How Do You ID Tube Type with No Marking?

[captouch]

How do you guys ID tubes with no markings at all?

I ran across a couple of tubes that have the Telefunken diamond on the bottom, but no visible markings on the tube itself. I tested them on my tube tester guessing they were 12AX7's, and they gave readings similar to other 12AX7's I've tested, but other than that, I can only guess what they really are.

I have another tube that was sold to me as a 12BH7, with faint Motorola markings on it, but no tube type. This one didn't give any reading at all on my tube tester set to 12BH7 settings, so I'm guessing it's not a 12BH7 after all, but no idea how to ID it otherwise.

I can post pictures, but not sure if tube type can be ID'd by visuals alone.

Thanks.

 

[cademan]

I can post pictures, but not sure if tube type can be ID'd by visuals alone.
thanks.

You can generally tell by the plate structure but you still won't know the pinout or heater voltage like a 6BQ5 compared to an 8BQ5, or 6AN8 compared to a 6GH8 or 7199 etc.

I think the 12BH7's are a little longer than your typical 12AX7/12AU7 tube. Same pinout though.

 

[nashvillebill]

Try putting them in the freezer for a few minutes, then take them out and look very closely under a bright magnifying light, sometimes they will form condensation where the lettering used to be. Also look very carefully on top of the tube as well as the sides.

 

[ferninando]

maybe superglu fumes.
works for fingerprints.

 

[heyraz]

Very similar to Nashvillebill..
In the same manner one would "huff a moist breath" on their eyeglasses to fog them before wiping, I fog the tube in question and look for any markings before the condensate clears. It works better if you can catch the lettering at an angle and should remain fogged longer if the tube is cold.
Tele's sometimes have a sequential number code, 163=12ax7, 162=12au7, 161=12at7

http://www.audiotubes.com/mullcode.htm
http://www.tubemaze.info/eia-codes-to-id-tubes/

 

[captouch]

Thanks guys. I'll huff and puff (though not superglue fumes) and/or freeze see what happens.


Heyraz, so the sequential number codes are on the painted portion or in the diamonds on the bottom. There are some numbers there, but I don't know if they have any significance.

 

[sKiZo]

Well, I'll be dipped and twiddled ... never noticed the Tele's don't have etched numbers ...

Don't know as this helps, but the center tube is a 12AU7 equivalent ...

 

[captouch]

Thanks for the pic. My tube looks a lot like the center tube, but don't 12AX7's also look like that as well internally? (I guess I'm hoping that it is a 12AX7, because I have more use for those than 12AU7's).

The other thing I can do, I suppose, is test a 12AU7 on my Hickok and see if the umhos reading is close to or much different from a 12AX7, because these two unknown Tele tubes give readings very typical of other 12AX7's.

 

[captouch]

Oh, and I did try the puff and freeze method.

My breath must not be hot and steamy enough, because just trying to moist puff on it doesn't really create any fog on the glass. The freezer did work, but it dissipate pretty quickly, so I have to work fast to try and see anything.

I guess I can try again - tubes can't be harmed by this type of temp cycling I guess?

 

[zandru]

How to find vacuum tube numbers

https://www.youtube.com/watch?v=XgdpAlarIvI&list=UUrKt6CZ5ZTBgbYcCiLGS2Lw&spfreload=10

Hope this helps......

Cheers.....have a Festive Holiday Season and New Year.......

 

[mech986]

Oh, and I did try the puff and freeze method.

My breath must not be hot and steamy enough, because just trying to moist puff on it doesn't really create any fog on the glass. The freezer did work, but it dissipate pretty quickly, so I have to work fast to try and see anything.

I guess I can try again - tubes can't be harmed by this type of temp cycling I guess?

You may have heard of the fad for cryo treating tubes (and other things like brake rotors)? So probably no sweat being in 0 deg. F freezer.

As for readings, the dual triodes 12AT7/12AU7/and 12AX7 are all of the same family and pinout, and have low to higher gain (gm) respectively. They may have similar setting on your tube tester, so if your tester give transconductance directly (gm) then you can figure them out, or test against reference tubes.

 

[captouch]

https://www.youtube.com/watch?v=XgdpAlarIvI&list=UUrKt6CZ5ZTBgbYcCiLGS2Lw&spfreload=10

Hope this helps......

Cheers.....have a Festive Holiday Season and New Year.......

Wow, this will be interesting to try. He didn't explain explicitly, but I guess forehead oil is literally oil you get from your own (or other oily complexioned pal's) forehead?

Sounds like a promising pick up line for single guys. "Excuse me miss, I'm trying to ID some old vacuum tubes. Can I borrow some oil from your forehead?". :scratch2:

Oh, wait. Most women probably wouldn't take it as a complement to be told they have oily skin. Never mind. . .

 

[captouch]

You may have heard of the fad for cryo treating tubes (and other things like brake rotors)? So probably no sweat being in 0 deg. F freezer.

As for readings, the dual triodes 12AT7/12AU7/and 12AX7 are all of the same family and pinout, and have low to higher gain (gm) respectively. They may have similar setting on your tube tester, so if your tester give transconductance directly (gm) then you can figure them out, or test against reference tubes.

Cool. If the oily forehead method doesn't work, I'll try this with reference tubes and see how different the Gm readings are between 12AT7's, 12AU7's, and 12AX7's.

Thanks!