I just did a quick Google search on 'tubestors' and lo, there does not seem to be any info jumping right out at you. Maybe later i should try another engine. I was curious how they handled dropping the B+ down from tube range to FET range. I have some lit "somewhere" but only like advertising, nothing revelatory about the actual innards circuit. These didn't seem to be on the market very long did they ? In Frank Zappa's words, they were gone like the elephant's footprints in the ocean.