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

@playbackmansion 

I can see how the Mullard won you over in the 12ax7 tubes. The higher gain provided in those tubes vs the 12au7, is probably a sweet spot.

In other words, when I listen to Mullards, the more volume I use the better the sound, which is the opposite of the Amperex. The Amperex are not bright, necessarily, but the top-end shine can be taken that way, especially at higher volumes. 

While it is an orange (preamp tubes) vs apple (amp tubes) comparison, the attributes of each are still readily apparent. Thanks for posting the video.

I clicked on the main page, scrolled down, and found this link to a film made in 1958 to promote RCA’s ’Living Stereo’, so serious

https://www.audioasylum.com/audio/general/messages/80/806082.html

the film, on youtube

https://www.youtube.com/watch?v=YT7JLEjusQE

I am almost certain that best Sylvania and RCA  are from 1950s, and the earlier the better. At least, that's what I read, I only have 1950s of both. Don't buy RCA from late 1950s, that's, later than 1956, they are a little dark and closed in; early/mid 1950s 12AU7 RCA sound excellent in my VAC, almost as good as Sylvania long black plates from 1950s.

After reading Joe’s Tube Lore years ago, I bought a trio of Sylvania 5751 for my then EAR 834. I thought they were bright sounding.

5751 are military with lower gain, not exactly 12AX7.