My experience with NOS tubes has been quite good. They have been screened and selected and have very little tube rush. Much quieter than most new production tubes. The NOS tubes come from an era when the process control was very good and metallurgy got the best materials. Tubes from that era were designed and tested to be very quiet.
Unfortunately, some "NOS" tubes are either used or in extreme cases - counterfeit.
Guess I was lucky to have the chance to collect these 25 to 30 years ago when stocks were still good.
The VT-231 and especially the 5692 red or brown base are typically very quiet. The 5692 and some of the VT-231 have additional support rods that help eliminate microphonic noise.
One of these days I need to sort through the tubes and sell off some of the abundant spares.
