Mullard 12ax7 testing?

[jcr1234]

I was lucky enough to pick up a couple pulls at a great price. When I tested one of them, one triode was 1325 gm, and the other was 2150 gm. There are no shorts. With 1250 being nominal, is the way high reading on the second triode anything I need to be concerned about?

[Phil_S]

It's probably faulty. Send it to me and I'll make sure to dispose of it in an environmentally friendly way, like maybe putting it in V1 of my 18W. I wouldn't want to put you out, but can you please remember to tell me which triode is which, so I can rewire my amp appropriately if needed.

[dotfret]

Mullard were usually quite tight about quality matters, but gain was not one of their major concerns, as long as there was plenty of it!

The release limit on a dual triode was +/- 20% of the pair average, so this one has strayed to another 4% off target. Should still be useful in a lot of applications, but be wary of noise.

The higher the gain, the more noise you are likely to get. Lower gain 12AX7 have better S/N than high gain, according to an old Mullard study I once read (and now cannot find).

It should be OK in a lot of guitar applications. If you're nervous about it, use it as the phase splitter, where the gain is a less critical factor.