[Amps] Thoriated tungsten lifetime when idling tspa

[k7fm]

Peter said:

"There can definitely be a problem running oxide coated cathodes for a long 
time without drawing current: it leads to a build up of cathode interface 
resistance, and according to one of my textbooks (I hesitate to admit owning 
one on pulse and digital circuits, being somewhat akin to admitting to 
owning pornography !) it was a major cause of failure in early tube 
computers."

This brings up a point from long ago (which I never fully understood). 
Tektronix faced the cathode intefface problem with some of their tube 
oscilloscopes.  Certain tubes in certain circuits seemed to have the 
problem.  As I recall, they solved the problem not by redesigning the 
circuit but by specifing special qualities in tubes used, finding that 
certains tubes were more prone to cathode interface.

But, this is from many decades ago and from events that happened in a 
vacuum.  The implication is that certain tubes may be more prone to the 
problem than others.

Colin  K7FM