Suggested Mods for Peavey Butcher - Negative Feedback Loop Tone control?

[apehead]

Hello all,

I have a Peavey Butcher that has become my mod test bed. I have been trying to get the amp to have more bass response and higher gain. I had previously added a choke, bias adjust, and changed the tone stack to be more similar to a Fender silverface than a Marshall JCM 800.

Recently, in order to increase low end I have also removed the bright cap and the "treble peaking" circuit between the first and second gain stages. This also entailed removing the 470k voltage divider and replacing with a single 1M load resistor.

I'd like to replace all the load resistors between gain stages with 1M audio taper pots for individual stage gain control.

In order to further increase gain, I wanted to convert the second half of the second 12AX7A, currently being used as the cathode follower for the tone stack, into a fourth gain stage. Concurrently, I wanted to ensure the tone stack does not excessively load the third gain stage.

Looking at the circuit design of many op-amp based preamps and pedals, it occurred to me to place the tone stack in the negative feedback loop between the output transformer and the phase splitter. Essentially I want to replace the presence circuit with a more comprehensive tone stack.

Questions:

Is this phesable? If so, does anyone know of any tube amps that utilize this circuit so I can cross-reference some existing schematics?

Are there simpler ways to increase gain?

What are some other things to try to increase the bottom end?

Should I still keep some sort of fixed resistive load between each stage, or is it OK to have pots as the only load resistors?

Are there any slick mods to simulate a Clean and gain channel (preferably footswitch operated) in a single channel amp? Maybe use a relay to shunt all the stage gain pots to full load resistance?

Thanks!

[apehead]

Here is the Stock Schematic

peavey-butcher.pdf

[Chuck H]

I think the fact that a tone stack operated by the typically late stage NFB loop hasn't already become a standard practice might be an indicator of the feasibility. If you never intend any PI or output tube distortion from the amp it MAY be ok. I've considered using the NFB loop for a mid control. Vox, of course used local NFB in the PI for a "cut" control which was a sort of treble circuit. I just think a lot of what tube amps do that sounds good happens in the PI and power tubes. If you relegate them to tone stack service, and therefor cannot clip them much, their affect will be lost. Maybe that's not an issue with your design.

I have to wonder what you're chasing with this.?. There is A LOT of gain available from two preamp tubes prior to the PI if uber gain is your goal. Maybe you just want to do it differently for it's own sake? Then sure, try it. Otherwise there are precedents that should give satisfying results I think.

[Chuck H]

P.S. Since your Butcher is already "butchered" (yuk yuk)...

I think altering the cathode follower to a gain stage is a good plan for more gain. Look at the Fender Prosonic schematic. Two preamp tubes. Uber gain fo shizzle. Once you strip out the effects loop and reverb the preamp only requires two tubes and sounds outstanding on both clean and dirty channels. I once wired that preamp into a Vibro Champ with good results. Hard to go wrong with that one.

[apehead]

Basically I'm trying to boost the available gain for a metal guitarist I know. Increase the "chug" of the fully distorted sound in other words. As a bassist, I've also been sort of disappointed with the percussive, clicky treblely sound of the amp since I bought it.

I wanted to convert the cathode follower to a gain stage in order to increase the gain, and considered the feedback tone stack in order to avoid a plate coupled tone stack loading the signal.

If there is an easier way to achieve that extreme metal type of gain without resorting to such extensive surgery, I am all ears!

It's just that the high gain descendant of the Butcher, the 5150, uses extra gain stages in the "Lead" channel, so I figured that was a sure bet.

I posted the same OP at the diyaudio forums, and I was pointed to an interesting schematic of an amp that uses a rotary switch to change cathode bias resistor values, which may be a much more economical option for this amp.