Patent GB-650667-A (GB)
Title: Improvements in electron discharge devices
IPC: H01J21/36
Filed: 1946-01-11
Abstract: 650,667. Thermionic valves. WESTERN ELECTRIC CO., Inc. Jan. 11, 1946, No. 1032. Convention date, Jan. 13, 1945. [Class 39 (i)] An ultra high-frequency electron discharge tube comprises an insulating support member 20 for mounting a cathode 22, 23, 24 and having a plane seating surface coplanar with the surface of the cathode prior to the formation of the cathode emissive layer, a second insulating support member 43 for mounting the anode 44 and having a seating surface coplanar with the surface of the anode, a plane grid 33 and means holding the cathode face, anode face and grid in aligned parallel relation with spacer means 38 engaging the seating surfaces of the support members and determining the spacing between the grid and the anode and cathode. The above-mentioned surfaces are made coplanar by lapping them simultaneously with an abrasive such as silicon carbide. The cathode comprises a platinum disc 21, a molybdenum stem 22 extending through and secured to the disc 21 and having an enlarged head portion 23, and a nickel disc 24 welded to the head portion 23. The surfaces of the member 20 and the cathode are lapped to be co-planar within 0.0002 of an inch and a thermionic material is subsequently applied to surface of the cathode to form a coating of the order of ¢ to 1 mil. The member 20 is seated against the annular corrugated diaphragm 17, preferably 'of material having small thermal expansion and conduction coefficients such as that known under the Registered Trade Mark "Invar," secured to a flanged, cylindrical, metallic member 16 having thereon a thin coating or glaze 18 of a ceramic having thereon a thin silver coating 19 forming with the closely fitting metallic cylinder 13 a cylindrical condenser. The cathode assembly includes a cylindrical metallic heat shield 27 joined to disc 21 and to a cylindrical metallic member 28 depending from diaphragm 17. The cathode stem 22 is encompassed by a helical heater filament 29 joined to conductors 30 passing through oversize apertures in a metallic heatreflecting shield 31. The anode 44 is of stainless steel or copper and is provided with an extension 45 fitting into an internally turned recess 52 in the split inner end of a leading-in conductor 46 sealed to a ceramic or glass disc 47 which is sealed to a cylindrical leading-in conductor 50 sealed to a glass or ceramic disc 14 sealed to the metallic cylinder 13. The conductor 50 is seated in a tinned channelled ring 39 soldered or brazed by a ring 40 in an annular corrugation in the spacer 38. The spacer 38 is provided with a plurality of depending sockets 41 extending into apertures 42 in the member 20 and preventing leakage of high-frequency energy through holes in the grid frame. The grid 33, Fig. 3, is formed of substantially parallel coplanar wires fixed to a molybdenum frame 34 and extending across a central circular aperture 35 therein. The wires are preferably 0.00033 inch tungsten, 1000 turns per inch, wound upon a pair of frames of the order of 5 to 10 mils thick, separated by a spacer 36. The grid wires are fixed to the frames by gold brazing in an atmosphere of hydrogen along the edges 37 and are severed adjacent these edges to form two grid units. The wire is maintained under a constant tension during winding. After brazing arcuate depressions 80 are pressed in the frame 34 to stress the grid wire to 20 to 30 per cent of its breaking point. The frame 34 is seated upon the metallic spacer 38 with the aperture 35 in coaxial alignment with the cathode disc 24. The cathode grid spacing is of the order of ¢ to 1 mil. and the grid anode spacing is of the order of 5 to 10 mils. The members 14 and 47 may be positioned substantially at anti-nodes in the cathode-grid and grid-anode systems. The cathode, anode and grid are constructed as a unitary assembly including also a ceramic body 53, the parts being clamped together by a plurality of ceramic posts 54 having vitreous beads 55 fused to the ends thereof. The enclosing vessel may be evacuated through an exhaust tubulation 59. A getter is carried by a wire 60 connected between two of the leading-in conductors 11. To effect outgassing radially extending grooves 70, 74 and 75 are provided in the cathode support 20, the anode support 43 and the insulator 53 respectively, and apertures 71 and 72 are provided in the disc 21 and anode support 43 respectively. Apertures 73 may be provided also in the grid conductor 50. An alternative anode assembly is shown in Fig. 7 and comprises a ceramic support 62 in which is moulded a disc 63 mounting a stem 64 to which is fixed a zirconium anode disc 65.. The surfaces 66 and 67 of the disc 65 and support 62 are lapped to make them coplanar.