GB682153A - Improvements in or relating to thermionic valves employing secondary electron emission - Google Patents

Abstract

682,153. Thermionic valves. ELECTRIC MUSICAL INDUSTRIES, Ltd. Oct. 8, 1947 [Nov. 12, 1946], No. 33556/46. Class 39(i) A thermionic valve used for amplification comprises a thermionic cathode 4, a control electrode 5, a screen electrode 6, a secondary cathode 1, a secondary electron collecting electrode 2 and beam focusing electrode 10, the secondary cathode 1 having a secondary electron emissive coating the emission from which increases with increasing temperature and the secondary cathode being of such small size and the dimensions and spacing of the other electrodes being such that under the operating conditions for which the valve is designed, the secondary cathode is maintained only as a result of bombardment by primary electrons, and by heat communicated from other electrodes of the valve, at a temperature which is more than half, but does not exceed, the temperature (on the Centigrade scale) at which the valve becomes unstable due to thermionic emission from the secondary cathode. The secondary cathode 1 is formed by coating a tubular nickel base with a mixture of refractory oxides containing at least 15 per cent by weight of alkaline earth oxides, e.g. magnesium oxide and barium oxide in the proportion of 2:1 by weight, the coating being between 10<SP>-4</SP> and 5 X 10<SP>-4</SP> inches thick, as described in Specifications 670,554 and 670,582. The electrodes 5 and 6 are formed with approximately elliptical turns of molybdenum wire, The focusing electrode 10 is of elliptical cross-section and is connected to the indirectly heated thermionic cathode 4. An arcuate nickel heat shield 11 is secured to one of the supporting pillars for the screen electrode 6. The ends of the electrodes and the supporting pillars therefor are insulated from each other by discs of mica 12 to prevent transfer of heat by conduction. Electrostatic shields 13, cooling fins 14 for the control electrode 5 and a gettering cup 15 are provided. The valve when in use is normally enclosed within a metal shielding can. Dimensions and voltages of electrodes are given. The thermionic cathode 4 in operation reaches a temperature of about 800-900‹C. The temperature reached by the secondary cathode 1 is about 500‹C, instability not occurring until about 600‹C. A multiplication factor of between 3.5 and 4 is obtained which is maintained approximately down to about 350‹C, but with temperatures considerably lower than 350‹C the factor falls to about 2.7 or lower. Specifications 489,923, [Group XXIX], 586,000 and 601,966 also are referred to.