Patent GB-859140-A (GB)
Title: Improvements in or relating to cathode ray tubes
Inventor: WOODBRIDGE LEONARD ALBERT
IPC: H01J31/12
Filed: 1956-05-08
Abstract: 859,140. Cathode ray tubes. ELECTRIC & MUSICAL INDUSTRIES Ltd. May 8, 1957 [May 8, 1956], No. 14206/56. Class 39(1). In a device in which signals such as a sound wave are recorded on film by means of a line of light generated by a ribbon beam of a cathode ray tube striking a flourescent screen at least one end of the line of light is given a sharp edge by generating two or more discrete ribbon electron beams one of which is completely stopped off by a mask except for a narrow beam at one edge and the other of which passes wholly to the screen one edge of the latter beam striking the screen at the same region as the narrow beam of the smaller beam, so that the line of light produced on the screen by the final ribbon beam exhibits a sharp cut off at at least one end. The tube has two separate cathodes 5, 6, Fig. 2 or a single cathode with an apertured mask 11, Fig. 3, in front to split up the electrons into two beams. Alternatively there may be three cathodes, Fig. 4 (not shown) to achieve sharp cut off at both ends of the line of light. If the latter device is used in a system requiring sharp cut off at one end only the cathode position is selected as the more favourable of the two possible positions, in view of manufacturing inaccuracies. The line of light is produced by means of a beam which is primally of ribbon cross-section (at the fluorescent screen). In one form Fig. 2, the two emissive coatings 5, 6 are carried by a strip 2 fitting into grooves in a circular projection from, and resting on diametrically extending knife edges 9a on an insulating disc 8 ; the strip 2 is supported at the centre by a collar welded thereto the collar being secured to a support rod or tube (not shown) and by a second collar on the other side of 8 but is free to expand at both ends so that it does not buckle under heat expansion forces. The insulator 8 has four apertures 10 through which pass ceramic gun support rods. To make the cathode structure the cathode support is sprayed through the mask 11 of Fig. 3 while the mask is attached to the disc 8 by four screws passing through the peripheral apertures. Alternatively, the two electron emissive layers can be mounted separately. The cathode in all cases is indirectly heated and is housed within modulator 20. Seven slotted gun electrodes and the cathode are mounted on ceramic support rods 19. Electrodes 22, 23, 24, 26 and 27 are connected together and one of them is connected by a welded-on contact to the wall anode (not shown). Plates 28 for altering the path or shape of the beam may be replaced if desired by a further electrode connected to 22, 23, 24, 26, 27. One transverse edge of the slot in electrode 27 serves as the beam stop mask and louvres 30 are welded to deflecting plates 29, which extend almost to the fluorescent screen, to trap stray electrons. The separation of the louvres decreases towards the screen as the angle of approach of the electrons to plates 29 decreases. The slots in the electrodes are shaped to correct for end effects as described in Specification 573,901. The slot performing the function of the mask may be in any gun electrode but is preferably in an electrode located as close as possible to the screen. 24, 25, 26 form a three element lens.