>
>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.
// Gas does not wax and wane -- despite what a self-declared expert
claims. Such leakage could be due to loose gold meltballs. There are
two ways to determine such. A high-potential I-leakage test and by
settling the loose gold meltballs. When tapped firmly on the top of the
anode connection with the flat side of a 2 to 4 ounce ballpeen hammer,
gold particles will settle into the base, safely away from the positive
attraction of the anode. Thus, remove each tube, hold it it vertically
in one hand and tap the top of the anode cap with the other. A dozen
firm blows should be enough to make a difference. / Note -- 3cx800A7s,
can stand many Gs in the vertical plane,/ After a gold-sputtered tube
is tapped, if it inverted, the gold will re-contaminate the anode
insulator so the tube must be re-hammered.
> I know the
>previous owner seriously overheated the tubes to the point where the
>delrin insulators around the tubes were melted.
>
// Delrin® does not have a low dissipation-factor, so it is subject to
RF-heating.
>Does this sound like a bad tube or tubes?
>
>Thanks in advance,
>
cheers, Bill
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
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