GB664347A - Improvements in and relating to methods of and apparatus for manufacturing electric discharge devices - Google Patents

Abstract

664,347. Thermionic valves. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 13, 1948 [Sept. 10, 1943], No. 4288/48. Class 39 (i). An electric discharge device is evacuated by supporting it in a closed chamber which is itself then evacuated, means being provided for adjusting electrode spacing during evacuation, either' to a predetermined distance, or by applying potentials and running the tube under operating conditions and adjusting the spacing until a measured electrical characteristic attains a predetermined value. The outer chamber is a bell-jar 36 mounted via a, rubber seating 37 on a duct bounded by metal plates 38, 40 and evacuated by a pump 3 through a large reservoir 43. The discharge device 1 comprises a metal envelope in two parts 14, 18 between which are supported two grids 12, 13. Insulating discs 15, 19 at either end support respectively anode 10 and cathode structure 11. The latter comprises a sleeve 17 within which slides another sleeve 25 flanged at its upper end with an aperture 97 therethrough, and attached to a fernico tube 26 surmounted by an emissive disc 27. A heatconducting eyelet 31 surrounds the heater 28. The cathode structure is supported. by rigid conductors 29, 33 clamped to rods 51, 52 secured in an insulating disc 55. An opening 56 in the plate 40 is sealed by a rubber gasket 57 with a thick central portion 62, and a thickened perimeter 58 enabling it to be clamped firmly against plate 40 by a metallic ring 61. Metal plates 64, 65 are clamped about the central portion of the gasket by a through rod 66 attached to disc 55. The lower plate 65 has external threads engaging with a nut 71 which bears against the underside of plate 61, rotation of the nut serving to raise or lower the whole cathode structure within sleeve 17 because of the flexibility of the thin portion 63 of the gasket, a scale 73 allowing of micrometer adjustment. The body of the device is supported from an insulating disc 47 by. sets of spring clamps 48, 49, 80, 81 which act also as contacts for applying operating potentials to the electrodes. These are applied during evacuation, the filament being over-run to assist degassing. Sealing'is effected by a ring of solder 96 between sleeves 17 and 25, which may be of silver-plated steel. If the temperature prevailing is not sufficient to melt the solder, an external H.F. coil 98 may be used. The device may have a preliminary evacuation. before being placed in the bell-jar, the solder 96 then being melted again to re-establish communication for further exhaustion. After sealing, while the seal is molten, the cathode/ grid spacing may be accurately set to a predetermined value, e.g. 0.001 inch, or less, by adjustment of the micrometer screw -71. Alternately, if a particular electrical, e.g. gridvoltage/plate-current, characteristic is required, the spacing is adjusted until the required platecurrent reading is obtained on ammeter 99. Fig. 2 (not shown), is an alternative circuit arrangement enabling a predetermined value of interelectrode capacitance to be used as the desired characteristic.