International projectionist (Jan 1943-Dec 1944)

slider of P.,. Thus we obtain potential on P,. Now if the slider moves upward in the figure, the voltage applied to terminal No. 1 increases while that on No. 2 decreases. The output level of machine No. 1 will increase while that of No. 2 will decrease and so the machines may be accurately balanced. Microphone Jack Provided Before we leave the first tube circuit let us look at J^. This jack is provided for the connection of a phonograph or microphone attachment. The contacts are shown with the attachment plug removed from the jack. In this position, the circuit is set up for film reproduction, Rj being shorted out and sound passes directly from the input terminal to the grid. When the attachment is plugged in the jack springs move upward in the figure and the junction of Rx and Cx is grounded, thus the machine outputs are grounded through Cx. The attachment input is across R, (500,000 ohms), its value indicating a high impedance pickup unless a coupling transformer is employed. We thus have a simple method of providing for the use of an attachment. Still in cathode circuit of the first tube in Figure 1 we find an arrow, which actually is a removable strap, pointing to C, and C3. This provides one step in high frequency equalization and is used in conjunction with the warping circuit provided in the inverse feedback circuit in the power amplifier. Let us see how this circuit functions. Current flowing from plate to cathode is modulated by the signal or in other words has the signal current superimposed on it. With the strap disconnected, the signal passes through R3 to the ground bus then through C4, Rt. and R9 to the junction with Cg, thus completing its path. Now when the strap is connected to C, and C, these two capacitors are in parallel with R3. The impedance of the capacitors decreases as the frequency increases and therefore the impedance of the circuit from the cathode to ground decreases as the frequency increases. Thus the higher the frequency, the more signal modulation in this circuit and the more through R9. In this way the AC drop across the plate resistor increases and thereby the gain of the stage is increased. C4 is 0.05 mf compared with 0.01 mf each for C2 and C3 so that its impedance is comparatively low. By proper selection of the values of C2 and C3 in conjunction with those of the cathode and plate resistors, an infinite number of high frequency curves may be obtained in theory. In practice the number is limited by several design considerations; such as, the gain required in accordance with overall systems requirements, the type of tube best suited for the purpose, limitations in the value of the plate resistor and the practical consideration of the family of high frequency curves usable in the types of theatres encountered. You may wish to apply this same analysis to \T± in Figure 1 to determine that a variation in the resistor R3 will change the gain of the amplifier. Electronic Sound Changeover Following the sound circuit from the input at the upper left of Figure 1 through Ca, VT1 and C2 we arrive at Pj, which is main system volume control — and remember that there is one for each machine. This volume has a total attenuation of 40 db in twenty steps of 2db each. Now follow the lower end of Px down through R6 to terminal "A" and note the adjacent terminals "B" and "C". These three terminals constitute the sound changeover circuit. Exciter lamp changeover also is employed in conjunction with sound changeover when this amplifier is used. The circuits used are of the three-way type, so arranged that only one amplifier can be used at a time and so that changeover may be made at either machine regardless of which machine is in operation. In the operating amplifier terminals ''A" and "B" are connected together while in the other amplifier terminals "A" and "C" are connected. In principle, therefore, changeover is accomplished by operating a single pole double throw switch in which the swinger is connected to terminal "A". When Terminal "A" is connected to terminal "B" the lower end of R6 is connected to the junction of R10 and R12 and the grid is negative with respect to the cathode by the amount of the voltage drop through R7. When terminal "A" is connected to terminal "C", the grid returns to ground through R13 and we find that the grid is negative with respect to the cathode by the drop across the two resistors R7 and RX1. The grid is then said to be biased beyond cutoff and the amplifier is inoperative. Also C5 is being charged by the voltage at the top of R„. When the "A-B" connection is made, this capacitor is discharged through R6, which retards the transition from the inoperative to the operative condition and makes the changeover quiet. Under this circuit condition C4 is now being charged by the voltage from the top of Rxl through R6. When the "A-C" connection is made this capacitor is discharged through R6 and R13. Here again we have the retarding effect conducive to noiseless changeover. C4 also acts as a filter capacitor. The rate at which these two capacitors discharge is known as the time constant of the circuit and is dependent, considering a given size of capacitor, upon the value of the resistor. The lower the resistor, the more rapid the FIGURE 2 INP O oa/r FIL —<56#D < » — vAA/W\ OPLAn 10 INTERNATIONAL PROJECTIONIST