Patent GB-605064-A (GB)
Title: Improvements in or relating to wide band thermionic valve amplifiers
IPC: H03F1/50
Filed: 1945-11-29
Abstract: 605,064. Valve amplifying circuits. RIBET & DESJARDINS. Nov. 29, 1945, No. 32303. Convention date, June 14. [Class 40 (vi)] In a wide-band thermionic valve amplifier with intervalve resistance capacity coupling, the resistance in the plate circuit of the first valve is of high value relative to the resistance in the grid circuit of the next valve. The coupling condenser may have a capacity of several microfarads. Fig. 3 shows an amplifier circuit in which the plate load resistance 2 is of the order of 25,000 ohms and the grid-leak resistance 5 is of the order of 1000 ohms; the coupling condenser 4 may be an 8 ÁF. electrolytic condenser. It is stated that the low value of resistance 5 permits this resistance to be made variable for the purpose of gain control without resulting in modification of the gain/frequency or phase/frequency characteristics. A compensating circuit to counteract changes in supply voltage is described (included in Fig. 8) which reduces the need for decoupling circuits. The screen of valve 1 is connected to a capacity potentiometer 15, 16, connected across the supply, condenser 15 being shunted by resistance 14, the circuit elements being chosen so that SRp=Ce+C<1>e/Ce where S equals the screen mutual conductance of the tube, Rp equals the value of resistance 2, and Ce and C<1>e equal the values of condensers 15 and 16; it is stated that the high value of resistance 2 enables this equation to be satisfied. The degree of compensation may be adjusted by means of a variable resistance in series with resistance 2. The effect of variations in the supply voltage is also substantially decreased by the potentiometer effect due to the high resistance 2 being in series with the large capacity 4 and low resistance 5. Fig. 5 shows a modified form of the circuit in which the grid resistance 5 is connected to the cathode of valve 1, a bias resistance not shunted by a condenser being connected in the cathode circuit of valve 1. A push-pull amplifying circuit is also described (Fig. 6, not shown) in which resistance 5 is made approximately equal to resistance 17, and the inputs to the push-pull pair of valves are taken from these resistors. A further modification (Fig. 7, not shown) provides for a variable capacity to be connected across resistance 5 when this is used as a gain control, a uni-control arrangement being provided such that the time constant of these two elements remains constant during adjustment, it is stated that this prevents phase variation at high frequencies, when the gain control is operated. Fig. 8 also shows a circuit for dealing with very low frequencies, wherein condenser 22 in shunt with resistance 24 is connected between resistance 5 and earth, and condenser 4 is shunted by resistance 23. It is stated that if the time constant of condenser 4 and resistance 2 equals that of condenser 22 and resistance 5, and the time constant of condenser 4 and resistance 23 equals that of condenser 22 and resistance 24, the transmission is independent of frequency when this approaches zero.