Seems to me that there would be some tube manufacturing equipment still floating around out there, or perhaps new/improved such could be easily made. Why do you think that nobody has gone into the repro tube market? If there is a dedicated following, it would seem to be a possibility. I mean, look at all the new parts for Flatheads these days...
Felony, the tube making process is much MUCH more difficult than you would imagine. If you don't have multi-million dollar machines, then it takes tons of trained labor. There is an hour long YouTube video floating around, of a vintage U.S. tube manufacturer from back in the '40s. Can't remember who it was, maybe RCA? But until I watched that, I had no idea what was involved in creating quality tubes. It really opened my eyes and explained why they are no longer made. Plus, throw in the coatings that are used in the processes and internal parts, add a dash of EPA, and you can imagine where that ends up in todays world.
Ok, took the time to find that video, and like usual my memory was only about 40% correct. It was Mullard (a VERY desirable vintage tube!) from the Blackburn plant in the U.K., and it was only a half hour. Still, a very eye opening video if you're into this sort of thing.
https://youtu.be/GDvF89Bh27YAnd like Shady said, China still makes them (no EPA, obviously), but the quality is nowhere near what is expected from audio people. I'm not sure, but I think Russia has completely quit making them, although they can still be found. And again, Shady is right, they are getting harder to find NOS. I have a stash of vintage EL34 and 12AX7 tubes, mostly Mullard and RCA with a few batches of Russian Svetlana, that I've bought over the years, enough to last me my lifetime. Overdriven guitar amps are very hard on power tubes, by nature. It's how you get the distortion that is coveted by rock players, so they have a limited lifespan. Pre-amp tubes will last a long time.
When most amps use 2 or 4 power tubes, and they should be replaced in sets, as well as matched in sets, you can see why tubes are such a big deal and why they dry up quickly. Not so much in radios, because they are not overdriven. The used tube market is a big market, but it's like throwing a dart at a dartboard (or your mother-in-law), blindfolded. You're lucky if you strike something usable.
