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
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