GB624832A - Improvements in or relating to circuits for cathode ray tubes and methods of operating the same - Google Patents

Abstract

624,832. Cathode-ray tubes. CINEMATELEVISION, Ltd., and BUCHANAN, W. H. July 10, 1947, No. 18362. [Class 39 (i)] In a cathode-ray tube of the type comprising a cathode heated by electron bombardment, a modulator electrode for controlling the electron emission from the cathode, and an anode, unidirectional potential pulses are applied periodically between the cathode and the source of the bombarding electrons. Only those electrons emitted thermionically from the cathode without including bombarding electrons or secondary electrons emitted from the bombarded cathode surface are then utilized to form the operative cathode-ray beam. As shown, the cathode 3, which may be shaped so that the emissive surface is a sharp point, is bombarded by electrons from a helical filament 4 surrounding it. The unidirectional potential pulses are supplied between the cathode 3 and filament 4 from a rectifier 7, 8 whose alternating potential input is derived from a time-base' circuit so that the cathode 3 is only bombarded during the flyback periods of a scanning process performed by the electron beam, for example in television, radar or oscillography. Potentials are supplied to the modulator electrode 5 across the resistance 9 to suppress emission from the cathode 3 towards the anode 2 in those periods when the cathode is under bombardment. To ensure that the modulator electrode is not held so negative that bombarding electrons are repelled from the part of the cathode 3 adjacent to the.modulator electrode, a charge-leakage path is provided between the electrode 5 and the filament 4 through the diode 14. In an alternative method, modulating potentials are supplied to the anode 2 to suppress emission from the cathode in the bombardment periods.