GB613530A - Thermionic tube amplifier automatic regulating system - Google Patents

Abstract

613,530. Automatic gain control. HOWARD, R. V. Dec. 21, 1945, No. 34719. Convention date, July 19, 1944. [Class 40 (v)] A slow acting A.V.C. system is combined with a fast acting A.V.C. system which operates only when the signal level is above a predetermined value. The invention is described for an audio-frequency channel comprising an input transformer T1, amplifiers 6K7 and an output transformer T2 feeding through an attenuator R5 to output terminals 4. Energy is fed from the input to an A.G.C. amplifier 5 feeding a double diode 6H6-1 having a load resistor R15, R16. A voltage-taken from tap 8 is fed to control the gain of amplifier 5. The full voltage across R15, R16 is fed through the upper diode in 6H6-2 to charge a condenser C6 shunted by high resistance R17, while the partial voltage across R16 is fed through the lower diode in 6H6-2 to charge a condenser C7. Condensers C6, C7 are connected by the upper diode in 6H6-3 and resistance R26. The voltage on condenser C7 is taken at point 10 and fed at point 12 to control the gain of the main amplifying channel. When the signal is increasing, the negative voltage on condenser C6 is greater than that on C7, and the gain of the amplifier is steadily reduced by the voltage on C7. If the signal falls, the gain remains steady, as C7 has no leak, until the voltage on C6 has reduced due to R17 so as to be less than that on C7. At this point, the upper diode in 6H6-3 conducts and condenser C7 then also discharges through R17 and the gain of the main amplifier rises at a rate determined by R17, the device giving a slow acting A.G.C. If, however, the signal ceases, the gain rises to a maximum in about 10 seconds, and the system is not sufficiently quick-acting to deal with a high signal level as soon as the input is restored. To deal with this, a quick-acting A.G.C. system 6 is supplied from the output of the main channel through transformer T5 which, through a triode in 6SR7 and transformer T4, feeds balanced diodes in 6SR7 to provide a voltage which is fed through a diode D4 in 6H6-3 to charge condenser C7 and thus provide a gain control voltage until the normal system becomes operative. The diodes in 6SR7 are biassed so as to conduct only above a predetermined signal level and act only for a short time at the beginning of a signal. A slightly modified circuit producing a similar effect is also described (Fig. 2, not shown).