Patent GB-442436-A (GB)
Title: Improvements in or relating to electron discharge tubes, indirectly heated cathodes therefor, and methods of making same
IPC: H01J9/04
Filed: 1934-07-07
Abstract: 442,436. Thermionic valves. KRAMOLIN, L. L. DE, 25, Nassauischestrasse, Wilmersdorf, Berlin. July 7, 1934, No. 20018. [Class 39 (i)] An indirectly heated cathode which is produced by the distillation of a vaporizable metal-for example by the azide or thermite process-is " formed " after the metal has been deposited on the body by a heating current flowing through the body, so that the normal heating means are not overloaded during the forming. In one construction the cathode body d, Fig. 1, comprises a spiral with spaces between the turns; leads x y allow a forming current to flow so that the normal heating filament a need not be used. During normal running the leads x, y may be connected to ensure an equipotential condition, where the cathode body is a tube the leads x, y are connected to the ends ; the forming current may be supplied by inducton to a coil so disposed within the envelope that the electrodes are not effected by this inductive effect and the coil may be of copper to be of greater conductivity than the platinum or nickel body. In other constructions the cathode d, Fig. 5a, and the normal heater a are at the foci of an elliptical reflector c, which as shown is a control electrode. The cathode is connected through a heat-frangible piece f to a wire forming the support for the grid, so that the forming current may be supplied independently of the heater a. At the end of the forming process a high current is sent through the body to fuse the piece f. As shown the anode b is in plate form not to obstruct the normal heating effect from the heater a. The reflector may be the anode or anode screen or independent reflectors may be used ; the magnesium gettering coating on the envelope may be used as the reflector or the latter may be outside the envelope and may be a coating thereon. These reflectors may be of copper or silver, or of nickel or molybdenum coated with copper or silver. Stiffening plates may be provided for the electrodes and reflectors or steatite &c. spacers may position the cathode and heater at the foci of the reflectors. The magnetic effect of the heater may be reduced by parallel conductors or screens ; if the heater is raised to emission temperatures it is given a positive bias. The heater is given greater bulk than the cathode by carbon or metal coating, the latter being subsequently oxidized for insulation effect; or wire may be wound on the heater in different amounts along its length so as to be thicker near the middle, to obtain uniformity of heating of the heater. The heater may be made entirely of ceramic material, i.e. a silite rod or it may be of carbon. As applied to valves of the kind described in Specification 415,079 which contain two systems, in one construction the heater a, Fig. 7, for the lower cathode d<1> is prolonged and used to heat the upper cathode d ; the forming device is the same as described with reference to Fig. 5a. The cathode d of the upper system supports the lower anode b <1> ; the upper system is to work at saturation and acts as a high alternating current resistance. The heater a is connected to the anode b and through a resistance r to the grid c ; the upper control grid may serve as the heater. In a modification of this construction the heater a, Fig. 18, is in the form of a resistance wire and may act as the anode for the upper system ; the control electrode c acts as the elliptic reflector and is supplied with a voltage at 180‹ to anode voltage of the tube. The heater a may be of the oxides of aluminium, zirconium or magnesium. In other constructions the heater a may be of spiral form within which the cathode d is situated. In further constructions two cathodes d<1>, d<2>, Fig. 9a with surrounding grids and it may be screens are placed at the foci of one or within two reflectors, which may also be electrodes ; one cathode acts as the heater for the other, and " forming " means are provided for each.