Why are older tubes so much sought after

The answer is simple.

Tubes made in the 40s, 50s and 60s were of higher quality...made to tighter tolerances than tubes made today. Why is this so? Because most of the military equipment depended on reliable, consistently high quality tubes to operate the radios, radar and other electrical equipment properly and in difficult conditions.

The high quality required for military spec tubes filtered down into the consumer tubes (which were often the same anyway) used in all the radios and TVs of the era.

Today, there is no need for high quality tubes in the quantities required before, and therefore manufacturing quality has slackened. New production tubes generally don't last as long, and the quality control suffers. Some new tubes are great. Other new tubes of the same type from the same manufacturer will fail after only a few hundred hours or less.

Also, NOS generally sound better. Why this is so is a question that has not been answered sufficiently, but any basic comparison will bear out the results.

Thus, NOS tubes are in high demand.

Thank goodness Tvad provided such a perfect response, now I don't have to reply other than thank you.

This is an interesting film on the production of Mullard Valves at the Blackburn plant.

www.techtubevalves.com/about_us/index.php

Click on vintage film reels

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.