Why do NOS Tubes sound better than new tubes?

Don't write EVERYONE that manufactures tubes off, just yet! These guys are making some VERY nice tubes currently: (http://www.euroaudioteam.com/en/products-000002.html) Much can be done with excellence(even these days), IF you take pride in your craftsmanship. YES- It costs!

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.

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!

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