> I have a Commander HF-2500 which uses a pair of 3CX800's. Recently I've
> noticed the plate current does not drop back all the way to zero when
> un-keying. The longer it's on and the hotter it gets, the higher this
> current is. It starts out around 50 ma and after a half hour or so of
> RTTY, will get up to 250-300 ma. If I let it sit with filaments on but
> not keyed, eventually the current will drop down again.
>
> I'm suspecting some out-gassing in one or both of the tubes. I know the
> previous owner seriously overheated the tubes to the point where the
> delrin insulators around the tubes were melted.
>
> Does this sound like a bad tube or tubes?
It is not necessarily at tube problem, especially if you can drive the
PA to normal output when it is acting acting up.It is certainly not
gassing, because that causes a HV anode to grid/cathode
flashover....a "big bang".
Only a grid-cathode short will do that, so far as tune problems
unless the amplifier unwisely dc grounds the heaters instead of
floating them with the cathode. In the case where the heaters are
grounded, a heater-cathode short could also cause the problem.
Other than that, it has to be outside the tube.
73, Tom W8JI
W8JI@contesting.com
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