GB446891A - Improvements relating to electrical sound amplifiers - Google Patents

Abstract

446,891. Valve amplifying circuits. PEDLER, R. D. CARTER-, 187A, West End Lane, Kilburn, London. Aug. 1, 1934, No. 22381. [Class 40 (v)] A high power audio frequency amplifier comprises a plurality of direct-coupled stages, each stage comprising a pair of valves connected in push-pull wherein each stage has an independent source of H.T. supply. In the arrangement shown in Fig. 1 the input from a microphone B is applied in phase opposition to the grids of power valves E, E1 which are balanced by a slider on the grid biassing resistance F connected to the common point of grid resistances D, D<1> and HT-. The anode of each valve is directly connected to the grid of the succeeding stage E2, E3, antioscillation resistances I, I<1> being included if necessary. The anode supply is connected in each valve pair to the centre of resistances G, Gl, G2, G3 &c. Individual anode voltage supply systems are provided for each stage of the amplifier even the valve heater supply being separate from each stage. For high power outputs, e.g. 50 watts, the last valves E4, E5 are directly heated. The grid-cathode circuit is then taken through resistance H3 to the centre of potentiometer H4 across the cathode. In an alternative arrangement the first of the three stages of Fig. 1 is replaced by a single power valve transformer coupled to the push-pull input of two double stages, Fig. 2, (not shown). A complete arrangement of this kind is shown in Fig. 3. The input is derived from a plug S1 supplied from a pick-up source connected across r1 or a microphone from transformer t1, mg being the microphone jack. The first amplifier V1 is transformer-coupled at t2 to the push-pull pair V2, V3 direct-coupled to the directly-heated output stage V4, V5. Valves V1, V2, V3 are supplied from one rectifier system t4, V6, and V4, V5 from the separate system t5, V7. Resistance r4 and condenser c4 are a decoupling combination, while c2, r3 shunted across the primary of t2 causes the amplifier to discriminate slightly in favour of the lower frequencies, thereby correcting for insufficient bass response with gramophone record reproduction. The anode supply to the directly heated pair V4, V5 preferably includes a thermal delay switch, to prevent application of anode voltage until the indirectly heated valves are taking load.