Calling All Tube Gurus

I find enjoyment and value in tubes made for hospital equipment. Often these have red, blue, or yellow painted tips . In most cases, these have been screened for critical use.. I find most of the red, yellow, and blue tips incredibly quiet and very three dimensional in sound. Sometimes they were made by Amperex, or Telefiunken, Mullard or even Matsushita but released under a different brand.. I generally feel the same about organ tubes (Baldwin, Wurlitzer, etc) often made by RCA that have gone through the extra screening process to assure low noise..

If the tubes were made with acid etched codes as were all the Philips Europe tubes then the code is all that matters. Same can be said of the silk screened codes, but they can be more easily faked and often rub off. Branding has no influence on the quality of the tubes. This is especially true of the major electronic companies like Magnavox or Philco. A rebranded Amperex made Mulllard is just as good at the Amperex branded tube. If it is a rebranded like National, I guess it is possible that these could be rejects.

I have experienced differences in sound from the same tube with different production dates from the same mfgr. I would attempt to get identical or very close production dates especially in high quality 2 channel applications. 

In closing look to things like the Telefunken diamond, acid etched codes, tube seams or top dimples when buying old stock tubes. Look for things that are difficult to duplicate. One more thing that buyers seem to get hung up on is the quality of the tube lettering. On many tubes, the lettering can be easily rubbed away. What you dont want to see is the lettering that wont wipe off easily. If the lettering is fixed or yellowed this tube has most likely seen heat.

I'm currently running two different branded NOS 2a3 tubes in my Wright monos matched by Tube Depot. One is GE the other is Raytheon I believe and they sound fantastic.

"NOS" means new old stock which indicates never used except for testing. Many of the tubes you refer to as NOS may just be old stock. True NOS without original boxes are uncommon.