I also have tubes that have no emission but no visible defects; I ran
across a 25AX4 like that the other day. I suspect one of the welds is
broken but the gap is too small to see.

The 12AX7 is a miniature tube. I have miniature tubes with pins that
have broken inside the glass base. Usually I notice it when the broken
piece falls out. If it didn't fall out, I would have a defective tube
with no visible problem. (This usually happens outside the seal area so
that air doesn't enter the tube.)

In any event, your tube doesn't work. You're going to have to replace
it no matter what happened to it. You probably didn't cause it. It's
just one of those things.


N_Cook wrote:
What would cause the failure of one half of a 4 year old 12AX7 ? A very
careful owner.
One half tests out fine but for half using pins 1,2,3 cannot get any zeroing
on the tester even setting current down to 0.5 mA or less and for any grid
voltage setting. Can't see any obvious internal weld failure.


--
Diverse Devices, Southampton, England
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--
Jim Mueller

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