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I have a box of preamp tubes and alot have no markings how can you find out what it is ???

Compare the inner structure to other known tubes that you have. You can also get all of the imagery you need from the net if required to help in the identification process. I've acquired a lot of pics this way which I have used to identify tubes by manufacturer. Pay particular attention to the plate structure and once you think you have identified a tube type confirm the pinout by carefully looking at the fine wires attaching the elements to the pins.

You might not be able to identify all of your unknowns this way, but it can at least help you eliminate tubes which are not from the 12a_7 family which I'm guessing that you are looking for.

Chappy

Seriously, put them in the freezer. If there is any residual marking, it will be visible briefly when you remove it and the condensation is present. This doesn't work all the time, but it does work.

Phil_S wrote:Seriously, put them in the freezer. If there is any residual marking, it will be visible briefly when you remove it and the condensation is present. This doesn't work all the time, but it does work.

This does work, Ive done it for years. I have tubes from the '20s with the marking worn off. Put them in the freezer for a while, take 'em out and blow hot breath across different areas, and *usually* you can se a faint image of the number.

in the freezer??? what a great tidbit But my wife will think Ive lost it big time!!!

hmm i think i'm gonna try this freezes trick to one of my tubes which lost his markings, it's a tube which is quiet smaller then the most tubes i've seen so wish me good luck in my freezing adventures

Hmm i've put them in my freezer and did this little trick.....
Nothing happened to be honest
Maybe my freezer isn't cold enough then?