Patent GB-668210-A (GB)
Title: Improvements in or relating to thermionic-valve power amplifiers
Inventor: PHILIPS NV
IPC: H03C1/20
Filed: 1949-09-27
Abstract: 668,210. Valve amplifying circuits. PHILIPS ELECTRICAL, Ltd. Sept. 27, 1949 [Sept. 30, 1948], No. 24748/49. Class 40 (vi). A high power amplifier comprises a grid current controlled Class B output stage which is driven from the voltage developed across an impedance in the cathode circuit of a Class B pre-amplifier stage, the impedance comprises a resistor and inductor connected in series, the control grid of the pre-amplifier valve is connected via a resistor to the junction point of the inductor and resistor and the grid bias for the output amplifier is supplied to its control grid via the cathode impedance of the amplifier stage. The Figure shows a push-pull amplifier in which speech currents generated in the microphone 1 are amplified in the stages 4, 4<SP>1</SP>; 14, 141 and in the Class B pre-amplifier and pre-output amplifier stages 22, 221 and 24, 241 respectively and are then modulated upon a carrier from a source 37 in the modulator stages 28, 281. The anode and screen stages voltage of stages 4, 4<SP>1</SP> and the screen grid voltages of stages 14, 141 and 22, 22<SP>1</SP> are supplied from a H.T. source 11 and a H.T. source 20 supplies voltage to the anodes of stages 14, 141 and 22, 221. The anode voltage of the output amplifier and modulator stages are supplied from a H.T. source 54. The grid bias voltage to the output amplifier stages 24, 241 is supplied from a source 45 via the cathode impedances 40, 41 and 401, 41<SP>1</SP> respectively and the grid bias voltage to the modulator stages 28, 28<SP>1</SP> is supplied from a source 33 via H.F. chokes 32, 32<SP>1</SP>. In order to economize in power consumption the amplifier stages 22, 22<SP>1</SP> and 24, 24<SP>1</SP> have a larger negative bias than for normal Class B operation and the consequent distortion is reduced by negative feed-back via leads 51, 51<SP>1</SP> from the output loads 50, 501 of the output amplifier stage 24, 241 to the control grid circuits of the input stages 4, 4<SP>1</SP>. By tapping the grid leak resistors 42, 42<SP>1</SP> on to the junction point of the resistor, inductor networks 40, 41 and 401 and 411 respectively a very high input impedance is obtained for stages 22, 22<SP>1</SP>. The inductors 41, 41<SP>1</SP> in the cathode circuits of stages 22, 221 form the primary windings of transformers, the secondaries 48, 48<SP>1</SP> of which are connected in the screen-grid circuits of these stages; in this way the screen grids are at substantially the same alternating voltages as the control grids or cathodes and the effect of stray capacities in the valves 22, 221 is considerably reduced. The capacitors 49, 49<SP>1</SP> reduce undesired effects at high frequencies due to parasitic couplings between the windings of transformers 47, 471. The hum level in the amplifier is considerably reduced by the high degree of negative feedback used and as the modulation choke 56, the transformer output impedance of the power amplifier and capacitor 57 are effectively in series with respect to hum voltages appearing in the supply voltages for modulator stages 28, 28<SP>1</SP>. Owing to the high degree of negative feed-back the transformer output impedance of the power amplifier is very low and as the A.C. impedance of capacitor 57 is preferably even lower the high A.C. impedance of choke 56 causes a very considerable reduction in the hum level. Specification 532,793 is referred to.