Inserting fets into opamp guitar amplifiers?????

[screamersusa]

Greetings, I'm working on another project along with the pedal one but they are related.
I picked up a Randall RX75 for $25, patched it up and am ready to make it "Mine".
I will use an external box for super clean and lead through the tape/aux in.
I want to inject a Spitfire, peppermill, or bor into the clean channel, (spitfire seems the nicest)
and replace the clipping section of the od channel with the Mosfet Engl mod 2.
Since the amp is opamp based I have questions regarding the insertion of said circuits.
Should I omit the first stage of each existing fet circuit? Will the engl mod 2 mosfet work on a 15v supply?
I know the SHO circuit will work but the russian mod2 mosfet uses a different bias scheme with a vref.

This thread might be a handy way to spice up an inexpensive opamp driven guitar amp for those who
kinda "Need" to.  I will start another thread about switching.

[WaveshapeIllusions]

It's doable but will likely require a decent amount of changes to the PCB. It will also require a correct schematic. If you don't mind desoldering and such, it shouldn't be too much trouble.

Some thoughts:
-For the clean side you could just use an external box into tape/aux in, if you were going to use that for clean anyways. The downside is that it will always be connected there, with the trouble that can entail. You would have to switch inputs between clean and dirty.
-For the OD side, you must find where it starts and ends. Chances are, it starts after the clean/dirty switch and ends at a mixing stage for both channels. It could also have a switching element after the dirty channel. After you figure out the start and end, a component at each end needs to come out to open pads. Your choice of dirt can just be grafted in where the OD channel was.
-Depending on preamp supply voltage, you can just run whatever from it. If it's 15V (likely +/- but you just need the positive end) you can borrow from the amp's supply and ground. 9V effects circuits shouldn't have a problem with the higher supply, aside from sounding cleaner. Make sure that it's disconnected from the mains and that all supply caps are drained before messing with the supply. You also want to find a good spot to tap off the supply. You might just have to solder to a component pad, as there are likely no free pads. Alternatively, you can take out something that goes to supply from the now disonnected OD side and use its pad.
-If you want to use the switching that the amp already has, the clean channel can be clipped out and replaced with your circuit of choice, the same as the dirty side. Just clip out the start and end and graft the circuit in there.
-Depending on how many holes there are in the chassis, you can just remove the unused pots and use the spots for your circuit's controls. I assume there's tone and volume controls, so you can just replace each channel's tone and volume with your circuit's. If there are not enough holes, more must be drilled or controls must be taken out. If the tone section of the amp is after the channel switching (it affects both channels) then you can just leave out the tone controls in the new circuits.

What you're suggesting is basically replacing the entire preamp. It's nice if you want it all in the amp chassis,  but it will take quite a bit of work. I've done the same with a similar amp (Fender Frontman), and there's a decent amount of PCB reworking. The upside is that it looks nicer, and you have a good amp for cheap. Remaking the entire preamps section is also much simper than putting together the whole thing by itself. You already have switching and a power section, so you don't need to build those. The chassis is alrwady put together too, so that saves time. I'd say it's worth it.

Though... it would be simplest to just put together an external box and use the aux in. That already goes straight to power amp. All you'd probably need was a dual rca (or stereo 1/8" depending on the aux in jacks) connector. It wouldn't look as nice, but it'd be a whole lot easier. Another upside is that you can use it with other amps if you end up selling or getting rid of that one.

Hope that this helps out.

[screamersusa]

Modding the rx75 is not bad at all.  I'm going to pull one resistor to interuppt the clean channel to insert
manual switching for the spitfire or peppermill circuit.
On the OD side I will remove the simple diode clipper to ground, and insert the mosfet engl3 there.
Thats pretty much it except for an external a/b switch to the clean etc digitech GNX3000, and I'll toss a
basic efx unit into the insert for delay.

I'm reusing the existing input and buffer amps. The board will be added to existing amp with
standoffs. 
A concern I have is with the input gain on the new circuits and if I need to omit the first stage on each.
Thanks for the reply