Why do old tubes sound better than current tubes?

"some old tubes sound great, some sound like crap, some new tubes sound great, some like crap"

I suspect this to be true.

I suspect that the aging process may have something to do with this. I have a number of very old globe tubes along with the newer equivalents with ST shape bottles. I consistently have found that the old globe tubes sound better. I doubt that the bottle shape makes a difference and I would be think that quality and manufacturing expertise would have been higher when the ST shape tubes were made (50-60's vs 30-40's).

In particular I have a set of blue glass Acurus 27's that were made prior to 1931. They sound notably better than the later 50's version.

IMHO the blanket pronouncement "NOS tubes always sound better than new production" is the mantra of salesmen and the deaf. That they can be, for sure. But as often only have that potential if they actually are any good, as in not used up and/or noisy.

More importantly it is ALL about utilization and tone. Their use is not needed if they don't actually improve anything or give you the tone you're looking for.

FWIW, I use a lot of tubes and I get good mileage out of new production tubes as well as NOS and often actually prefer them, especially on a cost/benefit ratio.

FWIW

I still think nostalgia is part of it.

Old tube gear had a certain sound different than modern gear. part of it was the tubes perhaps but also the overall state of eletronics design 50 years ago compared to now.

Old tubes in good condition still perhaps deliver that sound compared to more modern high end sounds.

The old tubes perhaps are more in the rhythm and smoothness camp and the newer ones more in the detail and resolution camp?

I'm sure there are exceptions, but that is my theory. I have too little actual experience with a variety of tubes these days to say for sure.

Similar to old low res TV versus newer HD TVs. You can see more details in HD but not everything is more beautiful in HD necessarily!