|[AMRadio] Re: AMRadio Digest, Vol 51, Issue 33|
k4kyv at charter.net
Mon Apr 28 18:33:18 EDT 2008
>>If there is visible black crud inside, that looks like the ashes of
>>burnt newspaper, the tube is probably NG. That stuff tends to peel
>>off the anode of tubes with many hours on them, and the tubes nearly
>>always arc over no matter what.
> Wow! It's interesting that you should mention this: the ones I
> remember trying quickly started to look like there was some kind of
> film coming off of the anodes (they certainly didn't start that way).
> It looked *exactly* as you describe. We could never get those tubes
> to behave and finally went back to the 3B28s. Any idea what it is/was?
> Kim Elmore. N5OP
I'm not exactly sure what it is. I have to assume there is some kind of
chemical reaction between the mercury and the anode metal, that forms a
layer of corrosion, and the corrosive layer then flakes off. No idea if
the black stuff is the cause of the problem or a symptom of the underlying
I have tried to carefully tap the tube in such a way that all the black ash
falls to the bottom, and usually the tube still arcs. So this stuff may be
be associated with some kind of poisoning of the environment inside the
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