What the heck are NOS tubes?!

I hear a lot of people talking about NOS tubes when I posted my question about the superior tube amp. BTW thanks for all the responses! But what are the NOS tubes your talking about!
NOS- New Old Stock, Refers to parts that are usually no longer made, but are still "new" in the box.
Grumpybb@aol.com provided a perfect answer. I would like to add that there are several countries like Yugoslavia, Russia and China that manufacture new tubes. The reason the NOS tubes are so sought after is that many were designed and manufactured to military specifications. It was not that long ago that tubes were as important to military forces around the world, as computers are today. Obviously, any product having that much importance to national security was built to standards that cannot be copied today. The budget for parts of the space shuttle ($600.00 hammer or whatever) is a perfect example. The subject of NOS tubes is complicated, and some of the new tubes manufactured today are equal to NOS. Many others cannot be duplicated, and therefore are available only as the original NOS. Unfortunately, this is a tube by tube and component by component test as to what is the "best." Long life and reliability not even taken into consideration! This would not be a bad thread for other Audiogon posters to discuss their favorite tube choices. You could post what worked the best, where is was used (equipment and position) and what it replaced.
Great topic Albert!! I bought my first all tube preamp with a phonoboard.The preamp is a Rogue 99 which is still a quite new piece from Rogue Audio. The first tubes in line in the phono stage were factory supplied Sovtek 12AX7LPS.This is a quiet tube and fairly decent.I was not happy.I was getting what is sometimes called "tube Rush" A low level hiss like noise.It was only audible during very quiet passages.I was told by several people this was normal and was "the nature of the beast"when it came to tube phono stages.I also was told this could not be corrected.Just a reminder,It really was very quiet and others could not hear the tube rush unless I pointed it out.Anyway,I was not happy!! I had just come from the Lehmann Black Cube outboard solid state phonostage.I was use to DEAD quiet.After consulting further with a "tube expert" I bought 2 Telefunken 12AX7's from Kevin Deal at Upscale audio.These 2 tubes have quieted my phono stage almost to the level the Black Cube was.I was told by 2 people who know the Rogue better than anyone that I was fighting a losing battle! WRONG! Besides silence,I now also have a much more revealing presentation of music,thanks to NOS telefunken 12AX7's!!
It's also possible that the sovteks were faulty; a lot of new tubes are bad. In general, if you're getting that 'rush' sound, it means you've got a bad tube in the preamp.
No way,the Sovteks were fine.I sent the phono board back to Rogue to have it checked out and they checked the tubes and tried other Sovteks LPS's in it then sent it back.Said tubes are fine,you wont do any better.No,I spent much time on this and I know what is going on.The NOS are just superior,period.
NOS tubes can bring levels of sound out of units that spending 1000.00 sometimes cant.I just put in 2 cv4003 mullards = 12AU7.I replaced the stock Sylvania that came with my Rogue 66 preamp.They where nice quiet tubes,I new i could do better and the NOS mullards have been a God send.Better bass ddead quiet and more clarity and space.If you use a tube preamp.Give some NOS a try you will be amazed.WWW.thepatrsconnection.com is a good source.Kevin at Upscale Audio is also very good at selecting the right tube for your application.
Another tube that you folks could try in place of the Mullard (or other) 12AU7's is the Amperex 7316. This was a military tube and very high quality. In the Atmasphere amps for instance, the Amperex was quieter, more dynamic and less distortion. All this, and a good price too! Most people are not perusing the 7316, so last time I bought some, they were relatively inexpensive, compared to the Telefunken and Mullards that are also extremely good.