How to tell 12ax7 from 12au7?

[cspyon]

I've got several Telefunken 12ax7 and 12au7 tubes, but due to the lettering on the tubes almost gone, I really can't identify which one is 12ax7 or 12au7. Is there any good way to identify one from the other?

 

[Dswankey]

Hopefully this help click on the link and the to the left where it says ECC83 Euro for 12AX7 & then ECC82 Euro for 12AU7. Once there click on the pictures which should enlarge and then compare & contrast to what you have

http://www.tube-classics.de/TC/Tubes/SieTel/SieTelIntro.htm

If that doesn't work post some clear close up pictures of what you have and I'm sure someone will be able to clarify for you

 

[cspyon]

Ok, these are close-up pics. Both test strong.

 

[peterh]

Mount them in a test rig and measure their properties.
Start with config for 12ax7

 

[cspyon]

Mount them in a test rig and measure their properties.
Start with config for 12ax7

Well, I've got a simple emission tester Sencore TC162, not so sophisticated as TV 7 D/U, etc. How do the test results show the difference?

 

[egellings]

If you can see the grids of the tubes through holes in the plates, the 12AU7 will have more spacing between the turns on the grid structure. 12AX7 has the turns closer together.

 

[Dswankey]

Well my eyes aren't what they used to be so I can't really tell from the pics you posted. Here's hoping someone with better vision comes along and can help out.

I'm not sure how one will able to tell the difference on an emission tester

 

[peterh]

Well, I've got a simple emission tester Sencore TC162, not so sophisticated as TV 7 D/U, etc. How do the test results show the difference?

I was thinking of building a simple setup as in the : https://frank.pocnet.net/sheets/030/e/ECC83.pdf page 3,
2 resistors and 250V. Compare with values in table. Repeat for ECC82 setup

Or, if you have a "tubetester", configure for 12ax7, test. If "green" then it is a 12ax7
Then configure for 12au7, tes, if green it's a 12au7 ( if tester is worth anything one tube
will fit at most one of the above )

 

[Tom Bavis]

12AU7 will test off scale tested as 12AX7. 12AX7 will test weak as 12AU7. That's on my B&K, SECO, Knight testers. Can't say about Sencore - if it tests emission using grid as the plate like many emission testers, they'll test about the same. But 12AU7 will have low gain in a 12AX7 circuit.

 

[cspyon]

Thanks for your feedback. It looks like the tube tester is the only way to tell the difference.

 

[tonyg88]

If you put it in a preamp section wouldnt the 12au7 just have lower volume? Maybe u can tell from that.