Patent GB-635585-A (GB)
Title: Improvements in or relating to signal transmission systems
IPC: H04B3/20
Filed: 1947-06-27
Abstract: 635,585. Thermionic valve amplifiers; automatic gain control. WESTERN ELECTRIC CO., Inc. June 27, 1947, No. 17023. Convention date, Nov. 18, 1939. [Classes 40 (v) and 40 (vi)] [Also in Group XXXIX] The negative feedback loop of a thermionic valve amplifier includes a frequency-variable impedance in the output circuit of a rectifier, the output of which includes frequencies at which the impedance has high and low values. As shown, the amplifier forms part of a gain increaser circuit controlling the gain of a vario repeater, which may connect a radio receiver with a telephone line or a loud-speaker. The vario-repeater comprises two valves 7, 8 fed in push-pull from a transformer 23 connected across a copper-oxide rectifier bridge 9. The gain of the repeater depends on the shunting effect of the rectifier bridge 9, controlled by the potential drop across resistor 36 caused by the current flow through valves 7, 8. The output of valves 7, 8 is fed to a negative feed-back amplifier 25, which feeds the outgoing line over a hybrid coil 26. A gain decreaser circuit is fed from a high-pass filter 37 connected to the hybrid coil 26. When the output of the repeater rises above a certain level, condenser 28 is charged through a rectifier 43 and resistors 46, 32 so as to reduce the current flowing in valves 7, 8 and thus increase the shunting effect of the rectifier bridge 9. A condenser 29, larger than condenser 28, is connected in series with resistors 30, 31 and 32 across condenser 28 to control the gain of valves 7, 8 for prolonged changes in volume on the line. Gain increaser circuit.-Part of the input to a repeater is fed through a filter 50, which selects the voice-frequency range, to an amplifier 52, the output of which feeds the diode sections of double-diode triode valves 54, 55 in parallel. The output circuit of the diodes comprises a frequency dependent impedance 66 and resistors 67, 68 in series. The triode sections of valves 54, 55 are controlled by the output of the diodes taken across resistors 67, 68 respectively, and have a common negative feedback circuit including coil 66, to eliminate the high frequencies. The output of the triodes is fed through the operating coil of a gain rate relay 71 to a transformer 72. A condenser 73 serves to select impulses of syllabic frequency, which are applied to the operating coil of the gainincreaser relay 74. Rectifiers 86, 87 ensure that relay 74 is only operated on the rising portion of a speech syllable. When relay 74 is operated a positive charge is applied to condensers 28 from battery 84 through the gain rate relay contacts, and resistors 91, 31, 32 to reduce the negative potential applied to the grids of valves 7, 8 and decrease the shunting effect of rectifier bridge 9. When the signals on the incoming line rise above a certain level, the output of the triode sections of valves 54, 55 decreases, the gain rate relay 71 is released, and the positive charge applied to condenser 28 on operation of the gain increaser relay 74 is decreased by the voltage drop across resistor 89. When the gain increaser relay 74 is released a negative potential is applied to the armature from a battery 35, to prevent leakage of any positive charge to the open contact.