Hey that's a good idea. I have been thinking the same thing about my 2550 clone. I have tired of the diode clipping and was thinking of dropping in a valve only preamp board such as the JCM800 2204. With the relay it should be possible to switch between the two inputs and so effectively create a clean and gain channel. Volume levels would be very different but the second pole of the relay could be used to switch between two different master volume pots. There would be a spare pot position as the lead master would not longer be required.
Questions would be around electrical compatibility (2550 power transformer has slightly higher voltages then 2204) and mechanical compatibility (would the board fit, would re-drilling be required).
I have not tried it yet but would be interested to know how your mod turned out? What circuit did Nik suggest?
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.
to keep the forums alive.
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