Sorry about the late reply ... at the end Nik suggested to wait until the release of his new King Kong amp (basically two Yetis or Chupas in one amp) ... he was too swamped with work to do custom one-offs. It's been a while though.

The idea does work and it really has been mentioned online in a few places including the Metroamp forum where I got most of my info from. No secret really, just seems odd to me that something like that is not commercially available ... always too many extra tone tweaks. You have to use relays for it to work easily and without problems IMO. I had trouble with signal bleeding and oscillation using optos/vactrols and FETs. My amp is pretty much butchered so I'm selling it as is to a local guy who is fully aware and I'll probably will start from scratch modding a new Plexi 50 or 2204 already built kit to have something reliable to play on ... more than reliable "clean" as the amp is perfectly functional but the chassis looks like Swiss cheese and I have too many knobs ans switches on the outside.

Posting as I am currently asking Nik about something like this but using the AFD amp as a base since it already has the relay.

The principle would be:

Plexi clean channel with a footswitchable modded 2204 OD mode.

Starting with a Plexi 50 amp for instance, one DPDT relay would switch between two volume (gain) controls, one of which would route the signal straight to the mixer (for plexi cleans) and the other would cascade the first gainstage into the second one, and then to the mixer for OD. A second relay would switch a pre PI MV in and out, being only active in OD mode for volume balancing. A post PIMV could be used for overall volume.

It's basically a switchable one wire mod (with tweaks for better tone) but also switching the pre PIMV in and out for volume balancing. The cleans side can be nice as Plexi cleans are or get to medium gain, and the OD mode could be tweaked from anything from a Jose style, to a Bogner Blue channel or whatever.

It works great, I've done it with some sucess but I'm a rookie and my amp looked a mess so I wouldn't trust its reliability ... it would be nice to get it straight from Nik for it to be wired properly! I don't really know why nobody offers and amp like this stock ... they always add too many bells and whistles.

The AFD is already almost there, you just need that the relay also switches the pre PI MV in and out for it to be active only in OD else you have too much of a volume difference especially if you want the cleans to be really clean. Also, instead of both modes sharing the first gain control, the relay should also toggle between two gain controls for proper setup of clean and OD.

If the base amp is a Plexi of course there'd be a stage gain less than on the AFD. With the extra tube the AFD allows more tone shaping ... you could even have a switch to bypass one of the stages in OD mode for a medium gain setting.

How loud are you playing? Is the master volume set above 40% or so?

How high do you have the PPIMV set? Keep in mind that the lower that you have the PPIMV the less negative feedback that you will allow the amp to have. As you go lower the first thing to stop working will be the presence. The amp will feel gainier and the other controls will begin to seem less responsive. If you're running the PPIMV low, then it's the nature of it. If you set it higher than halfway or so and still have the problem, then it could be something else like a broken connection of the mids pot to ground.

Hi there first post ... the only place where I could see some highs being removed based only on the schematic is on the voltage divider before V2, but that would only affect the clean sound.

You have the normal 470k/470pf //470K arrangement but with the way your switching is done you only have the bypass cap in the signal path when in OD mode so you get the full treble peaking effect there. When in clean mode the 470K/470p goes to ground and will cause the opposite effect, draining quite a bit of highs.

It should be easy to take it out just to see what happens to the cleans. If it improves some, you could install it again and put another 470pf bypassing the other resistor to get treble peaking on both modes ... this won't result in exactly the same treble peaking for the OD mode as before (because each side will be pulling a little treble from the other when shorted to ground) but I think the difference would be minimal.

About the overall darkness of the amp it could be the speakers as has been said before .... the circuit is straight up 2204/2203 other than the switching so it shouldn't be dark!

Btw, I'm using the same pot to ground on the MV method for volume boost in my amp (Plexi 50 made into switchable cascaded preamp), simple but effective.