I bought a lot of nearly 300 of those Russian 6SN7's, I marketed them, under the nick-name of "Russia Solders" because they were military tubes, but they were too hot or had to much
gain. I had them in a three 6SN7 tube based Cary Audio pre-amp.
The best pure 6SN7 GTB is the Hitachi 6SN7 with carbonized black plates, they were made on Mullard equipment purchased by Hitachi, stay away from the same tube in gray plates.
The best 6SN7 is not a 6SN7 it's a pair of 6J5-GT / 6J5-G, each of which equals half a 6SN7,
You can buy already made adapters on EBay, which convert two 6J5's into a single 6SN7,
so you need a quad to make a pair. I believe that the 6SN7 was created for economic reasons, but I believe it was a step backwards for high-end audio because I believe that a single triode in a bottle is better than two triodes in the same bottle as in a 6SN7.
It's really difficult to find NOS quads, but I have several and at least a hundred more tubes consisting of pairs or 3 of a kind, I hope to find the pairs and singles necessary to make more quads. It's as stupid as drawing one card to make an inside straight in poker, but I love the tubes.
