Identifying 12AX7 Tubes

neddyboy · 10th March 2010, 05:05 PM

I got a box of maybe 100 tubes from an old tech (for $10!), but many of the tubes I'd be able to use have had all their labels rubbed off. There are maybe 10 12AX7-ish preamp tubes in there.

First, is there any way to identify a 12AX7 (or AU7 or AT7 etc.) visually? I think they're somewhere in the family by looking at the construction, but beyond that I got nothing. Are there tubes that look like these tubes that could damage my amp?

I have a pretty basic Heathkit tester, and if I test the mystery tubes on the 12AX7 setting they come up as good. The tester just tells me if they're shorted or leaky, and whether they're good or bad. With this tester, can I assume if they test good as 12AX7s they ARE 12AX7s? I assume not. My guess would be that any suitable replacement for 12AX7 would also test good using the 12AX7 settings. True?

TheGimp · 10th March 2010, 05:18 PM

Plate sizes are different for the various 12A_7 family members. Although the plate size will also vary from mfg to mfg.

Differentiate by measuring Mu would work, but the emission tester won't tell you much. Look on the chart and see if there is a setting that is different for AU7 vs AX7 vs AV7 ...

Use it to bin the tubes.

TheGimp · 10th March 2010, 06:05 PM

Mu ranking is 12AU7 (20), 12AV7 (40), 12AT7 (60) and 12AX7 (100).

Plate power dissipation Ranking is 12AX7 (1.2W, 12AT7 (2.5W) 12AV7 (2.7W) and 12AU7 (2.75W).

neddyboy · 10th March 2010, 08:34 PM

Thanks Gimp. Now I have to figure out how to measure mu on my Heathkit IT-17!

Is Johnson City in east TN? I lived in Knoxville for 6 years.

TheGimp · 10th March 2010, 10:56 PM

90 miles from Knoxville. Just below Kingsport and Bristol.

The IT-17 is one of the Heathkit basic emission testers. Careful comparison of the setup for the different 12A_7 tubes with the emission strength may allow you to segregate the tubes in question.

I've not found the manual and scroll chart yet, but if I do I'll see if I can tell anything from them.

SY · 10th March 2010, 10:59 PM

Quote:

Plate sizes are different for the various 12A_7 family members.

I wouldn't count on that- there's SO much variation. In fact, the JJ 12AT7 and 12AX7 look identical. And I have a bunch of Eastern European 12AX7 from the late '70s-early '80s that have very tiny plates, visually identical to many 12AT7.

Really, it's pretty easy to set up a test jig to sort them.

Tom Bavis · 11th March 2010, 12:26 PM

12AU7 will have the most emission - any that seem to be low may be 12AT7, 12AY7 or 12AX7. Many 12AT7s WILL test low, as TVs and FM radios ran them pretty hot, so they were often the first ones needing replacement...

10th March 2010, 05:05 PM	#1
neddyboy diyAudio Member Join Date: Sep 2006	Identifying 12AX7 Tubes I got a box of maybe 100 tubes from an old tech (for $10!), but many of the tubes I'd be able to use have had all their labels rubbed off. There are maybe 10 12AX7-ish preamp tubes in there. First, is there any way to identify a 12AX7 (or AU7 or AT7 etc.) visually? I think they're somewhere in the family by looking at the construction, but beyond that I got nothing. Are there tubes that look like these tubes that could damage my amp? I have a pretty basic Heathkit tester, and if I test the mystery tubes on the 12AX7 setting they come up as good. The tester just tells me if they're shorted or leaky, and whether they're good or bad. With this tester, can I assume if they test good as 12AX7s they ARE 12AX7s? I assume not. My guess would be that any suitable replacement for 12AX7 would also test good using the 12AX7 settings. True?

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10th March 2010, 05:18 PM	#2
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	Plate sizes are different for the various 12A_7 family members. Although the plate size will also vary from mfg to mfg. Differentiate by measuring Mu would work, but the emission tester won't tell you much. Look on the chart and see if there is a setting that is different for AU7 vs AX7 vs AV7 ... Use it to bin the tubes.

10th March 2010, 06:05 PM	#3
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	Mu ranking is 12AU7 (20), 12AV7 (40), 12AT7 (60) and 12AX7 (100). Plate power dissipation Ranking is 12AX7 (1.2W, 12AT7 (2.5W) 12AV7 (2.7W) and 12AU7 (2.75W).

10th March 2010, 08:34 PM	#4
neddyboy diyAudio Member Join Date: Sep 2006	Thanks Gimp. Now I have to figure out how to measure mu on my Heathkit IT-17! Is Johnson City in east TN? I lived in Knoxville for 6 years.

10th March 2010, 10:56 PM	#5
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	90 miles from Knoxville. Just below Kingsport and Bristol. The IT-17 is one of the Heathkit basic emission testers. Careful comparison of the setup for the different 12A_7 tubes with the emission strength may allow you to segregate the tubes in question. I've not found the manual and scroll chart yet, but if I do I'll see if I can tell anything from them.

11th March 2010, 12:26 PM	#7
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	12AU7 will have the most emission - any that seem to be low may be 12AT7, 12AY7 or 12AX7. Many 12AT7s WILL test low, as TVs and FM radios ran them pretty hot, so they were often the first ones needing replacement...

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