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

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.

@inna 

AI informed me that with tubes, long plates have a wider soundstage and short plates are cleaner, faster with more headroom. Black plates are warmer and soft, while glossy plates are bright and direct sound (sounds more live or harsh). I guess you can take any valve feature and ask AI how it will affect sound. Interesting stuff!