Patent GB-299715-A (GB)
Title: Improvements in vacuum electric tube devices
IPC: H01J9/04
Filed: 1928-10-29
Abstract: 299,715. Westinghouse Lamp Co., (Assignees of Marden, J. W.). Oct. 27,1927, [Convention date]. Vacuum tubes.-A method of making a thermionic valve having an alkali metal as the electron-emitting material consists in (a) assembling the electrodes on a mount, (b) cleaning the electrodes by heating under oxidizing conditions, (c) sealing the mount into an envelope, (d) degasitying the electrodes, (e) forming a layer of electronegative gas such as oxygen on the filament electrode, and (f) introducing an alkali metal into the envelope. As shown, the electrodes consist of a tungsten or molybdenum filament 13 of M-shape, and an anode in the form of two plates 16, 17 on opposite sides of a control grid electrode 14. The plates 16, 17 and the grid are formed with horizontal vanes 21, 24 in the form of flat strips struck out from a plain sheet, the vanes on the two electrodes being arranged to overlap, so that the filament is protected from positive ion bombardment due to the field between the vanes. The upper loops of the filament are engaged by resilient supports secured to arms 36 extending from a glass bead 38 which is supported from the press 12. A pellet 45, composed of a compound of the alkali metal and a reducing agent, is supported by a loop 46 in a metal strip secured to the grid. support 43. Before mounting, the anode and grid are baked in hydrogen for 5-10 minutes at 1000 C. to remove oxygen, oxides, and grease, and the filament is straightened by flashing for 4 or 5 secs. at 1000 C. in an inert gas such as forming gas, consisting of hydrogen to which is added 80 per cent of its volume of nitrogen. The completed mount is heated for 30-60 minutes at 300 C. in air so as to remove grease and dirt by volatilization or oxidation. In order to prevent leakage, the press and bead 38 are coated with a solution in chloroform of petroleum jelly which has been purified by removing the volatile constituents; or shellac or other resinous material may be used. The mount is sealed into the bulb, which is then baked and exhausted; details of duration of baking and temperature are given. Long exhaust tubes should be used so as to prevent grease from passing back into the bulb. The anode and grid are heated by high-frequency induced current to drive off gas, the grid forming a complete loop for the current, and the anode being heated by radiation from the grid. In order to form a layer of electronegative gas on the filament, oxygen or air is admitted and the filament flashed at 2000 C. for about halfan-hour to remove grease and hydrocarbons so as to ensure retention of the oxide layer formed on the filament. Oxygen is evacuated from the envelope and the pellet 45 is heated by highfrequency induced current so as to drive off caesium. The loop supporting the pellet is mounted at right angles to the grid loop, so that the external coil may be arranged so as not to induce current in the grid. and anode. Instead of admitting oxygen, the pellet may consist of a mixture of caesium permanganate and silicon, and may be heated to a low temperature so as to evolve oxygen, and subsequently to 900 C. to drive off caesium. Preliminary activation of the filament is effected by heating by a current larger than that for which the filament is designed, and a potential may be applied to the grid and anode; for example, a filament designed for 50 milliamperes is heated with 140 milliamperes for 10 seconds. The current is then reduced to normal and the tube is operated for 10 minutes with 200 volts applied to the anode and a negative potential oi 17 volts applied to the grid. This treatment ionizes the residual gas which is cleaned up. The final activation is effected by Bashing for 20 seconds with a heating current of 170-200 milliamperes, and preferably with no voltage between the electrodes.