12ax7 v 12au7: Physical Diff?

[ivan100000]

Gentlemen:

Discarded tube consoles/wall units/TVs left at the curb for garbage collection were commonplace in the ivan100000 neighborhood @1990s, and I've plucked many a black plate preamp tube back in the day that have served me well for my various hi-fi units over the years. The problem is that I have 4 or 5 tubes rattling around my tube caddy with all markings rubbed off, and I am unsure if they are a 12ax7 or a 12au7. (I am 90% certain the tubes are one of these two, because I only pulled tubes in those pre-eBay days that my specific units could accommodate; looking back, I left many of the Telefunken EL84 and Mullard GZ34 rectifier tubes for the wolves.)

Trying to compare the inner guts/pinouts of these unknown tubes with either a known 12ax7 or 12au7 makes my eyeballs swim, as I'm not able to determine if I'm looking at construction differences of, say, two different 12au7 manufacturers, or if I am seeing two distinct tubes. Short of randomly plugging the tubes into a 12ax7 socket in my amp and hoping nothing blows up, is there a straightforward method of telling a 12au7 from a 12ax7?

Thank you.

 

[s-petersen]

I usually can't tell by eye with certainty, but you can tell with a tube tester.Swapping in the amp will not hurt anything,same pinout,same voltage,different gain

 

[sKiZo]

They're (more or less) electronically identical except for gain ... physically, same envelope and size, but as you've noticed, internal differences depending on who made them and when.

We're talking 5 times the gain on an AX over an AU, so easiest way to tell is to just plug em into the AX socket. Use a known AX as a reference, then test each tube and mark them based on the volume output without changing anything other than the tube. You should see a big difference in output on an AU.

 

[Audiovet]

Well - er - yes. But correct lighting and perhaps a magnifying glass are required.

Most of these tubes have two holes at opposite sides in the anodes. If such that you can see 'through' them past the grid, you will notice a finer pitch of spiral wires for 12AX7 than 12AU7. Better if you have known equivalent tubes to compare with. The difference is large enough to be noticable.

Risking in-circuit comparison, in most cases an amplifier will not blow up. (A power amp with NFB might oscillate if 12AX7s are substituted for 12AU7s.) Then measuring the cathode and anode voltages will certainly indicate which is which, when you refer to tube graphs.

P.S: Typed before reading sKiZo's post; agrees.