CATHODES

Coated Cathodes

Coated cathodes are of two general types - the directly heated filament cathode and the indirectly heated cathode. The coated-filament type of cathode consists usually of a nickel alloy wire or ribbon coated with a mixture containing certain alkaline earth oxides. This coating, consisting of a substantial layer on the filament wire, requires a very low temperature (a dull red) to produce a copious supply of electrons. Coated filament cathodes, therefore, require relatively little heat energy and have a high emission efficiency -many times that of tungsten.

A heater cathode comprises an assembly of a thin metal sleeve, coated with an active material similar to that employed on coated-type filaments, and a heater element contained within and insulated from the sleeve. The heater is usually made of tungsten wire, or of a tungsten-molybdenum alloy, and is used solely for the purpose of heating the coated sleeve (actual cathode) to an electron-emitting temperature. The sleeve is heated by conduction and radiation from the heater. Due to the fact that the coated cathode is isolated electrically from the filament heating source, it is also called a unipotential cathode; unlike filament-type cathodes, it has no voltage drop along its length due to a heating current.

Advantages of the coated cathode are its high emission efficiency, relative freedom from filament or heater burn-out, low operating temperature, and its comparatively low hum level (especially in the unipotential-cathode type).

A disadvantage of the coated cathode is its tendency to contaminate adjacent electrodes with small quantities of active emitting material, so that emission from these electrodes may take place at relatively low temperatures. Despite their high emission efficiency, coated cathodes have been used in transmitting tubes principally in small, low-voltage types where operating temperatures of the electrodes are relatively low.