tube's cathode — in the case of the pushpull tubes, in series with the center taps of the filament supply windings of the power transformer. The negative side of each bias resistor is con^ nected to ground, and the control grid of each tube is also in some way connected with grid. Thus in every tube the voltage of the control grid is more negative than that existing at cathode, by the extent of the voltage drop through the tube's bias resistor. That voltage drop, of course, corresponds to the amount of plate current flowing through the bias resistor — which is identical with the amount of space current flowing through the tube. The space current of each tube varies momentarily according to the volume of speech current which the tube is amplifying. To prevent these variations from appearing as momentary changes in grid bias, each bias resistor is by-passed by a condenser that exerts a filtering action. In the case of the left-hand tube regard C-3 as a source of voltage, the positive side of which is connected to cathode and the negative side, through R-2, to grid. C-2 is the filter condenser. In the circuit of the next tube C-6 is the by-pass condenser; R-7 is the bias resistor wired positive to cathode and negative (through R-6 and a variable tap connection ) to grid. R-10 is the bias resistor of the next tube, wired negative to grid through R-9, and C-10 is the by-pass condenser. In the case of the pushpull tubes, R-26 and R-27. as already seen, are the bias resistors. They are connected positive to their respective filaments through their respective filament supply secondaries; while their negative ends are grounded to chassis, and the grids of the pushpull tubes also ground to chassis through the secondary of the coupling transformer. Speech Circuits The direct current flowing through the photocells pulsates in strength according to the light and dark areas of the moving sound track. Thus each cell can be regarded as the source of a pulsating voltage or a.c. component. Supposing cell A to be in operation, the circuit connected to that cell can be traced from anode right through C-l, down through R-l and thence to cathode. A branch circuit can be traced as follows: Positive leg — right through C-l and the upper terminal of S-l, right through the closed contacts of jack 954, to the control grid of the first tube. Negative leg — from cathode through chassis, up from chassis to the bottom of R-3, left, up through C-2 and right to the cathode of the same tube. The tube's control grid and cathode, regarded together as constituting a condenser, are the load in this circuit. Switch S-l is the changeover between cells A and B. An extension of the shaft of this switch appears behind the casing of the amplifier and, of course, revolves through an arc when the switch is thrown from side to side. Projector changeover devices can be attached to this protruding shaft, and in that way visual changeover is effected simultaneously and automatically with the switching of the sound. Another interesting feature of this amplifier is that S-l has extra terminals (also designated S-l and drawn to the right of R-5). While changeover is actually in process these additional terminals momentarily short circuit R-5, cutting off all sound for the fraction of a second involved. Microphone input is connected directly (through receptacle 20001) to the blade of the sound changeover switch, and therefore can actually be superimposed upon (mixed with) the film sound if desired. Regarding R-17 and the microphone (in series with each other) as the source of pulsating microphone output current, the positive end of that source is wired to the grid of the first tube and the negative side, through the chassis connection at the lower end of R-17, and through C-2, is wired to the tube cathode. If the microphone and its adapter are of the dynamic type the output will be a.c. and not pulsating d.c. In that case one leg connects to grid through the blade of the changeover switch, and the other leg connects to cathode by virtue of the ground connection at receptacle 20001. If a phonograph is plugged into jack 954 both the microphone and the photocells are completely disconnected by the opening of the jack's auxiliary contact. One side of the phonograph line connects to grid through the tip of the jack, and the other side connects to chassis through the sleeve of the jack. The cathode and plate of the first tube may correctly be regarded as the terminals of a source of amplified pulsating current, and the circuit connecting to this source may be traced up and right from plate, down through R-5, right through C-5 to chassis, left through chassis to the lower end of C-2, and so back to cathode. Since this current flows through R-5, that resistor may be regarded as the source of a pulsating voltage drop, and the circuit connected to it can be traced right through C-4, down through R-6, left through C-5. (This source, R-5, can be regarded as the point of origin of all speech current appearing iurther on in the amplifier, and it is completely short circuited, as explained, during instants of changeover.) Source of Pulsating Voltage Since the current just traced completes its path through R-6, that resistor in turn may be regarded as a source of pulsating voltage drop. Any desired portion of that voltage drop is tapped off by the variable connection, which is the volume control. The source of pulsating voltage consisting of that portion of R-6 between the point at which the sliding contact is set and the lower end of R-6 is wired to grid and cathode of the next tube through C-6 ; the tube's grid and cathode together, considered as a condenser, constituting the load. The amplifier's speech circuits continue in practically identical manner to the primary winding of the coupling transformer. However, there are two identical devices inserted in those circuits. The tone control consists of resistor R-9, together with R-28, coil 15096 and C-8. Coil 15096 and condenser C-8, in series, constitute a tuned bypass circuit through which some of the speech current output of the second tube is shunted or short circuited, and thus prevented from appearing across the grid and cathode of the third tube for further amplification. The very small capacitance of C-8 indicates that it is principally the higher frequencies that will be short circuited by this device, but the volume or extent of h.f. attenuation will depend on the setting of the slider of R-9. With that slider at the bottom of R-9, h.f. attenuation will be maximum. In series with the plate circuit of the next tube there is a low frequency attenuator. With switch S-3 set at the bottom terminal the output circuit from the plate of that tube runs through coil C-100-B directly to the coupling condenser C-ll and to the primary of the coupling transformer ; but with S-3 set at the upper (Continued on page 30) 12 INTERNATIONAL PROJECTIONIST