Patent GB-407934-A (GB)
Title: Improvements in or connected with the operation of thermionic valves
IPC: H03C1/52
Filed: 1932-09-19
Abstract: 407,934. Valve modulating circuits. DEHN, F. B., 103, Kingsway, London.- (Elwell, C. F., and Terman, F. E. ; Stanford University, California, U.S.A.) Sept. 19, 1932, No. 26056. [Class 40 (v).] Relates to valve modulating and frequency - changing circuits in which one frequency is applied to the plate-filament path and the other frequency is applied to the grid to vary the A.C. resistance of that path. Triode or tetrode valve circuits may be employed either singly or in push-pull pairs. Valves may be used in which two heated electrodes function alternately as anode and cathode. Fig. 2 shows a triode valve circuit with a carrier frequency source 13 in its grid circuit, and a modulating source 1 in its plate circuit. The grid is positively biassed, and the curves showing plate current as a function of plate voltage may have the shape shown in Fig. 4, each curve representing a different value of grid voltage. The plate is biassed (negatively for the case represented in Fig. 4) to a point 14 or 141 at which the curves intersect ; with the result that grid voltage changes alter the slope resistance of the valve but do not affect the plate current. The resistance presented to the source 1 by the plate-filament path is thus varied at carrier frequency by the source 13. Side bands or side frequencies appear in the plate circuit and may be taken off by the transformer 2, whilst the carrier frequency in some cases is suppressed. When a valve with two heated electrodes 16 is used, Fig. 1, the carrier source 13 is connected between the positively-biassed grid 17 and a point between two small condensers 20, 20<1>, shunted by chokes 19, 19<1>. The modulating source 1 is connected across the two elements 16. In a modification (Fig. 10 not shown), a valve similar to that of Fig. 1 is employed, but the carrier frequency is applied to two separate grids through a transformer with two independent secondary windings. When a tetrode valve is employed, the outer grid may be held at a fixed positive voltage and the carrier applied to the negatively-biassed inner grid (Fig. 5 not shown) ; or the inner grid may be held at a fixed negative voltage, and the carrier applied to the positively-biassed outer grid (Fig. 6 not shown) When two triode valves are employed in pushpull relation, Fig. 7, the modulating wave is applied from the source 1 through a transformer 35 with centre-tapped secondary so that the plates are energized in phase opposition ; whilst the grids are excited in phase agreement by the carrier source 13. The two valves may be replaced by a duplex valve with a single filament, a single grid, and two separate anodes. (Fig. 11 not shown). Fig. 9 shows another arrangement employing two oppositely-connected valves 56, 58 with the plate of each connected to the filament of the other, and each including the modulating source 1 in its plate circuit. The grids are oppositely excited by the carrier source 13 and are positively biassed by batteries 61, 611. Rejector circuits 57, 57<1>, 60 prevent current from the source 1 passing to the grids or to the source 13, whilst chokes 55, 55<1> keep the carrier current from the plate circuit. The output is taken from the transformer 2. The grid circuits may be kept separate, and may be connected to independent secondary coils of a transformer which has its primary coil in the circuit of the source 13 (Fig. 8 not shown). Specifications 184,875 and 186,138, [both in Class 40 (v)], are referred to.