They’re not a KT66 (as the original Bluesbreaker had), nor a 6L6GC and they’re not even a 5881; they’re a former Soviet military design. That said, having been designed for MiGs, they are very tough and longer lasting than some. If you like the sound of them, then perhaps I’m being unfair but the Chinese KT66 is getting to be an acceptable replacement for the old GEC and worth trying as an experiment. I can’t resist swapping glass around; sometimes getting a big improvement but sometimes not.