ax7 or au7 ?

[stoo]

I have a several tubes that have all the paint worn off. I know they are either 12ax7 or 12au7. If I plug them into my amp can I ID them by comparing voltage drops across the cathode or plate resistors or some other electronic way? To me just listening is not conclusive enough. My ears haven't been the same since the '69 Canned Heat concert
Thanks for any/all advice!
Stew

[fishy]

Might be wrong here but here goes.
The Ra for the 12ax7 (~70) is 10X that of a 12Au7 (~7) so maybe looking at the output impedance of the stage might help. All things being equal, I would expect the au7 to be lower.

[Structo]

Yes, gain should be much lower.

[stoo]

Yes, gain should be much lower.

right, but is that something I can measure with a volt meter or do I need a signal source and a scope? I was sort of hoping that the tube characteristics would be different enough ( idle current ect.) that plate or cathode voltages would give away there identity.
Stew

[ic-racer]

I have a several tubes that have all the paint worn off. I know they are either 12ax7 or 12au7. If I plug them into my amp can I ID them by comparing voltage drops across the cathode or plate resistors or some other electronic way? To me just listening is not conclusive enough. My ears haven't been the same since the '69 Canned Heat concert
Thanks for any/all advice!
Stew

I have some similar tubes. One of them got put into my ODS build when I first fired it up and checked voltages. I got the following plate and cathode voltages:

12??7 =

V2A = 133v and 2.5v
V2B = 149v and 2.3v

With a 12AX7 I got

V2A 205v, 1.6v
V2B 193v, 1.8v

So, yes the voltages should be different enough to tell them apart. But I don't know about distinguishing AT7 from AU7 though.

[VacuumVoodoo]

Compare with a known 12AX7. In LTP PI slot without NFB ( or presence at max) it should be pretty clear difference in volume between thw two types, just don't change any settings.

[muchxs]

So, yes the voltages should be different enough to tell them apart. But I don't know about distinguishing AT7 from AU7 though.

Telling an AT7 from an AX7 or an AU7 is easy. AT7s have relatively small plates that are frequently teardrop shaped in cross section. Tiny plates = AT7. Even short plate 12AX7s don't have a plate structure that compact.

[Firestorm]

Find somebody with a tube tester. The differences will be very evident. You can also do it with a signal generator, but you'll have to build a (or use an existing) preamp circuit and measure the gain.