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

[David Moylan]

Here it's probably better to listen rather than watch.  Most valves are 
a little microphonic, since the elements can't be perfectly stabilized, 
but there are definitely differences and one that would be labeled 
microphonic will ring much louder when tapped.  If it's really bad and 
you have a high gain circuit you can get them to feedback.

I used to work for a custom amp shop that used a lot of octal pre tubes: 
6SL7, 6SN7 and we had to test every one no matter who supplied them. 
12AX7's and the like were usually better in that regard.

Dave

Paul Perry 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).
> 
> paul perry Melbourne Australia
> 
> 
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