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

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. 

I see. Certainly can use them in VAC amps then.

Sad to hear Brent Jessee is closing the doors. He was my go-to resource. 

Now I really regret not backing up my pair of Mullard 12AX7's...

Hopefully the shop will reopen with a good steward.