GB248731A - Improvements in thermionic valve amplifying systems - Google Patents

Abstract

248,731. British Thomson-Houston Co., Ltd., (Assignees of Rice, C. W., and Kellogg, E. W.). March 3, 1925, [Convention date]. Thermionic amplifiers. - In a valve amplifier particularly for use with rectified A.C. supply, the polarizing winding of a telephone acts as a smoothing choke, and the grid bias is obtained from a potential drop in the plate circuit and varies with the current in the latter. The input is applied to a jack 11, its D. C. component being shunted through the choke coils 35, 36, wound upon two cores 37, 38, which can be fitted together to form a closed magnetic circuit as shown,. The coils are oppositely wound so that stray magnetic coupling with the output of the system may be prevented, and the choke may be screened by an earthed metal casing 41. The grid G1 of the first amplifier 1 is connected to a variable tapping 34 on the input resistance 33, and the amplifier 2 is resistance-coupled to the amplifier 1. The output is applied through a transformer 26 to the moving coil 27 of a receiver, this coil being adapted to actuate a diaphragm (not shown) and located in the airgap of a pot magnet 29 which is polarized by the coil 15. The circuit of the coil 27 is closed bv the switch arm 40 after the switch 39 has closed the filament heating circuit of the valve 1 and the primary circuit 5 of the transformer which supplies filament current and plate voltage to the remaining valves. The plate circuit of the valve 2 passes from the plate P2 through the primary of the transformer 26, magnetizing coil 15 of the receiver (which acts also as a choke) middle point of the winding 8, rectifiers 7, winding 6, earthed line 20', resistance 30, winding 9, and filament F2. Condensers 24 and the choke 25 serve to smooth the plate voltage derived from the rectifiers F. The grid G2 is connected through the resistance 31 to a point which is separated from the filament F2 by the resistance 30. The potential drop along this resistance due to the plate current thus determines the grid bias, and an excess plate current produces an increased grid bias, so that the arrangement is self-regulating. Similarly the grid bias. of the valve 1 is determined by the potential drop in the resistance 19 which carries the plate current of that valve. The winding 15 carries current additional to that passing through the valves, the extra current following a shunt path through the resistance 32.