GB504960A - Improvements in or relating to audio frequency thermionic amplifiers - Google Patents

Abstract

504,960. Valve amplifying circuits. FERRANTI, Ltd., SEARBY, N. H., and THOMAS, G. 1. Jan. 12, 1938, No. 965. [Class 40 (v)] An audio-frequency push-pull amplifier comprises two valves 9, 11, the valve 9 being fed from the preceding stage 18 and the valve 11 by the potential developed across the resistance 10, which is included in the output circuit of valve 9 and is connected in series between the two similar sections 7, 8 of the primary of the output transformer 6. Since the grid circuit of valve 11 does not contain an inductive element, the stability is increased. The transformer sections 7, 8 are wound so that the fluxes due to direct currents are in opposition and resistance 10 is such that the potentials across it are equal in magnitude whilst opposite in phase to the input potentials of valve 9. In order to balance out any hum in the output caused by ripple in the high tension supply P, half ot the hum potential of the supply is applied to the control grid of valve 9 and consequently an equal hum potential is applied in opposite phase to the control grid of valve 11. Since the whole of the hum potential is directly applied to the control grid of valve 11, the net hum potential on the grid of valve 11 is equal to and in phase with that on the grid of valve 9 ; hence none appears in the output. The required hum input to valve 9 may be developed across one arm of a potential divider formed by a preceding valve and its associated circuit elements, Fig. 4 (not shown). In the figure the hum input is obtained by adjusting the hum input to the preceding valve 18 by the capacity potential divider 19, 20. Alternatively the capacity potential divider may be replaced by a resistance potential divider, Fig. 6 (not shown). In Fig. 7 (not shown) the control grid of valve 11 is connected to a tapping on its grid leak so that the hum applied directly from the source is twice that applied from the output of valve 9.