Patent GB-355346-A (GB)
Title: Improvements relating to circuit arrangements for electrical amplifiers
IPC: H03F3/32
Filed: 1930-05-23
Abstract: 355,346. Valve circuits. NUMANS, J. J., 45, Ten Hovestraat, The Hague. May 23, 1930, No. 15955. Convention date, Feb. 25. [Class 40 (v).] Push-pull amplifiers are operated at the lower end of the straight portion of their characteristic curves, the grids being prevented from swinging in the negative direction by the use of rectifiers. Distortion is thus prevented, and as the average value of plate current rises with an increase of control or input voltage, the valves function more efficiently. Each valve receives practically no input during one half-cycle, and to prevent the inoperative valve from putting a load on the other, its internal resistance is raised by applying to its control grid or other electrode a compensating voltage derived from the plate circuit of the operative valve. If pentode valves are employed, their high internal resistance renders this compensating voltage unnecessary. In the arrangement shown in Fig. 1, the input circuit of the pentode valves 1, 2 includes the balanced transformer 11, a pair of diode valves 3, 4 and resistances 6, 7, normal grid bias being applied by a battery 13. The positions of the resistances and diode valves may be reversed and any other form of rectifier may replace the diodes. In Fig. 5, the rectified voltages across resistances 6, 7 derived from the diodes 3, 4 and applied to the grids of the triode amplifiers 1, 2 are supplemented by voltages across resistances 17, 18 derived from coils 15, 16 wound on the output transformer 10. Alternatively the compensating voltages may be introduced at c1, c2. In another arrangement, the compensating voltages are applied to the grid circuits by including therein the respective halves of the secondary coil of a transformer, the divided primary coil of which is connected at P1, P2. Adjustment is effected by a variable resistance connected across the primary coil. In Fig. 10, the rectifiers comprise triodo valves 3, 4 with high resistances 15, 16 in their grid circuits and with no grid bias. Grid current rectification gives rectified currents through the plate resistances 18, 19, and rectified voltages are tapped therefrom and applied to the control grids of the valves 1, 2. The plate voltage for the valves 1, 2 is taken across neon tubes 14 which, in series with other neon tubes 17, are connected across a mains supply system Va. Compensating voltages are transferred from the plate to the grid circuits by resistances 5, 6 connecting each plate to the opposite grid. Instead of introducing rectifiers in or before the grid circuits of the main valves, the average plate current may be caused to rise with a rise in the input voltage by rectifying the valve output and causing this rectified output to affect grid bias or plate voltage. As shown in Fig. 11, a dynamo 13 supplying plate voltage to the valves 1, 2 has its field coil 15 connected in series with rectifiers 3, 4 and shunted by a smoothing condenser 14. The rectifiers 3, 4 are in series with the coils 8, 9 of the output transformer 10, and are connected across choke coils 5, 6 in the plate circuits of the valves 1, 2. Thus with an increase in signal input, the plate voltage is raised. As an alternative, the field coil 16 may take the average rectified output from diodes 17, 18 in a circuit coupled to the load circuit 12 ; or as shown in Fig. 14 the output from the diodes 17, 18 may be applied across a resistance 20 in the grid circuit to reduce negative grid bias as the signal strength rises. The omission of a condenser across the resistance 20 causes the pulses supplied by the rectifiers to give intermittent operation of each valve in turn. Fig. 15 shows another circuit, in which compensating-voltages are supplied to the grid circuits by coils 15, 16 transformer-coupled to coils 19, 20 in the plate circuits, shunted by variable resistances 5. The compensating-voltages are rectified by diodes 3, 4. In an alternative arrangement, the valves are operated at the upper end of their characteristic curves, and the rectifiers allow only negative half-cycles to affect the grids. As another alternative, the compensating voltages may be applied to the grids in opposite phase to that referred to above, their effect then being to reduce the internal resistance of the valves. This arrangement is used when the valves have separate output trsnsformers, with the secondaries in series with each other and with the load, the reduced internal resistance of the valves then serving to reduce energy losses. The invention is applicable primarily to the operation of loudspeakers &c., or to the amplifiers preceding a modulation system but is stated to be applicable also to oscillators, modulators, and detectors.