GB472256A - Improvements in and relating to thermionic amplifiers - Google Patents

Abstract

472,256. Valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd., and ROCHE, A. H. March 18, 1936, No. 8109. [Class 40 (v)] In a negative feed-back amplifier incorporating an approximately balanced output bridge so that the output circuit is conjugate to the feed-back path a portion of one arm of the bridge is adjustable to vary with frequency the amplifier output. The Figure shows a threestage amplifier VI, V2, V3, in which feed-back is taken through condenser 17 from one diagonal of the output bridge comprising V3 and R15, R16, R17, the output transformer L6 being connected across the other diagonal of the bridge. The feed-back voltage is applied across resistances PM, R22 which are variable to control the gain, resistance pads A1 in the input may be used to supplement the gain control. A variable condenser C18 is provided to maintain the balance of the output bridge at high frequencies and used to make the output impedance increase or decrease at high frequencies. The resistance R15 is made variable in steps over a small range to facilitate matching of the output circuit. A variable condenser C, parallel with the input transformer is provided and adjusted so that the imput impedance remains constant over the operating range. Jacks J1 ... J4 are provided for metering and a relay S1 to indicate valve or supply failure. The output valve is of the co-planar grid type positive bias being obtained by the potentiometer R20, R21 across the anode supply and from the low tension battery. The intervalve couplings comprise transformers with their windings in series with the anode resistance and grid leak.