Visual identification of tubes?

[hiegdk]

Any advice for for visually identifying tubes that have worn or non-existing markings? I found a handful of noval tubes in my stash that either never had proper markings or they've worn off and I'm trying to figure out what they might be. Are there any tell-tale things that would let you differentiate a 12AX7 from a 12AT7 or whatever?

[WimWalther]

It all depends upon who made them. But without some deep & detailed knowledge, it can be very difficult to determine what's what - and even so, it may be nearly impossible.

Best idea would be to test them and see how they read. Of the most common audio dual-triodes, AU have the lowest mu, about 22. AT are next, at about 70. AX are the highest, at 100 or so. 6DJ8 / ECC88 have a very different construction.

One hint.. if you fog the glass and hold it at the right angle under a light, it will often reveal latent traces of markings. Try this before deciding that the marks are totally gone.

[WimWalther]

If all else fails, you could send them to my good friends at TCtubes.com and have Tyler ID & test them for you. He maintains the most versatile tube testing setup in the Midwest, including a pair of Amplitrex AT-1000s.

If there are any rarities in there, he may offer to buy them as well, though these days he's heavy into the equipment repair side of the biz.

Send them an email, link on their site.

[HotBluePlates]

Any advice for for visually identifying tubes that have worn or non-existing markings? ...

Apart from one or two notable exceptions, you'll just be guessing.

Instead, plug the tube into the input stage of a Fender-style amp.  A comparison chart I made tells us how to ID the tube.  Feed a 100mV signal to the grid (verified by measurement), then check the AC volts at the plate:

    12AX7  ---> 5.8 volts
    12AT7 ----> 3.4 volts
    12AU7 ----> 1.4 volts

[tubeswell]

There’s also the tube museum site (where they have a huge catalogue with pictures)

http://www.r-type.org/


http://www.r-type.org/static/find-me.htm