They run pretty close to the max plate dissipation, at least as my kit is built?
It's interesting, if you look at the Western Electric datasheet for the 300B, the maximum operating conditions that they warn about are all relative to how high the plate voltage is rather than the plate dissipation itself. In that regard the 550V rating the plate in section 2 carries is way over what's necessary in the SEX amp. But yes, we do run the plate at 95% of its dissipation rating. These tubes were designed to go inside of hot, cramped televisions that would operate for hours on end every day of the week, so there's some extra margin in there compared to something like an EL34.
Should I wire the transformer to cut the B+ a bit, or does that matter?
I don't think that's necessary.
With it running about 3-4 hours a day, if that, do I have to plan to re-tube often?
Once a year? 1000 hours? 2000 hours Anyone have any idea?
I have an amp in my workshop that runs a cousin of the 6FJ7: the 6FM7. I never turn this amp off, it runs 24/7/365 and has done so for about the last 8 years. I retubed it about 6 years in. Those tubes still passed signal, but they were starting to sound a bit rough on the edges. That's 50,000 hours of use. You could argue that you'll get a little less with power cycling, but I think PJ still uses his original SEX amp with the original tubes in his TV room and that amp has been around a decade or two.
