Visually identifying preamp tubes?

[vibratoking]

Let's say you had a box full of unmarked tubes. Probably many 12AT, 12AX,12AY, 12AU and possibly others. Is it possible to discern what type each tube is visually? If so, can any of you explain how to do this? Are there special structures, features, etc... that are unique to each type that can be used to identify them? If not, do you have any suggestions on any other methods that could be used to separate them?

[selloutrr]

Because it's difficult to Identify models with in a pin series and silk screening rubs off over time. Most Manufactures embedded or etched designations either in the top ring of the glass or on the bottom of the tube.

some plate structures are different but often the connection that would identifiy the tube model are not visual from the ouside.

this might help
http://www.tubedata.org/

[lzzrdgrrl]

The 12av6 - 6av6 family of valves - tubes, have a very distinctive "frame grid" structure which makes them very easy to pick out. All the dual-triode octals can be thrown into the same box and that does for almost half of a sort through random material.....

[Firestorm]

It's hard to be absolutely sure without a tester. Unfortunately, all of the noval twin triodes have examples that look identical to each other depending on manufacturer and date of manufacture. There are many 12AT7s that have a tear drop shaped plate structure, but there are also examples with the familiar box structure you see most commonly on 12AX7s, etc. Then you have the later GEs with the very narrow box with huge "wings."

Once you sort out however many are clearly not 12A*7 types, I think the next thing would be to try and verify the pinout for the heater connections: see that you have continuity between pin 9 and pins 4 and 5. That will narrow the possible basing to 9A (12AX7, etc.), 9CF (9- or 12BR7, which should look different), 9CZ (these have only the cathodes and grids swapped, but should be in a taller bottle), and 9EN (twin diode/triode types so should look different).

Once you know it's a 9A base, you can run it in a test circuit with a signal generator and measure its voltage gain. That should help identify what type it is. Also measure how much voltage is developing across Rk to see how much current the tube draws; that should help discriminate between tubes with a similar mu.

Of course, none of this will tell whether the tube is crap or not...