Patent GB-402431-A (GB)
Title: Improvements in series coupled thermionic amplifiers
IPC: H03F3/44
Filed: 1932-02-25
Abstract: 402,431. Valve amplifying, detecting, and generating circuits. DUBILIER CONDENSER CO. (1925), Ltd., Victoria Road, North Acton, London.-(Assignees of Nyman, A. ; Genehurst Apartments, Dobbs Ferry, Westchester, New York, U.S.A.) Feb. 25, 1932, No. 5660. Convention date, Feb. 25, 1931. [Class 40 (v).] A valve amplifier comprises a number of valves T1, T2, T3 with their anode-cathode paths in series across the supply voltage 1, 2 and with the input applied to the grid 4 of the first valve, the remaining grids being at fixed potential and coupling means comprising a constant resistance or a tuned rejector circuit connected in series with the last valve. The changes of potential between grid and cathode in successive amplifiers are in phase so that coupling between successive stages tends to stabilize the system. To avoid leakage between the cathodes and heaters the heater secondary windings 19, 20, 21, Fig. 3, may be centre tapped and connected to the potentiometer R3 from which grid bias potentials are taken so that the heater potential is slightly below that of its cathode. Choke coils may be connected in series with the heater leads to avoid leakage of radio frequency potentials due to the capacity between the cathode and its heater. The invention is applicable to low frequency amplification in which case the tuned circuits S1 S2 are replaced by a resistance. The input to the first valve may be biased by a resistance R2 in the input circuit. Fig. 3 shows an arrangement in which a pair of tuned circuits are used to obtain a band-pass effect and the selectivity is stated to be that of the number of tuned circuits used multiplied by the number of valves in cascade. Due to the high selectivity an untuned aerial circuit may be used. The detector 27 is resistance direct coupled to the output valve P. Automatic gain control is obtained by grid biasing the input valve from the detector resistance R6 instead of from resistance R2. Condenser C3 in conjunction with the resistance shunted by it gives the required time constant. Hum due to the high tension supply may be compensated by providing a condenser 36 adapted to apply pulsations to the cathode of the valve 27 equal to those applied to the grid through grid condenser 7. Triode, screen grid or space charge grid valves may be used. Fig. 4 shows an arrangement in which the valve T3 serves as a detector by the provision of grid condensers and leaks in both the control and screen-grid circuits though these may be provided in one only of these circuits. To prevent the audio frequency from being applied to the valves T1 T2 the audio-frequency bypass condenser 49 is provided in series with radio frequency choke 50. To exclude radio frequencies from the output valve P bypass condensers 51-56 are provided. Fig. 5 shows a reflex arrangement in which the coupling for the amplifying valves T1 T2 T3 comprises the tuned circuit S1 for radio frequencies and the resistance R1 for audio frequencies, the detector D being coupled to the tuned circuit S1 by the grid condenser 7<1> and leak 8<1>. In a modifica- Fig. 7 not shown) the anode-cathode path of the detector is in parallel with the first radiofrequency amplifier the detector anode being connected to the anode of the latter through a radio frequency choke. With this arrangement automatic gain control is provided by biasing the amplifier valve from a resistance in the cathode circuit of the detector. The sum of the anode currents of the valves in parallel is arranged to be equal to that of the other valves or else shunt resistances are provided to supply the additional current necessary. Fig. 8 shows an arrangement of high frequency amplifiers T1 T2 T3 and a detector D and low frequency amplifiers T5 T6 in which automatic gain control is obtained by varying screen grid potentials by resistance R13 arranged in series with a resistance 73 in shunt to the detector D. Alternatively or additionally control grid potentials may be derived from this resistance for further gain control. In the arrangement shown increase of signal strength increases the screen grid voltages which are so chosen initially that this decreases amplification. If an anode bend detector is used the resistance R<1>3 is tapped to resistance 75 instead of to resistance 73. The resistance 73 may be connected to the anode of one of the valves T5 T6 if higher gain control potentials are necessary. In another reflex arrangement (Fig. 9 not shown) part only of the high frequency amplifier is used for low frequency amplification in which case the remaining valves serve for the coupling impedance. The invention is applicable to glow discharge valves in which the controlled potential is applied between the grid and anode and in this case the coupling element is preferably connected in the cathode instead of in the anode circuit. Super-heterodyne reception : Fig. 11 shows a detector oscillator T utilising a space charge grid. Alternatively a separate oscillator may be used (Fig. 10 not shown). Automatic gain control is effected by diode rectifier 119 in series with, and controlling the bias of the detector D. Alternatively the bias of the first detector may be controlled for this purpose. Fig. 12 shows an oscillator T2 and detection takes place in the series amplifier T1 T2 T3. The second detector T4 controls the bias of the screen grids of the series amplifier T4 T5 T6 through potentiometer R13 to give automatic gain control. In a further modification (Fig. 13 not shown) radio frequency amplifiers, the first and second detectors, the oscillator and the intermediate and low frequency amplifiers are connected in series across the supply. The Specification, as open to inspection under Sect. 91 comprises also an arrangement. Fig. 10 (Cancelled) in which the detector D is arranged at the positive end of the amplifier beyond the tuned circuit 51 and automatic gain control is obtained by means of an assymetric resistance 96 in series with the potentiometer R3 controlling the grid potentials. The resistance 96 is shunted by a condenser 97 to form a combination of large time constant. This subject-matter does not appear in the Specification as accepted.