I have had a fair share of valve trouble. I had just bought a new guitar and it sound really really bright and harsh, turns out it was a power valve going south and I didn't realise until I'd sold the guitar that it was the valve at fault. It was a really good brand of valve too, and I swapped it out for a nondescript Chinese one that surprisingly sounded amazing, so I tracked down the brand on eBay and I always use them now. I've heard good and bad about NOS. It mentions them in the Gremlin manual, and says that they can be hit and miss, and to stick with new ones from Russia or China I think, from memory, but I will give them a try on your recommendation next time I swap them out. Any particular brand you think are good to try??
You know, I've had far less hit and miss with NOS than with new tubes. Going back to the late '60s when I started using amps, I never had a tube blow - until the last couple of decades. I spoke with one of Mesa's techs about it in a conversation about tube rattle in one of my amps before installing NOS, and he said there's no question that newer tubes are being made worse and worse each year, and that they've seen far more issues with new tubes.
They've got a pretty large customer base, so I'm guessing their info is based on a large enough sample to be worthwhile.
If you look closely at a quality US, UK, or German NOS tube and compare it to even a decent new Chinese or Russian tube, you can easily see that on the new tubes, corners are being cut in the subassemblies and wiring gauges. The assembly quality is also worse, with things like tilted and bent parts in the tubes. These contribute to problems like increased microphonics, shorter life, and how the tube sounds. For example, many Chinese preamp tubes sound extremely 'ringy' to me due to microphonics.
Remember, too, that new tubes are made with very old equipment that was originally purchased by Eastern countries from Western factories when the West stopped producing tubes by around 1980. Whether the equipment is properly maintained or worn out now is anyone's guess. Perhaps that's a factor.
On American style amps, I use NOS GE and RCA preamp tubes, and with 6L6 power tubes, I like GE, RCA, Sylvania, and a few others. I like the NOS JAN (Military Spec) Philips 5881s. I've also had good luck with other US-made NOS JAN preamp tubes. If you can get them, the RCAs sound great in V1, but the GEs are no slouches and they're considerably less expensive. The NOS GEs I like have been the JAN military versions, but I'm told that they're identical to the civilian version, they were off the same line, and simply tested to meet military spec back in the day. I've been using JAN GE and NOS RCA 12AX7s for years, and never had one go bad. Doesn't mean it can't happen, of course, but my experience has been very positive. And of course, the sound is nicer.
On British-style amps, I use NOS Mullard or Brimar in v1, sometimes NOS Siemens in other spots, sometimes Mullard or Brimar, and lately always NOS Siemens EL34s, because the NOS Mullard EL34s have gotten so expensive. I've had pretty good luck with NOS '80s manufacture Russian EL84Ms for my DG30 amp (the EL84M is a different tube than a standard EL84). The Brimars are not as consistent as the Mullards for some reason, which is surprising, since many of them were branded Brimar and made by Mullard. However, I have never had an issue with a Mullard 12AX7.
You might wonder about the tone of these things, so here goes:
About 18 years ago, I became a Two-Rock player. At first, the amps came with NOS tubes, but after a while, it became an option. However, when I got a new Two-Rock, I would install NOS tubes. The amps' tone improved, but more importantly, there was greater satisfaction with each amp long-term.
I equipped my HXDA with NOS, and even though it was a keeper with the stock tubes, the NOS tubes made it even more satisfying as a long-term #1. My DG30 was one of the first 20 made, and came with NOS from the factory. I love it.
I also have two Mesa amps. I bought a Lone Star because my son was touring with one, and there were times he wanted to cut tracks at my studio when he visited from LA. At first, I really didn't love the tone. So I installed NOS GE, and the amp changed in subtle but audible ways. To my surprise, however, the amp became a keeper, and a studio staple I really like now. The harshness is gone, the tube ringing is gone, the brittle high end is gone, replaced by a lovely treble. It went from an amp of convenience to a keeper. I've had it for about 5 years, and since installing NOS, the amp hasn't needed a tube change. I record a lot of tracks with it when I want a more pure clean tone.
I found that putting NOS in my Mesa Fillmore improved it in a similar way - it sounds like a more expensive amp now.