12AU7 and 12AX7 long black plates vs short black plates. Any difference ?

Best of the best 12au7's CBs Hytrons of the 1950's or the square getter rca JHY12au7, I will lookup the other super 12au7 tube in my stash later this evening. You need to locate yourself and buy a  NYAL George Kaye Small Signal Tube Tester if you are serious.

https://www.audiotubes.com/audtube.htm

Jessie Brent Tubes closing March 5, 2026.

Better look and stock up quick ...  BUT great information on audio tubes, well worth the time to browse. One cannot help learning a great deal about our beloved vacuum tubes and the variants!

https://www.audiotubes.com/12au7.htm

https://archive.org/details/InsideTheVacuumTube

Have fun!!

I read what Brent Jessee has to say as well. You'd be hard-pressed to find anyone who knows more about tubes.

5814 (A) tubes draw more heater current than 12AU7. I guess, it is not always a good idea to use them as a substitute. Better ask the manufacturer of your equipment.

5814 (A) tubes draw more heater current than 12AU7. I guess, it is not always a good idea to use them as a substitute. Better ask the manufacturer of your equipment.

The difference is so minimal that you amp is more likely to be affected by sunspots. Happily use any 5814 in a 12au7 slot. 

If an amp maker DID size the filament transformer so minimally that it was operating right at the limit which could theoretically be impacted by the extra 0.05A, then shame on them, and you should get a different amp anyways. 

And with a tube power amp or integrated, the vast majority of heater draw is from the power tubes. You're getting into a fractional % of a delta on the filament tap from this.