GB634639A - Improvements in or relating to television tubes incorporating electron multiplier units - Google Patents

Abstract

634,639. Cathode-ray tubes. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 18, 1946, No. 8365. Convention date, Sept. 16, 1944. [Class 39 (i)] A television tube in which an electron beam from an electron gun including a first anode is projected through an aperture in said first anode towards a target and electrons not collected thereby are returned to the first anode, is characterized in that the first anode constitutes the first dynode of an electron multiplier which includes at least one additional dynode and a collector electrode disposed coaxially around the electron gun and to the rear of the first anode. In the construction shown in Fig. 1, the first anode 21 surrounds the indirectly heated cathode 19 and the control grid 20 and has its outer surface adapted to act as the first dynode of the multiplier stage. The dynode 21 is preferably integral with sleeve 23 around which is placed an insulating sheath 24 made of glass. The sheath may be omitted and the insulation obtained by air space. A plurality of dynodes 25, 26, 27 and 2.8 surround the gun and may be mounted on the sheath 24 by frictional contact and may be spaced and supported at their peripheries. All the dynodes have surfaces of high emissivity for production of secondary electrons. A collecting anode 29 of mesh or screen type is placed in front of the last multiplier dynode 28. A ring anode 34 extends from near the outer surface of the dynode 21 towards the target end of the tube for a sufficient distance to shield the multiplier stages from the wall-coating voltage, and may be given the same potential as the first dynode. The usual wall-coating electrode 36 which may be a metallic film may extend from near the ring 34 to near the decelerating lens 13. The dynode units 25, 26 and 27 are similar and each consists of a metal plate having a secondary electron emissive surface and so constructed that the electrons emitted therefrom pass therethrough to the rear of the tube. The dynode 25 shown in Figs. 2 and 3 is stamped from a disc of silver magnesium alloy so as to have 32 substantially radial vanes 37 separated by slots 38. The vanes 37 are inclined at an angle of about 30 degrees to the plane of the disc and the direction of inclination is preferably reversed for alternate dynodes so as to form a tortuous path for the electrons. The stamping die is so constructed that the outside ends 41 and the inside ends 42 of the vanes are left integral with the outside ring 43 and the inside ring 44. A wire mesh or screen 45 is mounted at the side of a dynode unit receiving electrons from the preceding stage and is given the potential of the associated dynode and can therefore be mounted directly in a metal frame 46 to which the dynode is conductively secured. The dynode and screen are mounted on wire rings 47 and 48 for rigidity. The screen may be made by knitting together wires of 1 mil. diameter having openings between the wires totalling about 90 per cent of the screen area. The last multiplier dynode 28 is a plane disc of metal without vanes. When only two multiplier stages are used the last stage 28 takes the place of the second stage 25. In a modification, Fig. 5, concentric rings 49, 50 are maintained at a potential equal to or slightly less than that of the final dynode 51 and a collector grid 52 is provided. In a modification the final dynode is formed with the concentric rings integral therewith and a fine screen at or near the potential of the dynode is placed in front of the collector grid. In a further modification the concentric rings are electrically connected and given the collector potential so that they serve as the collecting anode. Other types of dynode such as perforated discs or mesh screens may be used instead of the vane type.