Roger and I had some great discussions way back when about Futterman circuits. The genius was getting a pentode to operate at low plate voltage with high screen voltage, operated by a bootstrapped driver pentode. The oscillations partly came from paralleling the output tubes to get enough drive current. I’ve built a few over the years with PL519s and 6HB5 compaction tubes and can vouch for their transparency. The frequency compensation on the H3 I serviced was a bit of a compromise so I tweaked it to get better stability and make it more linear in the extreme top end. The gas filled regulator tubes sounded far better than zener diodes and solid state regulation in the screen driver circuit and so I decided the circuit did need to be as complex as the original. They look extremely cool too.
Clio09, build it! If you need a hand to interpret Roger’s take on it let me know!
Clio09, build it! If you need a hand to interpret Roger’s take on it let me know!