GB529441A - Improvements in or relating to thermionic amplifiers - Google Patents

Abstract

529,441. Valve-amplifying circuits. HADFIELD, B. M. April 17, 1939, No. 11581 [Class 40 (v)] Direct-coupled thermionic amplifiers for giving a balanced output from an unbalanced input comprise a pair of valves having a common cathode resistance, the anode of one valve having a D.C. connection to the grid of the second through a resistance potentiometer of such value that the gain of the first valve at the grid of the second is unity. The input is applied across a potentiometer R, G1 to the grid of a valve VI the anode of which is coupled by a potentiometer P1, P2 to the grid of a second valve V2 in such a way that the voltage swings on the two anodes are equal and opposite. Grid bias is provided by separate resistance r1, r2 in the cathode leads and by a common variable resistance r3. Resistances R1, R2 are provided in the anode circuits and a central sliding contact D connects these to the positive bus-bar, to which the valve screens (when pentode valves are used) are also connected through a resistance S. Feedback effects due to the resistances r<SP>1</SP>, r<SP>2</SP> may be reduced by closing a switch SW connecting the cathodes through a resistance r4 ; or a condenser may be connected between the cathodes for the same purpose. , Small inductance coils may be placed in the anode leads to raise the upper frequency limit response. The grid lead of the valve V2 may include a resistance G2 of such value as to neutralize interference voltages induced in the grid circuit of the first valve. Gain control may be effected by varying the series input resistance R. When the circuit is used for supplying cathode-ray tube deflecting- voltages, the deflecting-plates X are connected to the anodes, while the initial spot position is adjusted by varying the tapping adjustment D. Deflection coils may replace the anode resistances or a part thereof.