GB273826A - Improvements in or relating to telephone transmission systems - Google Patents

Abstract

273,826. Standard Telephones & Cables, Ltd., and Hinton, L. T. April 12, 1926. Thermionic amplifiers.-A thermionic valve used as an amplifier or repeater in a telephonic or like transmission system has an extra control electrode for the purpose of automatic gain regulation. As shown in Fig. 1, the amplifier V is connected in the usual way between an input circuit I and an output circuit O. The incoming currents are partially shunted and applied to a rectifying-valve K. The plate circuit of the rectifier includes a resistance R and a choke L for suppressing speech frequencies, which are bypassed by a condenser C1. The resistance R is included between the earthed filaments and the extra control grid G2 of the amplifier. The voltage thus applied to the grid G2 is varied in proportion to the strength of the incoming signals; and the gain of the amplifier is thereby varied so as to maintain a standard strength of signal in the output circuit O. In the modification shown in Fig. 2, a pilot wire a, b extends adjacent to the telephone lines so as to be subjected to the same climatic and temperature variations. Direct current is sent through the pilot line from a source D through a variable resistance R, and the circuit of the extra grid G2 includes the resistance R. Any alteration in the resistance of the telephone lines will be accompanied by a similar alteration in the pilot line, and will be reflected in the voltage applied to the grid G2. The pilot wires a, b may form a complete loop, with the battery D and resistance R connected across the loop. The pilot wire circuit may comprise a phantom circuit.