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

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.

@inna 

when using AI, use the fewest words to ask a question. Kind of like negotiation with your wife, you don't use words that will upset the apple cart. AI is absolute with results that pertain to science, math, medical data, etc. Ensure your not using an AI Overview, which can be wishy washy.