Patent GB-780838-A (GB)
Title: Improvements in electronic tube and apparatus therefor
IPC: H01J21/06
Filed: 1951-03-27
Abstract: 780,838. Thermionic valves. MACHLETT LABORATORIES, Inc. March 27, 1951 [March 25, 1950], No. 7076/51. Class 39(1). [Also in Group XL(c)] A double-ended tube comprises an evacuated chamber, a cathode electrode and co-operating control grid, screen grid and anode electrodes in the form of surfaces of revolution, the cathode control grid and screen grid having corresponding first and second edges respectively, connection members in the form of surfaces of revolution connected to the first edges of the grids and extending to the exterior, and an internal radiofrequency capacitive coupling between the second edges of the screen grid and cathode. In Fig. 1 the cathode in a cylinder 12 heated by four heating coils 10, of which two are shown ; the control grid 44 is supported by a conical connection member 43, and the screen grid 37 is supported by a conical connection member 36; all the connection members extend through the envelope and are joined together by ceramic discs, the grid support having an external coating 50 of dielectric material so that a coaxial line may be joined to it for R.F. purposes but insulated from it for D.C. The capacitive connection between the cathode and the screen grid is provided by a dielectric coating 39 on the cathode support surrounded by a cylinder 38 connected to the lower end of the screen grid through a hairpin bend 40 which allows for thermal expansion of the grid and prevents conductive material depositing on the dielectric 39. The anode 23 is provided with cooling pins 27 and is insulated from supports 30, 31 and connecting cylinders 32, 33 by dielectric rings 28, 29 so that an R.F. connection can be made through the cylinders 32, 33 without there being any D.C. connection. Since the screen grid is capacitively connected to the cathode an input may be applied between the screen and control grids and an output taken between the screen grid and anode ; the screen grid may be earthed. To prevent resonances in the space between the grid support 43 and the cathode this space may be made smaller by providing the cathode with a metallic end cap, or alternatively the conductive end 41 of the grid support may be replaced by an insulator so that the grid support becomes a waveguide with a cut-off higher than the desired operating frequency. The axial length of the electrodes should be less than 1/16 of a wavelength. In a modification, Fig. 4, a double tetrode is provided comprising a cylindrical cathode 81, control grids 84, 95 on each side of the cathode, screen grids 85, 96 and anodes 86, 97. The screen grids are capacity coupled to the cathode by cylindrical members 109, 113 and dielectric members 111, 114; the control grids may be coupled by either two (as in the Figure) or four neutralizing capacities 115, 116 to the anodes of the opposite electrode system. The central anode 86 may be provided with a cooling coil 87, and the outer anode 97 may be provided with fins or may have a cooling coil 98. The connection members extend through the envelope and are separated by ceramic rings as before. In a further modification, Fig. 6, the electrodes are planar and comprise a cathode 121, control grid 132, screen grid 135 and anode 139 ; the inner edge of the screen grid is attached to a supporting conducter 143 which is separated from the centre of the cathode by a dielectric disc 122. The centre of the cathode is non-emissive and is here equivalent to a second edge.