Preamp tubes that have no letters on them?

[Blueswede]

Is there any way to identify preamp tubes that are basically blank? No lettering on them.......? I have about 6 that I'd like to know if they are AX, AU, AY......Thanks in advance. Steve

[mbratch]

Some tubes have a code etched in the glass you can look up. It would be close to the base somewhere. But you have to look closely, perhaps with a lighted magnifier.

[rockon1]

Been there before! A bit of elimination can do it sometimes. Are they NOS or new production? bob

[Blueswede]

I think most are NOS.....

[SatelliteAmps]

They wouldn't be NOS if there were no markings. They would be used. NOS means NEW OLD STOCK, meaning vintage tubes that have never been used or handled. Even used for an afternoon means they are no longer NOS.

Tubes that are used are usually referred to as pulls, because they were pulled from an amp or other circuit in the course of a repair. A lot of those have their lettering rubbed off, and can be hard to identify.

You can always test them in an amp or with a tube tester to see which version they are.

[Blueswede]

I stand corrected.....they are used.

[rockon1]

They wouldn't be NOS if there were no markings. They would be used. NOS means NEW OLD STOCK, .

Ok somehow I knew somebody would jump on that. Just trying the simplest way of finding out if they are old stock or new production.

OP: Visual ID: Generally speaking old stock 12AX7's and 12AU7's are virtually identical in appearance internally.Fortunately the gain is much different and can be easily noticed by swapping them in an amp. Having a large collection of known 12AX7's I can use this to ID some unmarked 12AU7's and 12AX7's. A 5751 would be much harder to distinguish in this manner.
12AT7's often had an "aerofoil" shape to the plates fairly unique to them but also came with flat plates. Both are much smaller than 12AX7 or 12AU7 plates. I would say the "aerofoil" shaped plates would would probably be 12AT7's while the flat plates could be 12AV7's or 12AT7's. These would be difficult to distinguish from eachother in an amp based on gain factor alone.
All the 12AY7's I own are are short plates and similar looking to short plate 12AX7's. They could be distinguished from a 12AX7 by gain in an amp.

So you see this gets a bit tricky/difficult to do. Perhaps if you posted detailed pics I could help more.

Short of expensive testing equipment I wonder if thiere some way to check resistance or something with a MM. Sounds to easy... Anyhow thats all Ive got for now. HTH Bob

[BigPapiFan]

I have had a similar situation. I came into a rather large lot of tubes including some Mullard 10M "Master Series" pre-amp tubes which had been re-branded for scientific instrument use without any indication of type. Here's what I did:

1. Examine the tube and compare with known tube types. If you don't have a selection of NOS or used tubes, don't fret. There are a number of internet resources that present nice representative images of a variety of tubes:

Tubemonger library: http://www.tubemonger.com/Pre_Tubes_s/1.htm
Tubeworld: https://www.tubeworld.com/12ax7.htm

Other forums where knowledgeable people are usually happy to help with identification include:

The Gear Page Amp Technical Information Page
Tubes Asylum: http://www.audioasylum.com/audio/tubes/bbs.html
Plexi Palace Tube Forum: http://vintageamps.com/plexiboard/viewforum.php?f=13&sid=e143224aad29ffcc5e197fb5fbfa9585

2. I tested the tubes as 12AX7s this would indicate to me whether or not the pinouts and specs were in the 12A_7-family.

3. I then put the tubes into V1 or V2 of my Fender amp and compared gain, sound, volume with known tube types ( and suspected manufacturer, if possible).

After all that, you should be able to get a fairly accurate identification.

Oh, yeah...They were 10Ms and they sounded like Mullards.

[Timbre Wolf]

I have had a similar situation. I came into a rather large lot of tubes including some Mullard 10M "Master Series" pre-amp tubes which had been re-branded for scientific instrument use without any indication of type.
True Mullards are easy to i.d. by their etched codes. Just look for the "type" code (I63, for instance, for short-plate 12AX7A, or 654 for 12AT7WA/CV4024).




OP: want to post some photos? That's the easiest way to get an i.d. fast around here.

- Thom

[rockon1]

OP: want to post some photos? That's the easiest way to get an i.d. fast around here.

- Thom


Yep or at least closer. Bob

[BigPapiFan]

True Mullards are easy to i.d. by their etched codes. Just look for the "type" code (I63, for instance, for short-plate 12AX7A, or 654 for 12AT7WA/CV4024).

- Thom

Indeed, they had the I63 and a Blackburn code as well!

[Timbre Wolf]

Indeed, they had the I63 and a Blackburn code as well!
There y'go! :BEER

- T

[rockon1]

Sweet. If your trying to ID "unmarked" (which isnt the case here really) 12AX7's it doesnt get much easier or nicer than that. Bob

[Blueswede]

Thanks for all of the great info....I'll post pics as soon as I have the opportunity to take some.