NFB/Presence control for a Cathodyne PI build?

[Gainzilla]

One other possibility to propose... What about a pair of 6mb8s? They have a built in triode, so theoretically with a pair I would have the triodes for a LTPI. I have no idea how they sound though... My guess is somewhere between a 6v6 and el84, but that's just total speculation.

Thoughts on that approach? Any experience with those?

Cheers!

[J M Fahey]

if the mosfets are truly an option, I'm not opposed to giving them a try.

I would plan on using a MOSFET for cathode follower like in the second attachment, and I guess the loop driver stage like the first attachment.

The tone control driver Mosfet replacement is fine, but keep the original 100K or 56K cathode resistor, itīs part of the sound.
22K is "technically better" but we want the original sound.

As of the loop, use the Mosfet for the drive/send side, because there itīs "transparent" (as in: unity gain. plus it never ever approaches clipping) but keep the tube "recover" triode.
because that one , given its gain, *can* be overdriven by loop return signal.

The unwritten general purpose rule is: SS can replace tubes impunely *if* it never distorts, not even approaches that.

[Gainzilla]

UPDATE:

It turns out I had a copy of the above loop with the credits cropped. Attaching a version with credits intact. Schem should be the same. Apologies for any confusion.

[Wombaticus]

I'm just going to pop in here in defence of the cathodyne. Theoretical limitations aside, there are some very nice sounding amps that use the cathodyne PI, the 5E3 tweed deluxe coming immediately in mind. It's true that in "standard configuration" when overdriven hard they can become ratty sounding. But as Merlin has pointed out, the situation is much improved by slapping a really big grid stopper on the cathodyne itself (input impedance is so high that even 470K-1Meg is OK with respect to HF), and also larger than normal grid stops on the power tubes --- like 22K or so. The resulting circuit tames the worst tendencies toward rattiness, while still retaining a unique character that, for a certain type of amp, can work quite well. It's eminently worthy of experimentation.