Why are older tubes so much sought after

Good answer Tvad! There are less than a handful of companies now producing tubes that compare with NOS in quality of materials, care in manufacture and sonic performance. Here's some info and an idea of what that quality and care cost in today's dollars:
(http://www.kraudio.com click on "tubes")
(http://www.elusivedisc.com)
(http://www.euroaudioteam.com The only company making quality small signal tubes now)
(http://www.westernelectric.com)
(http://thetubestore.com)

A great question and very interesting answers above.

In my experience of NOS in new equipment the results have not been that great. I have found in the valves I mainly used (power tubes 6550,KT88,and 12AX7,12AU7,ECC81,ECC82) the older valves never had the clarity in the highs or the low noise floor or transparency.

The last time I used some NOS Brimar and Jan philips in a very high end preamp they were very rolled off, dark and fat sounding, even a little metalic. The new valves with gold pins where much cleaner, extended and detailed in comparison. Maybe this is very valve dependent.

I had the same experience with Svetlana 6550C power tubes. I found the 6550c from Svetlana to be the best 6550 I could find, leaving the others sounding murky.

I can understand that maybe the new old stock valves work well in older equipment or as a tone control.

I agree with Tvad.

So, the answer to your question is a simple one - because the old tubes sound better. And, the results are not even close.

I just built a Dynakit ST70 to play around with, and I was running the stock Sovtek 7199 driver tubes. Well, the amp was OK, semi-pleasant to listen to, and I threw the, "What do you want for $600?" tag on it. But, I couldn't get past how slow, plodding, and constricted it was. It simply would not get out of the way of the music. As for a letter grade, I gave it a C Minus.

I see several areas where the performance could be upped for free, but before doing that decided to buy an old set of RCA 7199 driver tubes to establish a baseline for what the amp should actually be.

Now, I'm often here preaching the benefits of maximizing one's output and driver tubes to wring as much performance out of their power amplifiers as they can get. I'm also not much of a fan of most Russian tubes at all. Wow, even I was not nearly prepared for the transformation! I've never heard such a performance increase from a tube change in my life. Right away, the sound opened up to where you would hope it would, that EL34 magic midrange arrived in spades, and bass is more extended and tight, and incredibly much more defined. It finally sounds like a tube amplifier. The amp now grades out to a solid B. Borderline B Plus. Perfect? No, not hardly. But I could sit down and be happy long term with it now just as it is. And, I didn't even start tinkering with it yet...

My recent experiences echo Chadeffect's. I have come across a number of new-production tubes that I find superior to their NOS counterparts, or at least good enough. This has been the case for 6L6GC, 12AU7, 12AT7, EL34. The only ones that seem to be consistently better in NOS form are the 12AX7 and 12BH7.

And it is a blessing! Just as the fun-factor of hunting for NOS tubes was wearing off, I discovered the new Groove Tubes KT66C (Shuguang - the Chinese make some great tubes) is fantastic. Hands down better than the NOS RCA black plate 6L6GC bottles - and for a lot less money and risk. Similar story for the ElectroHarmonix 12AU7 and 12AT7 - they are dirt cheap but if you can put that aside, they have a beautifully balanced sound that rivals all my old tubes. The venerable NOS RCA Clear-Top is terrible in my system with rolled off highs and mushy bass. And then some of the Brimars were horribly grainy.

But then there are the NOS 5751 black plates which will never be fully replicated, IMO, as they are just about perfect. Too bad my set is about worn out. The new Tung-Sol 12AX7 is very good, so I will make do with it, but on certain recordings I will sorely miss the imaging those Sylvanias could conjure up.

But let's not overlook the fact that these comparisons will be VERY tube dependent. There is lots of overlap in performance and which is better will totally depend on the exact tubes in question, the circuit they are plugged in, and the human brain doing the analyzing....

Arthur

Just one more comment from the peanut gallery. The Soviets in particular continued to use tubes in a lot of their military equipment long after the US military had moved much of their equipment to solid state. (In fact, they felt vacuum tubes were more resistant to the EMF effects of a nuclear bomb and gave them an advantage in the survivability of their electronics.)

One can get into the endless loop debate of whose military had stricter QC standards for electronics manufacturers (and the US has had its share of equipment procurement scandals over the years) but the military angle alone does not explain the differences in tubes.