GB735632A - Improvements relating to cathode ray tubes and arrangements therefor - Google Patents

Abstract

735,632. Cathode-ray tubes. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Jan. 24, 1952 [Feb. 15, 1951], No. 3644/51. Class 39 (1). A cathode-ray tube arrangement comprises a cathode-ray tube having a plurality of independent cathodes capable of being selectively energized at will to generate separate electron streams, means for setting up a constant deflecting field which causes said electrons to be deflected in a direction towards or away from a path along which it is desired that they should pass, a second means for setting up a constant deflecting field for changing the direction of said deflected electrons, the arrangement being such that with any one of said cathodes energized and with the deflecting means suitably oriented the electron stream from said cathode will be caused to move first in one direction and then in a changed direction so as thereafter to follow said desired path. In the tube shown the cathode structure comprises four cathodes 7 and heaters 8 supported by means of an insulating disc 6 within a modulator electrode 9 the end wall 10 of which has four small apertures 11 each coaxial with a corresponding cathode 7. The cathodes may be shielded from one another by welding to the underside of the wall 10 two V-shaped metal strips which project towards the insulator 6. An anode 13 is provided with four apertures 14 each several times the diameter of an aperture 11 and located so that a projection of an aperture I would touch or almost touch the inner edge of the corresponding aperture 14. One leg of each heater 8 is connected to a common base contact pin and the cathodes 7 may be connected to a common pin or to separate pins. In operation, the beam from a cathode proceeds in a direction, as indidicated at a, across the axis of the tube due to the asymmetric electrostatic field between the apertures 11 and 14, and a magnet assembly 15 is provided to deflect the electrons along the axis of the tube so that they are brought to a focus on the fluorescent screen by focusing means 5 which may be a coil or an annular permanent magnet. Any ions in the beam are substantially undeflected and may be collected by attaching to the anode 13 a tubular extension which extends in a direction away from the cathodes 7. The magnet assembly 15 preferably comprises a pair of permanent magnet pole-pieces mounted 180 degrees apart on a soft iron ring so as to make a close fit with the external wall of the neck 1 and is rotated about the neck to position the pole-pieces correctly with respect to the electron beam issuing from the particular cathode being used. The magnet assembly is shown spaced from the anode 13 but in practice is positioned close to or in line with the anode. The tube has an anode 4 in the form of a coating of metal or graphite and is provided with deflecting coils 16. In an alternative arrangement the anode 13 is provided with a single central aperture of a radius at least as great as that of the circumscribing circle of the four apertures 11. In this arrangement the beam from a cathode is deflected away from the tube axis by the asymmetric electrostatic field between the apertures and is deflected along the tube axis by one or two magnet assemblies similar to assembly 15. An additional anode having apertures corresponding to the apertures 11 may be placed between the anode 13 and the modulator electrode 9 to provide an electron gun of the tetrode type.