GB466831A - Improvements in and relating to the control of thermionic devices - Google Patents

Abstract

466,831. Valve amplyifying and modulating circuits. BRITISH THOMSONHOUSTON CO., Ltd. Feb. 8, 1936, No. 3872. Convention date, Feb. 9, 1935. [Class 40 (v)] The anode supply voltage, and the grid bias, of an amplifier or modulator valve operating on the straight part of its characteristic curve, are varied automatically in response to the volume level, varying at syllabic frequency, of the audio signals impressed thereon ; the object being to increase efficiency by economizing anode current. The Figure shows a choke modulating-circuit in which the carrier-wave output of the valve 11 is modulated by a valve 18, the two anode circuits being fed from the same source of high-tension current through a common choke 31. The grid of the valve 18 is fed with modulating- current by a source 13 and amplifier 14 through a transformer 16. Modulating-current also feeds a rectifier 32 the output of which is smoothed by a filter 33 designed to remove the modulating frequencies but to retain syllable frequencies, which appear as variations of voltage across the resistance 35. Anode voltage for the valves 11, 18 is supplied by mains rectifiers 23, 24 shown as full-wave rectifiers of three-phase supply current. The voltage across the resistance 35 is applied through a polarizing-source 36 to grids 27 in the rectifiers 24, thus causing variation in the D.C. output of the rectifiers. This is applied to the valves 11, 18 through a smoothing circuit 28 designed to remove the supply frequency, but to retain the lower syllablefrequency caused by the grids 27. In this way the supply current to the valves 11, 18 is raised and lowered at syllable frequency. A self-biassing resistance 20 shunted by a condenser 21 may be arranged to cause a corresponding variation of grid bias. A voltagelimiting diode 38 is connected across the resistance 35 in series with a fixed voltage source 39. The system has the effect of reducing carrier strength when the modulating signals are weak. The Specification as open to inspection under Sect. 91 describes modifications in which only the anode voltage, or only the grid bias, is varied at syllabic frequency. This subjectmatter does not appear in the Specification as accepted.