So, I'm one of those nerdy engineer types in real life. I've read the Peavey Transtube patents and they're pretty interesting. Sondemeyer was Peavey's chief engineer for decades. Hartley Peavey hired him directly from RCA's solid state division, back in the days when RCA was a giant manufacturer of tubes, transistors, integrated circuits and even microprocessors, as well as a huge maker of TV and other consumer electronics.
Sondemeyer was a brilliant engineer. He created many things, including the CS-400 sound reinforcement amplifier. That was the world's first truly bullet proof professional audio amplifier. I still sometimes see these in venues that have probably been in use since the 1970's. Sondemeyer passed away about 10 years ago.
I've actually bread-boarded out the original transtube preamp circuits from the patents and the first-generation amps. It is extremely clever, (getting nerdy here) a modified Darlington pair of transistors, and a couple of other tricks, to mimic the characteristics of vacuum tube triodes, like the 12AX7 tube used widely in guitar amps. That generation of guitar amps are actually quite good. Reliable too.