I've been buying 6SN7's for more than 30 years and have hundreds of them, with examples of all of the above.
So what is my personal preference , what is the Holy Grail of 6SN7's?
Well maybe surprisingly I'm not that keen on old NOS tubes, mostly because most of them are not NEW OLD STOCK.
The only way you can be sure that a tube from the period 1930- 1980 is a genuine NOS tube is if it comes in a sealed box that has not been opened .
Anything you buy from ebay is suspect, even genuine tube sellers really have no way of knowing if a tube has ever been used or not, or how many hours, and under what sort of operating conditions a tube has been used.
They like to assert quality by using a tube tester and give some vague measurement that really doesn't mean anything.
Quite often the tube tester is more than 50 years old and long past its use by date.
The only way to tell if a NOS tube will give optimum service is to actually measure it in the circuit it is being used in.
Gain and channel balance can be checked and if its a highly sophisticated circuit like the Supratek, the operating parameters can be adjusted for perfect performance. On the proviso that the tubes were in excellent condition to start with.
New production tubes aren't that exciting, the current Sovtek and Chinese 6SN7's aren't bad, but they aren't quite as good as the tubes made when cost cutting wasn't more important than quality.
The tubes I use in my preamps are Russian military NOS tubes with OTK certification.
I buy them in boxes of hundred , never used and they have what I value the most, near identical parameters and performance. They are constant and I know they will work as they are intended to.
You get the occasional bad one, but it would be less than 1 in a 100. And they are all checked in circuit to ensure the preamp has identical performance in each channel.
This is very important for phase response and resultant sound-staging.
Ever wondered where the imaging and sound-staging went with that pair of warm sounding NOS 6SN7 you bought off ebay?
Most likely the tubes are operating quite differently from each other and there is a mis-match between channels.
NOS 6SN7, particulary the early ones from 1930 on are usually microphonic, sometimes quite badly .
Some of the 6SN7 derivatives like the CV181 and the ECC33 aren't really 6SN7s. They have different plate resistances and will give quite different electrical voltages in a circuit designed for 6SN7's.
This can result in different operating points and distortion figures , which might sound "warmer" or "romantic" .
Nothing wrong with that if it is pleasing to the ear, but it is not what the preamp was designed for.
So playing around with NOS 6SN7's can be a minefield, sometimes you'll get a blast you like, sometimes your shooting yourself in the foot.
