GB755072A - Improvements in or relating to direct-voltage electronic amplifiers - Google Patents

Abstract

755,072. Valve amplifying circuits. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Aug. 3, 1954 [Aug. 5, 1956], No. 22496/54. Class 40 (6). The first stage of a direct-coupled D.C. amplifier comprises a valve with four or more grids, the signal input being applied to the third grid 6. A resistor 11 is provided in the cathode circuit, the first grid 4 is given a constant biasing potential, and the second and fourth grids are coupled together by means of a common H.T. feed resistor 10. According to the invention, in such a circuit the cathode resistor 11 and the coupling provided by the resistor 10 are so proportioned as to suppress microphony and any effects due to variation in cathode emission, and in addition give an increase in effective slope between the third grid and anode. The arrangement is shown as applied to a push-pull amplifier, the first stage of which comprises a pair of triode-heptodes, the triode portions of which are utilized for balancing the input stage: the triode portions have cathodes in common with the corresponding heptodes and obtain grid bias via individual resistors 29 and a bridging potentiometer 28, by which the currents can be balanced. The heptodes are direct-coupled to pentode amplifiers 16 which are in turn direct coupled to cathode-follower output valves 20. Overall negative feedback is applied from the cathodes of the output valves 20 to the first grid 4 in each corresponding input heptode, the . degree of feedback being adjusted by means of shunt resistor 26, while the balance is adjusted by means of a balanced T-network comprising resistors 22, 23 and a potentiometer 24 with its slider connected to earth. In a modification, Fig 2 (not shown), the fourth grid of the heptode is coupled via a resistor to the second grid, to which the series H.T. feed resistor is connected.