Adding a switchable clean boost switch to a distortion circuit

[guitarlynn]

How would one go about modifying an existing distortion pedal circuit to add a clean boost option that is foot switchable? My thoughts would be along the line of the difference between a Tim & a Timmy circuit, although I imagine that it would be consistent between any common distortion pedal circuit. TIA

[Mark Hammer]

Depends on the design of the existing circuit.  If the pedal has some sort of output buffer or gain-recovery stage to compensate for a passive tone stack, then a component-value or two can be switched to change the gain.  It the pedal is something like a Fuzz Face where the distortion-creating part goes directly to the volume pot and output, it would necessitate adding a gain stage of some sort.   

[Ben Lyman]

I wonder if it can be done by simply wiring two volume pots to a footswitch. It's not gonna give you a "clean boost" but I should think it will give you two output levels to choose from, right?
All the distortions i've owned or made seem to have way more than enough output to handle this task

[robthequiet]

If the box has a diode clipper section it would be fairly trivial to add a switch to lift the diodes, although there may be some issues with switch popping or level changes.

[GibsonGM]

I wonder if it can be done by simply wiring two volume pots to a footswitch. It's not gonna give you a "clean boost" but I should think it will give you two output levels to choose from, right?
All the distortions i've owned or made seem to have way more than enough output to handle this task

Yup.  A version of that was done by Fulltone by switching in FEEDBACK pots to change gain. Doing the output pot might be more involved as they may be interactive....maybe a resistor switched in and out (shorting) would do the job?
http://generalguitargadgets.com/wp-content/uploads/fulldrive_2.jpg

I've always approached this by attentuating the effect and adding a boost circuit right on the PCB, like an AMZ Mosfet boost.  Seemed easiest to me...all volume mods seemed, to me, to alter the original tone too much...YMMV

[Mark Hammer]

If the box has a diode clipper section it would be fairly trivial to add a switch to lift the diodes, although there may be some issues with switch popping or level changes.

There is generally no popping issue in my own experience, however there can be a substantial, and often jarring, difference in output level, not to mention a big difference in tone.

It often doesn't take much resistance between the diodes and their contact point, whether that is ground or in an op-amp feedback loop, to alter both output level and tone.  For instance, inserting a 5k trimpot between a pair of diodes to ground, and ground, would allow one to dial in how much reduction in clipping you want.  A footswitch can then be used to bridge and unbridge that trimpot, by connecting between the ground side of the diodes, and ground.

[GGBB]

If you are asking about modifying a circuit - not an already built pedal - you can easily just tack on a clean boost section to the output of the pedal (or before the input of you like) with appropriate bypass switching. This wouldn't be much different than simply having two pedals in one enclosure. Otherwise, or if you want to get clever about it, it depends entirely on the circuit as Mark noted. Can you post a schematic?

[guitarlynn]

Thanks for the thoughts. I was thinking along the lines of the Tim pedal that simply had a separate Boost section that was independent of the scaled down Timmy pedal without the Boost option. I had been considering either bypassing the diodes and/or building a feedback loop, however that would be an either/or option with the distortion engaged which isn't what I was hoping for. In regards to this it would seen that a separate boost stage would be required unless there is enough play room to manipulate an input/output buffer for this purpose. I was hoping someone may have already done this and was willing to share, but if not I'm willing to spend some time working on it when I can do so. Ultimately I don't care to add another gain stage requiring an additional transistor or IC.

[PRR]

> switchable clean boost switch to ... .... ..... ... ...  Tim pedal

[guitarlynn]

Looking at a Layout for the Tim, there is a "Boost Volume Pot" connected from the Inverted Input on stage 1, but then it apparently returns via a "Boost Tone Control" at the Non-Inverted Input to the Output Buffer. Obviously the layout is missing the actual circuitry and values to these components. Clearly a schematic would be far more useful, but it's a starting point anyway...

[induction]

Clearly a schematic would be far more useful...

Ummm...

Obviously the layout is missing the actual circuitry and values to these components...

Nope. They're all there. 

The Boost Tone control isn't shown in the schematic, but it's just a pot and a resistor. Easy to reverse from the layout.

[guitarlynn]

Lol, you are correct induction. For some reason the text at the bottom didn't show until I linked it in the post. The text answered the remaining questions. Many thanks for pointing out the obvious and for all the helpful suggestions that could provide similar solutions! :-)