How to identify 12AX7 and 12AU7

[karsavuran]

Hi all,

I have (hopefully) two 12AX7 and a 12AU7. All Telefunken. Their lettering is completely gone. They look completely identical to me. I only have a DMM. Is there a way for labeling them?

Best,
Ozan

 

[knockbill]

Put em in a working amp and see which one has the most gain??

 

[karsavuran]

Put em in a working amp and see which one has the most gain??

Would this be harmful to tubes or amp?

 

[Tom Bavis]

>> Would this be harmful to tubes or amp?

No.

 

[Mr Boochie]

Hi all,

I have (hopefully) two 12AX7 and a 12AU7. All Telefunken. Their lettering is completely gone. They look completely identical to me. I only have a DMM. Is there a way for labeling them?

Best,
Ozan

"Thanks a million for the link!!!
I've solved the mystery of the distinction between 12AX7 and 12AU7 telefunken.
It depends on the position of the getter support.
I've checked a dozen telefunken tubes, and found out the consistent location of the getter support of the two:
When the getter support is on the 1st pin, it's 12AX7,
When the getter support is on the 3rd pin, it's 12AU7.
Thanks all for your feedback and input, everybody, especially aclime!"

 

[karsavuran]

Put em in a working amp and see which one has the most gain??

Thank you that idea worked.

"Thanks a million for the link!!!
I've solved the mystery of the distinction between 12AX7 and 12AU7 telefunken.
It depends on the position of the getter support.
I've checked a dozen telefunken tubes, and found out the consistent location of the getter support of the two:
When the getter support is on the 1st pin, it's 12AX7,
When the getter support is on the 3rd pin, it's 12AU7.
Thanks all for your feedback and input, everybody, especially aclime!"

And I verified with this information. Thanks.

 

[egellings]

Another possible way that would work if you can see a part of the grid is that the 12AX7's grid will have the turns of wire spaced more closely together than those of the 12AU7.