International projectionist (Jan 1943-Dec 1944)

through Cs to the upper end of Rs. It will be remembered that the plate current is modulated by the signal and that it flows through Rs and Re. These modulations are in phase with the signal and are impressed on the grid of VT* through Cs. Thus, since the output signal of a tube is 180° out of phase with the input signal, the signals on the grids of the two push-pull tubes are likewise 180° out of phase, which meets the requirement for pushpull operation. One more factor, however, must be considered in the design of a circuit of this type and that is that the signal strength on the grids of both push-pull tubes must be the same. This is accomplished by proper selection of the cathode resistors with respect to the plate resistor, or plate load, and considering the characteristics of the tube employed. Note that the AC voltage applied to the grid of VTi is that across the two resistors Rs and Re. This voltage is equal to, but 180° out of phase with the AC voltage applied to the grid of VT3 across Inverse Feedback Circuit Now follow the signal through the pushpull output transformer to the secondary terminal designated 24 ohms. There are three connections to this terminal, one is the output, the second goes to the jack, Ji, and the third to the inverse feedback and warping circuit. This latter circuit may be traced to the left and down to the cathode of VTi. So a portion of the signal is returned to the cathode of the first tube, together with any noise or hum that may have been introduced into the amplifier from external sources or within the amplifier itself. Since this return signal is 180° out of phase with the signal on the grid of the first tube and is, likewise, out of phase with the modulations in the cathode circuit it will not reinforce the signal, but will cancel out a portion of the signal. Of course the feedback circuit is so designed that only a small percentage of the signal on the grid of the first tube is cancelled, just enough to obtain as quiet and distortionless an amplifier as desired. The gain of the amplifier is thereby reduced somewhat, but this factor is taken care of in the design. That is, the amplifier would be designed so that with say a 10% feedback it would meet the system requirements for gain. If it is desired to feedback all frequencies, the amount of feedback is limited by a resistor in the feedback circuit. If, on the other hand, frequency selection is desired, then a resistor-capacitor mesh of the type shown in Figure 1 is used. By adjusting the values of the components of this mesh many combinations of feedback frequencies may be passed or cut out. In this way, undesired pickup may be suppressed or the frequency response of the amplifier adjusted to conform to the acoustics of an auditorium. This latter statement should be qualified to point out that not all acoustical defects can be corrected by changing the frequency response of an amplifier. It is safe to state that sound can be improved in any auditorium by this means, but if the conditions are too extreme acoustical treatment will definitely be necessary. The first mesh we come to in this circuit is composed of R2 and C6 in parallel with a removable strap across them. This combination is for low frequency equalization. With the strap removed, the higher the frequency the closer Co approaches a short circuit so that at the higher frequencies there is less and less selection of frequencies or loss through this combination. As the frequencies decrease, the value of this combination approaches the resistance of R2 (100,000 ohms ) . Thus we find that, as the frequencies decrease, the amount of the signal fed back decreases and so the frequency response of the amplifier will rise at the low end. Actually a rise of 8.5 db at 55 cycles may be obtained. The constants have been selected so that the frequency response is not affected above 1000 cycles. Continuing to the left from R2 and Co we come to a more complex mesh, which i05125 V ... so-eo crcte ™ POWEP VL SUPPLY IIVTERNATIONAL PROJECTIONIST