Why do NOS Tubes sound better than new tubes?

As far as the sound is concerned, I'll attribute the decline to significantly decrease competition and demand (like many of you have already indicated).

For the build quality, durability, and lifespan, I think it's the shift to a "disposable" society. Cars, lower-end receivers, washers and dryers, telephones, televisions, mass-produced furniture, and so much more just don't last as long as they used to. That's not to say that they're not functionally better in many ways; they just don't last as long. I've had a cheap gas stove from the 50's, and a cheap gas stove from the mid 90's, and while the newer stove is safer (avoiding pilot light issues), the older stove might still be functional in another 60 years while the new one might crap out tomorrow. It should be no surprise that vacuum tubes (no longer built in the UK, Germany, or North America) also have shorter lifespans.

Rodman makes a good point. There are a couple companies out there that make excellent new productions; however, EAT for instance is limited to the KT88, 300B, ECC88, and the ECC803S, and that's not all that uncommon. Heck, the only current production 5751 is the Sovtek (as far as I've found). There are some very good options out there, but not for everyone (certainly not for the rollers).

Personally I prefer the NOS tubes. They also seem to last a lot longer. The exception is EAT. The problem I have with EAT KT88's is I bought a quad of used original KT88's that have serial #'s that start with 'D'. 1 is bad after 25 hrs and 2 won't hold a steady bias after 100 hrs. The pair I have that start with 'E' seem rock solid. So as of now I only have 3 good ones. That being said the sound from the EAT's is something else. Just got some Gec KT88's that sound real close to the EAT's and being vintage tubes I hope the last a lot longer.

I wouldn't buy used power tubes, of any brand/vintage, unless I personally knew they were not abused(over or under biased). Either of those conditions can shorten a tube's life dramatically. Besides that; there is no warranty. I recently purchased a premium matched octet of NOS GE 6550A's. One tube began to increase it's own bias(10-20mA/min), and blew a HI-FI Tuning fuse(ouch), after the third week. The tube was returned, and replaced within three days. There's no guarantee that NOS tubes will last longer, SO- it's nice to have coverage.

Rodman - How does under biasing a tube shorten the life? And does a very slight under bias cause problems? It was recommended by Andreas Hoffman from Octave corp to slightly under bias the tubes (biasing is done with leds and not a meter). I'm fairly new to tubes (about a year) and I do know they have to be treated with care. Constantly driving them hard will shorten the life too. And if man made it - it will break (not a matter of if but when).

As far as vintage lasting longer I have TS 6550 solid grey plates where the getter flashing is pink and almost gone but the quad still sounds great and strong. Those I got from E-Pray for a great price and when compared to my NOS quad they are very close (NOS sounding a bit better). Same with the solid black plates (the best TS sounding tubes BTW).

My Octave came stock with SED KT88's and I bought SED 6550's and EL34's from Octave. All good tubes but the EL34 has a mid range magic I love - just not enough low end. Then I got some GL KT88 re-issues' and they are better (like a cross between the SED KT88 and 6550's but the mid range wasn't there). Next up was TS 6550 bp's Ei KT90 type 2 and EAT KT88 diamonds (early version - just had to hear if the rave was true for myself). Anyhow the TS 6550 bp's have the mid range and low end. Very musical and tubey sounding. KT90's nave the mids and the low but a somewhat steely high end (wrong input tube) BUT power like I have never heard. The EAT's are a different animal all together. Much more detailed and linear yet retaining the tubey sound with the mid range which is very important to me. The highs just sparkle. Last I just received NOS KR Audio branded Tesla KT88's and Gec KT88's. The Gec's are very close to the EAT's and the Tesla just trails the Gec's.

For drivers the Octave came with Rca 12at7wa. BUT I really like the Brimar 6060 yellow T and Gec CV6091's with the high transconductance tubes (EAT - GEC and Ei outputs) (slight edge going to the Gec's).

The stock input tube was a Sovtek 12ax7. I tried the GL 12ax7 reissue Ecc83 and ts 12ax7 reissue. Not much difference there. Rca 5751 black plate really smoothed out those KT90's. I'll be trying a Brimar CV4004 soon.

Any advice? I'll probably try a Telefunken input at some time. For current production output I'll want to try either the Shuguang Treasure Z or Psvane. Problem with vintage is they have been out of production and the supply is dwindling forcing prices up LOL.

Thanks