Long time ago I breadboarded the 1624 valve and liked the sound. The 807 is famous and I never ended up with a permanent system with this valve. Yet, I like it. And if you have them at hand, use them. Even for guitar amps are amazing.
This is what I used. A 1:4 SUT with the LL7903 driving a PMOS – any of your choice will do. The output would look something on these lines:
Which was very pleasing to my ears as far as I could remember. Now, look at the driving signals:
You can see that the grid will swing about 20V. but the source of the MOSFET will go from nearly -180V to -40V so about 140-150V swing.
I never tried this, but if you’re seeking for lowest distortion profile, you can implement this, not cheap but possible:
The depletion FET (IXCP10M90S in simulation but can be anything you like that can stand 1kV or so) needs a 700V source voltage to allow full swing of anode voltage albeit not loading the anode with the feedback loop. Clever arrangement but expensive, or audiophile-stupid level proof.
If you do this, you would improve the distortion of this stage to this:
And yes down from 1.1% to 0.35% for 4W output. Worth looking into.
Happy Sunday.
Ale, If you are willing to use a 6L5/807s, and use SS, along with feedback, then you owe it to yourself to try the Blohbaum MTA circuit from Linear Audio vol 6. You’ll get rid of those huge distortion levels.
Hey Tim, MTA is a different circuit indeed. Nice but different, just sharing what I’ve done
Hi Al, I don’t have any SUT transformer available. Can you suggest a tube or SS driver circuit for the front end?
Hi Bing, sure. Any DHT of gain around 8 – 10 would work very well. There are few examples on my blog to look for around the 01a, 26, 4P1L, 2P29L, etc.
The input capacitance isn’t a problem in a source follower. Look for the theory of this topology to understand as this capacity is bootstrapped. I use other MOSFETs these days rather than SiC due to availability, etc. look around for examples as there are many.