The removal of plate and grid voltages can be readily accomplished by the construction of an adapter socket with filament connections only. The voltage applied to the filament during rejuvenation mush be carefully controlled to the values given. The accompanying graph shows the results of various voltages applied to a thoriated tungsten filament during rejuvenation. It shows that a voltage lower than the recommended value will eventually result in a fairly good tube, while too high a voltage will result in a tube which will remain weak. The vacuum and the original outgassing of the elements in these older tubes was not near the present day standard, therefore, they contain considerable residual gases.
Vacuum tubes have essentially three basic types of emitters.
The type of emitter in a given tube can be determined by its operating color at rated filament voltage. The pure tungsten filament operated bright white, the thorated tungsten filament runs orange to yellow, while the oxide emitter operates in the dull red region. The pure tungsten filament needs little rejuvenation as its operating temperature makes it self-cleaning. This type of filament was used in such tubes as the UV200, UV201, and in many types of transmitting tubes. The thorated tungsten filament is probably the major one to be dealt with by the collector.