GB313521A - Improvements in electrical apparatus for amplifying alternating voltage - Google Patents

Abstract

313,521. British Thomson-Houston Co., Ltd., (Assignees of Farley, T. S.). June 13, 1928, [Convention date]. Thermionic amplifiers.-An electric discharge device of the thyratron type described in Specification 223,921 having a plate-current grid-voltage characteristic of the form shown in Fig. 1 is utilized for signalling over high-voltage transmission lines. As the negative grid voltage moves along the abscissa 10 towards the zero, a point is reached at 16 where plate current commences to flow as indicated by the curve 13. For any further increase in grid voltage the plate current remains steady along the line 14. With an A.C. plate voltage the plate current is only cut off during each negative half-cycle, but when the A.C. grid input reaches a sufficiently negative swing, no plate current will flow. As shown in Fig. 2 the discharge tube 17 is supplied with alternating filament current across the leads 21, a mid-point tapping on the potentiomoter 26 serving to balance-out voltage variations. The signal voltages to be amplified are applied across the terminals 22, whilst alternating-current plate voltage (which is preferably an exact multiple of the signal frequency but must be of considerably higher frequency) is supplied by the transformer 36. Fig. 3 shows the relation between the input and output currents. The output consists of A.C. impulses, each having the frequency of the plate supply voltage, divided into groups 44 having the frequency of the applied grid voltages. The peaks 42 represent the frequency of the plate supply, and the curve 41 that of the applied grid or signal voltage. Only the positive values of the plate supply are effective within the brackets 44 owing to the rectifying action of the discharge tube. A high grid impedance is used to minimize the power consumed by the tube. A band filter 31, Fig. 2, having a cut-off just above and below the signal frequency passes the latter to the working circuit 34. The arrangement is suitable for signalling with comparatively low power over lines where considerable voltage variations may occur.