Tube ID help

[cryptozoo]

Does anyone recognize the type of tubes the two to the right of the 6V6 in these pics are? 8-pin, no markings on the glass.

[Enzo]

Look like twin triodes, my guess is the one on the right might be a 6SL7 and the center one maybe a 6SN7.

You can see inside the tube, you can probably see where each internal element is connected to the base. LOok up the tubes I mention in a data sheet, and see if the plate grid and cathode connections show the same pattern.

[Chuck H]

Also, WRT older tubes, there is almost always a certain marking that remains even after the marketing print is long gone. It may be nearly invisible unless you hold it just right in the light. Usually surrounded by an elongated octagon, like a rectangle with the corners cut off and a tube designation number inside.

[Enzo]

And while we are used to the number being on the side of the tube, back then, a lot of them had the type number on the top of the glass.

[madkatb]

Have you tried the "put them in the fridge for a few minutes and then breathe on them" trick? The condensation produced sometimes shows up faint labeling invisible to the naked eye.

[cryptozoo]

I'll be damned -- that worked! After a stint in the fridge, I could faintly make out markings on both tubes. One too me two go-rounds, but finally got 'em. Thanks, guys!

GE = 6SL7
RCA = 6SN7
- Good call, Enzo

[Enzo]

Get out a Sharpie and mark them permanently.

[Austin]

Are 6sl7 and 6sn7 pretty much interchangable?

[Chuck H]

Same pinout. Would probably "work" in each others sockets. Different voltage ratings and amplification factors. So... Not really interchangeable as direct replacements for one another. No more than the typical 12XX7 tubes that people are fond of subbing for each other.

[Enzo]

Sure, they are just as interchangable as a 12AX7 and a 12AT7.

[Chuck H]

Or... A 12au7 and a 12at7.