GB734196A - Improvements in or relating to indirectly heated cathodes - Google Patents

Abstract

734,196. Thermionic valves. STANDARD TELEPHONES & CABLES, Ltd. June 11, 1952, No. 14675/52. Class 39 (1). An indirectly heated thermionic cathode comprises a heater formed of a helix of suitable wire, such as tungsten, embedded in a tube of refractory insulating material, such as alumina, a further helix of, for a example, nickel wire or ribbon wound over the tube, and a coating of electronemissive material covering the further helix, in which the turns of the first mentioned helix are spaced apart and the turns of the further helix are laid in troughs of the tube lying between the turns of the first helix. In making the cathode, a thin tungsten core 13, Fig. 1, is provided with an insulating coating 14 of alumina, magnesia or beryllia, over which is wound tightly a helix 15 of molybdenum or tungsten wire forming the heating element. This helix is then provided, preferably by electrophoresis, with a coating 16 of alumina or the like, and a further helix 17 of nickel wire is laid between the turns of the helix of heater wire. The assembly is then coated with an alkaline earth carbonate mixture to provide the emissive coating. The warming- up time of such a cathode is 10-15 seconds compared with the normal 60 seconds and may be further reduced by dissolving out or otherwise removing the core 13. To provide a 6.3 volt heater the cathode may be formed in two portions each having a 3.15 volt heater and mounted on the same core which may then be bent into hairpin form and mounted as shown in Fig. 2 in which the cathode 18 is suspended at its mid-point from an upper insulating disc 6 by means of a resilient support 19. The lower ends of the cathode are located in a lower insulating disc 7, the two discs 6 and 7 being mounted by means of rods 9 from a glass base 11.