Patent GB-251041-A (GB)
Title: Improvements in thermionic valve circuits for wireless signalling and like purposes
IPC: H04B15/00
Filed: 1925-02-04
Abstract: 251,041. Western Electric Co., Ltd., (Western Electric Co., Inc.). Feb. 4, 1925. Thermionic generators, amplifiers, and modulating systems.- The input circuit of a thermionic valve comprises an inductance coil and a capacity in series with a resistance, across which conmection is made to the grid and filament of the valve, with the object of reducing inherent capacity reaction effects. The filament is supplied with alternating current, and the resulting hum is neutralized by applying to the grid a voltage of the same frequency but adjustable in phase. As shown in Fig. 2, a thermionic or other oscillator 1 is loosely coupled to a circuit 3, 4, 5 which is tuned to the oscillator frequency, but is damped by a series resistance 5. One side of the resistance 5 is connected to the grid of the amplifier 6 and the other is connected to the filament through a biassing-potentiometer 11, 12 (shunted by a condenser 22 and adjustable resistance 23). Alternating current is supplied to the filament through a transformer 15 from a source 14 which is also connected to the primary coil of the transformer 27. The relative phase of the potentials supplied by the source 14 to the grid and filament circuits is adjusted by varying the resistance 23 and a condenser 60 across the filament. The plate circuit may be connected to the filament through the transformer secondary coil 27, so that the compensating voltage is applied to the plate as well as the grid circuit. Reaction effects of the plate circuit on the grid circuit, due to internal capacity &c., are reduced by the damping resistance 5, and the frequency of the oscillator 1 may thus be kept substantially constant in spite of variations in the impedance of the aerial circuit 26. In a modification, the resistance 23 and condenser 60 are omitted, and the phase of the compensating voltage is adjusted by variable impedances interposed between the source and the primary coil of the transformer 27. Fig. 3 shows the invention applied to a radio telephony transmitter, comprising a control oscillator 1, power amplifiers 6, 38, 39, and speech amplifiers 40, 41, 42. Grid bias is applied to all the valves from a source 11 through leads 28, 29, potentiometers and filters 30. Plate current is supplied from a direct-current source 16 through a filter 31. The plate circuit of the parallel power amplifiers 38, 39 includes a choke 46, and that of the parallel speech amplifiers 41, 42 includes a choke 55, a large condenser 56 being connected across the chokes. The filaments are connected in parallel across leads 36, 29 energized through a transformer 37 from one phase of the three-phase supply 35. To supply a compensating voltage wave to the grids of one or more of the valves, resistances 58 are connected across the other two phases of the source 35 and a transformer coil 60 is connected across adjustable portions of these resistances, the secondary coil 61 being connected in a grid circuit. As shown, the compensating voltage is applied only to the grid circuit of the first microphone amplifier 40.