On Nov 22, 8:36*am, "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
electronic hints and repair briefs , schematics/manuals list onhttp://home.graffiti.net/diverse-

You will almost never see the weld crack since it usually only shifts
a thou. or two. I tested a tube for failed cathode ribbon connection
by connecting a 'good' tube filament - cathode [cold] across an RF
inductor on a Q meter and tuning for resonance, then connecting the
'bad' tube in it's place. The capacitance, tiny as it normally is, was
much less as shown be a significantly higher resonant frequency.

Neil S.