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

I would always use analogue source to compare anything.

Anyway, it is good that in the age of AI and streaming we are going into detailed discussion about vintage tubes.

There is a set of three Sylvania 5814A triple mica NOS for $150 on ebay right now, if anyone wants to try them. I'll pass, need resources for Mullards XF2.

@billpete 

Mine aren't gold pin. I don't know a lot about tubes.

@slaw I'm not claiming to know a great deal about them either but I have been enjoying them for about the last 25 years or so. I would guess that the non gold pin Sylvanias would not sound as good as the 3 mica Gold Pins as they were their "top of the line" and have commanded pretty high prices over the last 20 plus years. I used to buy them for $15 to $30 apiece back when I started buying them. I bought NOS and lightly used ones at those prices. You'll be hard pressed to find any Sylvania Gold Pins for less than $80 to $150 now, with 3 micas bringing the highest prices. Must be something to it. I never had any Sylvania 5751's that were not gold pins. I do have others, like 5814A's in steel pins and they are fine but I imagine if you found the same in a gold pin, it would be even better. I don't think it's so much the pins themselves that make them better (although it can't hurt) but what went into the rest of the tube as well that made them a premium tube. My 2c.

@billpete If I am reading it correctly, according to Joes Tube Lore the Gold Brand 5751 3 mica black plates are identical to the gold pin 3 mica black plate tubes other than the color of the pins.

Per google: " Gold is used on tubes for its inert properties, specifically to prevent oxidation and corrosion on the pins over time. It is not for significantly better conductivity compared to nickel or copper, but rather to ensure a reliable, quiet connection."