Line scanning valves and circuits

The post-war trend towards higher operating voltages and wider scanning angles in cathode ray tube design has necessitated the development ot high efficiency scanning circuits in order that the advantages of these tubes may be realized without unduly high power consumption. This economy has been effected by making use of the resonant return type of time base which recovers a part of the energy stored in the deflection field at the end of each line. This paper discusses the requirements of scanning valves arising from these circuits. Generally speaking, pentodes have to withstand high anode voltages for a fraction of the cycle with the cathode current off, and thermionic emission from the screen must be kept at a low value under these conditions. This has been achieved by designing the valve so that the screen runs cool (i.e. aligned grids) and also by previous treatment of the screen coupled with suitable processing. Diodes have to withstand a high inverse peak voltage and should have a low forward impedance. The voltage requirements are not completely new but these are mass produced valves intended for commercial sets, whereas previously such valves were regarded as special low production types. Insulation properties of mica have been investigated under these conditions, and valves using all mica insulation have been rated at 6 kV (design centre) on the anode.