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!
Hi Ale,
That’s a nice 2-stage PP design ! Good source of inspiration 🙂
Great use of your source followers.
I like the local feedback, I’ve found different names for this kind of feedback:
Partial, Trans, E-Linear, …
https://tubecad.com/2018/11/blog0447.htm
https://www.mennovanderveen.nl/images/onderzoek-ontwikkeling/publicaties/ETF-2021_Trans_versus_Global_nfb.pdf
https://wtfamps.com/2019/07/05/e-linear-an-interesting-kind-of-feedback/
Regards,
Danny
Hi Danny,
Great to hear from you! It’s been a long time. Hope you’re doing well.
Yes, there are multiple versions and names. I had the inspiration from Gary Pimm and a few “local” feedback designs and builds I made on the workbench. I hope to continue reading Menno’s work on the topic as he has taken a detailed and systematic approach which I always liked from his work.
Cheers, Ale
Hi Ale,
DIY has been a bit slow for me, too much other stuff.
But now I start planning some new projects for after the summer and I just saw your nice PP design.
Time to play with it in LTspice 🙂
Grts,
Danny
Sure, same here. Keep me posted on your progress. I’m sure you will come up with some interesting designs.
Cheers, Ale