[sdiy] A simple 12AX7 vacuum tube tester schematic..

[Travis Shire]

> "Paul Perry" <pfperry at melbpc.org.au> wrote:
> >There is little mystery to testing a valve for microphony, I imagine one
> >simply runs an audio signal through it, and taps the valve itself, while
> >watching the output.
> >Any unhappiness will be only too visible (or even audible, if you do not
> >have a scope).
>
>
You don't even need an audio signal. Just scope the plate (through a
coupling cap) while thumping on the tube. Usually most preamp tubes will be
microphonic to some degree....you'll always get some sort of a clunk out of
'em. But they shouldn't ring excessively or make obnoxious noise. Its easier
to hear this than to see it on a scope if you know what you're listening
for.

I've thought about building a jig for testing preamp tubes...I have a couple
boxes full that I need to sort out. And it'd be nice to ID all the ones that
the printing has rubbed off and the type # is not etched on the glass. Not
uncommon to have a box of tubes brought to me with the usual " I was gonna
throw 'em out but I remembered you were into that old stuff" comment. Quite
a few Mullards rollin' around here I'd like to know the condition of.