Patent GB-460562-A (GB)
Title: Improvements in and relating to thermionic valve circuits
IPC: H03F1/20
Filed: 1935-07-24
Abstract: 460,562. Valve amplifying circuits. PERCIVAL, W. S. July 24,1935, No. 19674/36. Divided application. [Class 40 (v)] Valve amplifiers particularly intended for handling a.wide band of frequencies comprise a number of similar valves 6, 7, 8, Fig. 1. with their control grids connected in parallel with the interposition of lumped impedances, e.g. inductance coils 16, and with their anodes similarly connected, in such a way that the impedances 14, 16 together with the effective shunt capacities form two wave filters with the same speed of propagation, and the voltage or power fed to the output circuit is the sum of the individual outputs of the several valves. Fig. 1 shows three screen-grid valves 6, 7, 8 connected in this way, with their output fed to a similar set of valves 9, 10, 11. The input is applied at the terminals 15, and the low-pass filter comprising the coils 16 and the grid-cathode capacities is terminated in a network comprising inductance coils 17, 18, a condenser 19 and a resistance 20, which is designed to avoid reflection. The input capacity at the terminals 15 is such as to maintain the input impedance of the filter at a constant value substantially up to the theoretical cut-off frequency. The anode filter comprising coils 14 and the anode-cathode capacities is similarly terminated at the end where the input is applied. At the output end, the anode of valve 8 is connected through a coil 14 to the grid of the first valve 9 of the second set 9, 10, 11 which is arranged similarly to the first set, the final output being taken from the terminals 22. The resistances 20 may be employed to feed anode and grid bias voltage to the valves. A suitable network may be inserted between the anode of the valve 8 and the grid of the valve 9 to provide impedance matching when the anode capacity differs from the grid capacity, Fig. 2 (not shown). The same object is attained in Fig. 3 by connecting the anode of the valve 8 through a coil 14 to the grid of a screen-grid or triode valve 30 which has in its cathode lead a resistance 31 high compared with the reciprocal of the valve mutual conductance. The gridcathode circuit of the first valve 9 of the second set is connected across the resistance 31 through a coil 32 which has half the inductance of the coils 16. A transformer may be interposed between the resistance 31 and the following grid circuit. The effective grid capacity of the valve 31 is made equal to the anode capacity of the valve 8 by adjustment of the resistance 31 or by connecting a condenser between the screen-grid and the cathode. The filters may be arranged as band-pass filters by connecting condensers 46, Fig. 4, in series with the inductances 45 and by using shunt inductances 47. As shown in Fig. 4, the filter is used in the grid circuits of a set of three valves 41, 42, 43 operating as anode-bend detectors. The grid filter is designed to pass a desired carrier wave and its side-bands ; whilst the anode filter comprising coils 48 is of the low-pass type cutting off near the highest signal frequency. The valves may operate as grid-leak detectors if resistances are connected in series with the coils 47. One set of valves arranged as a filter may be connected in push-pull relation to a second set and neutralizing condensers may be crossconnected between anodes and grids. When the valves are used for supplying power to a load, the characteristic impedance of the filter is made equal to the impedance of the load over a desired operating range of frequency. Both the signal source and the load, when capacitative, may form elements of the filter. The filter connection between the several valves of a set has the effect of increasing the signal-to noise ratio as compared with ordinary parallel-connected valves.