GB515426A - Improvements in or relating to television receivers - Google Patents

Abstract

515,426. Television. FERRANTI, Ltd., TAYLOR, M. K., and WOOD, H. May 30, 1938, No. 16040. [Class 40 (ii)] Adjustment of the gain of a receiver may give rise to a change in the illumination of the black parts of a picture. To prevent such variations, the whole or part of the potential drop across an amplifier valve which varies the over-all gain, 'is used to control the bias of the cathode-ray tube, either directly or by way of a directcoupled amplifier. In one arrangement, the control grid of the cathode-ray tube 15, Fig. 1, is connected over diode 22 and resistance 27 to a tap on the anode resistance 24 of valve 25 whose gain is variable. If its gain is increased its anode current rises, the potential of the tap falls and increases the negative bias on the cathode-ray tube. In a modification, the potentiometer 19 is connected between resistance 24 and N and the cathode of the cathoderay tube is energized by signals (Fig. 2, not shown). In another modification, the varying potential from 24 is applied to one grid of a valve, and rectified signals to another grid, and the output is applied to the cathode of the tube (Fig. 3, not shown). In a further modification, the resistance 24 is connected via a potentiometer 30, Fig. 4, to the anode of the diode detector 3 and provides a negative potential for the grid bias of the video frequency pentode valve 9 controlling the cathode-ray tube. Specifications 475,728 and 494,502 are referred to.