THE GREAT MULLARD MAGIC BLOG — DULL EMITTER

A filament current drain of 1A was a real problem and the search was on to achieve good electron emission from a reduced filament current.  Early efforts were based on the use of alkaline earth metalled filaments where tungsten was doped with thorium, barium or strontium oxides before drawing into filament wire.  Development progressed  until a final configuration of tungsten wire, overcoated with a thorium oxide layer was settled upon.     This greatly reduced filament current consumption and had the side effect of a valve loosing it's bright but homely glow.   As a consequence, these new low current...