This design is inspired on the great work from Gary Pimm. The 47 is a sterling directly heated pentode and worth using it. I have several on my stash, including some good globe versions.
The following topology is well known. The push-pull output stage has local feedback (a.k.a. “A la Schade”) between anode and grid. Obviously the drivers are pentodes. Subtle difference is that I added a source follower in between grids to provide negative bias and grid current drive.
The drivers are based out of the beloved 6J49P-DR. Need to finalise it. The stage can do 8W at less than 0.6% THD with an OPT with Zaa 8KΩ.
I prototyped a new PCB with a simple circuit to provide Screen voltage regulation based on a VR. Quite handy in a circuit like this. Let’s see how it turns out.
Looking at my notes and archive from 4-5 years ago, here are the “Schaded” anode curves:
Quite a lot of FB applied here to get the anode resistance down to 800-900Ω.
This will make a nice sounding amplifier!
