GB429854A - Improvements in or relating to cathode ray tubes - Google Patents

Abstract

429,854. Cathode-ray tubes. McGEE, J. D., 9, King's Avenue, Ealing, London, and CONDLIFFE, G. E., Thorncombe, Fulmer Road, Gerrards Cross, Buckinghamshire. March 19, 1934, No. 8644. [Class 39 (i)] In a cathode-ray tube provided with a cathode 3, deflecting means 15, 16 and tubular electrodes 5, 6, the electrode 6 being provided with an apertured diaphragm 13, means are provided for maintaining the electrodes 5, 6 at different positive potentials to form an electron lens which focuses the beam upon the screen, the potentials being such that the ray is accelerated to its maximum velocity by the electrode 6, and the diaphragm 13 is located between the lens and the deflecting means and substantially outside the field of the lens. The tube is formed with cylindrical and frusto-conical portions, and comprises a cathode, which may be indirectly-heated and consists of a shallow body of pill-box shape, a modulating electrode 4 and the electrodes 5, 6, housed in the cylindrical portion. The electrode 6 is connected by a wire 14, an annular ring, or by an external connection, to a portion 12 which forms an internal coating on the tube or a sliding fit therein and comprises a frusto-conical part 19 formed as a continuous sheet or with fingers. Electrostatic or electromagnetic deflecting means are used and are located in the field-free space within the portion 12. The electrode 5 is provided with diaphragms which do not obturate the beam to any great extent and the diameters of the apertures in which increase with the distance from the cathode, while the aperture in the plate 13 is such that spherical aberration is avoided by allowing only the passage therethrough of the axial portion of the beam. The electrode 4 is provided with an apertured diaphragm 7 near the cathode and with the electrode 5 forms an electron lens. The tube is highly evacuated and the member 12 extends towards the cathode at least as far as the end of the electrode 6. The cathode is earthed and the electrodes 4, 5, 6 are maintained at 20, 1000 and 5000 volts respectively. The screen effect of the diaphragm 13 may be increased by the provision of a fine-wire mesh in the aperture. The electrode potentials may be obtained from a potentiometer and the current to the electrode 5 should therefore be kept small. If the current to the electrode 5 need not be small then any or all of the apertures in the diaphragms thereof may be used to limit the ray to the axial portion and the diaphragm 13 may then have a larger aperture. Additional electrodes, e.g. a cathode shield and an accelerator may be provided between the cathode and the modulator and the diaphragm 13 need not be located at the end of the electrode 6. The electrode 5 may be divided into sections the diameters of which may be the same or increase with the distance from the cathode, or the diameters of the central sections may be greater than those on either side. Further the potentials on the sections may increase with the distance from the cathode. The Provisional Specification states also that the member 12 consists of a copper-nickel alloy and the member 6 of nickel and that the deflecting plates are connected through resistances to the electrode 6 and are supported by members passing through the member 12.