Why do NOS Tubes sound better than new tubes?

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

Mr X- Actually; it depends on semantics. Read the first paragraph of this article: (http://www.svvintageamps.com/tubebias.php) Most often; people refer to, "over biased" as being too hot, rather than the other way around. I simply covered both bases(sorry for the confusion*). Of course; running a tube slightly cooler than it's design parameter, will extend it's service life(as with virtually any other electronic component). Happy listening! *Having reread my earlier post: I did say, "either of those conditions" which was a brain-fart on my part!

Rodman - Thanks I was starting to worry about the slight under bias. I know tubes won't last as long as transistors and want to get the most life out of my tubes. As I've stated overall I prefer NOS (EAT is the only exception and maybe the Shuguang Treasure Z or Psvane). Most of the NOS and A(lmost)NOS I have are very close to EAT Kt88 prices. Thanks again for clarifying the under bias - I'll sleep better tonight LOL.

I need to sleep better AND stop posting at 3:00am! Again: Sorry for the anxious moments.