In the S-30, the rear tube has a little more voltage on it. If you are using NOS tubes, a -GTA or -GTB is recommended. A GT will also work but will not last as long- it is running right at its limits. So save your expensive KenRads or whatever for the front two positions.
Modern 6SN7s are marked 'GT', like the Chinese 6SN7; they work fine (that is what we use of course) in any driver tube location.
Modern 6SN7s are marked 'GT', like the Chinese 6SN7; they work fine (that is what we use of course) in any driver tube location.