CATHODES

Thoriated-Tungsten Cathodes

Thoriated-tungsten-filament cathodes are drawn from tungsten slugs which have been impregnated with thorium. In processing, a surface layer of thorium is formed; as a result, these cathodes liberate electrons at a medium temperature (a bright yellow). They have an emission efficiency between that of pure tungsten and coated cathodes.

Thoriated-tungsten filaments are suitable for use in tubes operating at a fairly high voltage. They are not used in tubes operating at extremely high voltages because the surface layer of thorium may be sputtered off by positive-ion bombardment; this results in loss of emission. Thoriated-tungsten filaments are substantially free from grid-emission effects and possess the unique capability of being reactivated (in many cases) after their emission has been lost because of temporary tube overloads.

The choice of cathode material for a transmitting tube depends upon the service for which the tube is designed. The plate voltage to be used on the tube is an important factor, as is also the cathode emission required. In general, coated cathodes have been employed in small, low-voltage tubes; thoriated-tungsten 6laments have been used in medium-power tubes operating at fairly high voltages; and tungsten filaments have been used in large, high-power tubes operating at very high voltages However, design requirements control the choice of cathode material for specific RCA tube types.