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

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!

goodlistening64, I prefer human interactions. As for the accuracy of the information that AI provides via general search, I tested it, though not extensively. It is often hit or miss, and sometimes it's a sort of a combination of both. Not reliable. Besides, you phrase your question differently and it sometimes doesn't get it. It is going to improve, though, no doubt.

I run RCA 5751 black plate triple micas in my tube DAC and 1950's Mullard  12AX7 halo getter long plate  in my phono stage. Very happy with both sets sourced from Brent Jessee.