How to identify an unmarked tube?

[Ripthorn]

Hi guys, a friend of mine gave me some tubes he doesn't use, 3-5751's and an unmarked tube that he thinks might be a 12ax7, but he isn't positive. I was wondering if there is any kind of simple test (short of using a tube tester, which I don't have) to identify it.

[rfgordon]

The first thing to do is check to see if you have continuity, with a very low resistance (<100 Ohms) between all the filament pins: 4,5 & 9.

If that is the case, and there is no other continuity among pins, it may well be a 12A-series tube.

[Ripthorn]

Well, there is about 12 ohms between 4 and 5 and about 6 ohms between 4 and 9. Anything else I can do?

[David Root]

If you can post a pic of this tube that would help. 12AT7 plates, for example are very short.

[rfgordon]

Plug it in and see what happens!

I mean that, seriously. If there is no other continuity in the tube, you shouldn't have a disaster.

Here's what I would do: plug my stereo into the amp, so there's a signal on it. Stick the tube in the socket and turn on the mains. Does it seem to glow like the other 12AX7s?

If so, flip off the standby. If the amp makes good sound, then you have your answer. If there's no sound or something weird happens (like blowing a fuse), well, you also have your answer.