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

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. 

Sylvania’s quality unfortunately seemed to drop off the cliff sometime late 50s / early 60s. These tubes go from sounding amazingly dynamic, neutral, and 3D - to sounding bright, a bit lean almost solid-state like (typically gray plates).

@mulveling 

Hopefully, the ones I purchased will be the amazing sounding ones. I bought a pair of Sylvania 5814A matched tubes with triple mica and square getters a couple of days ago. They are not black plates, but have short pinched gray plates. 

As you stated, these features may fall into the late 1950’s era; early 60’s. I guess the features changed over time for the worse for Sylvania. I think all mil spec tubes have triple mica while commercial tubes have two mica spacers. I have a pair of 60’s era GE Mil Spec 5814’s and they sound great - hoping Sylvania’s will provide a little more air like Amperex or RCA Clear Tops. No heat issues. 

I understand, RCA 12AX7 long black plates from early 1950s might come close to Mullard long plates, though I myself have not tried them. At least, Brent said that they were the best US made 12AX7. Difficult to find too.