Projectile Overdrive mods?

[jwyles90]

This came up again in the wish-list page for a MkII version, and @Feral Feline brought up some great questions for what a mod to this might look like. I'm relatively new to the world of modding existing circuits, but I'm curious if anyone has any insight or guidance on how to complete the following:

1) Adding a boost before the main circuit to push the gain stage.

2) I'd want to double up the gain and volume pots, so a Vol. A/Vol. B, Gain A/Gain B type deal

3) One extra footswitch to swap between the A/B sections, and a toggle switch (SPST I think?) to toggle that boost circuit on or off

If I'm understanding this correctly, the majority of these mods would probably just require some additional hardware and extra wiring from the main board, correct?
As for adding another gain stage, I'm a little more lost on that one. The MkII of the actual 385 just has a switch that kicks things up a notch, so would that require a separate little board wired to a switch, which is then wired to the main PCB?

Apologies if making a post of just questions isn't very helpful, but I'd love to try my hand at modding this circuit and would appreciate any insights y'all have to offer!

 

[Feral Feline]

Huzzah! Me thinkering that this'n is will be fun-fun-fun project for you.

I guess the first order of business is to figure out all the bits and how big of a box you need.

I'd want to squeeze it into a 1590BB, portrait, but maybe you want 1590XX landscape.

7 knobs
1 Toggle
2 Stompers

Mockup:




Missing anything?


1) BOOST
Decide on the type of boost you want:

Clean vs Dirty​

BJT​

JFET​

Op-Amp​

There's advantages to each kind, maybe explore that a little and see what best suits your end-goals.
The circuit itself is basically a boost (Q1) pushing a FuzzFace (Q2&Q3) into a tone stack with a recovery stage after (Q4).

Adding the boost on a toggle is easy enough, as is adding an order-switcher.
Maybe focus on the main circuit and its mods first?


2) DOUBLING GAIN & VOL POTS

I'd use the BASS and TREBLE pots to support the PCB as well as use hard-wires to support the connection to the bypass to also help support the PCB, because you're going to have to ...

​

- Offboard wire the GAIN and VOL pots.​

The GAIN LUG1 has a minimum-limiting resistor below it, R4, of 33k. GAIN LUG2 feeds into R5 470k... not sure what the interactions are going to be for having two pots stuck to those resistors.

One way of getting around that would be to switch out two different R5 values instead of two different GAIN pots. In fact, let's say stock value of 470k is R5a and the mod is R5b — you can make R5b a variable resistor. Another advantage to the R5b-method would be having the stock GAIN pot supporting the PCB further.

The VOL is straight-forward, as its LUG1 goes direct to ground, so setting up a 2nd VOL will be a doddle.

Another advantage to swapping out R5 as a stock fixed item R5a and rheostat R5b means you'd have a an easy time with adding an LED.

Here's what I've done similarly in the past:

You don't need the PCB in the pic above, everything can be offboard wired — two poles of the 4PDT handle the swapping of the VOL pot, 3rd pole handles the bi-colour (or regular single) LED channel-indicator and the last pole is for swapping between R5a and R5b.


See how DGWVI does a pot-swap, his diagram is in this thread:

Thread 'Expandora Forbidden toggle with its own volume knob?'

Feb 18, 2025

Hey guys. I've been working on a modded Expandora aionfx board. I moved the forbidden mode onto a footswitch with an LED indicator, but when playing live I didn't always like the huge volume jump. So i wanted to put in an alternate volume knob just for when i'm in that mode. Does all this wiring look kosher to you?





Thanks!

• nate433
• Replies: 4
• Forum: Modifications

3) FOOTSWITCH & TOGGLE

Really, both the A/B Channel foot-switch and Boost-Toggle are part of and belong with points "2)" and "1)" respectively.

 

[jwyles90]

This is soooo helpful, thank you!! I've got to put an order in for some parts/pcbs soon, so I'm adding the Projectile to that list. Will keep you posted as I begin my tinkering.

I like the 1590BB vertical layout, so I'll probably go that route. Feels like a good opportunity to practice using one of those multi-color LEDs with the three prongs as well.

I've also never made a variable resistor (and to be honest I'm not entirely sure I'm wrapping my head around how that would work). Would that go in the place of r5 while still having the two separate gain pots?

 

[Feral Feline]

...
I've also never made a variable resistor (and to be honest I'm not entirely sure I'm wrapping my head around how that would work). Would that go in the place of r5 while still having the two separate gain pots?

A potentiometer is a variable resistor. It can be wired as a voltage divider, or as a rheostat.

If you haven't already read it, do read: The Secret Life of Pots — by R.G. Keen. Awesome info.
Once you've got the PCB, you can mock up how it's going to sit in the enclosure, either with the enclosure itself or sometimes I just use a piece of cardboard or cereal-box cut to the size of the box's footprint. If going the R5b-route below, you may find it better to mount the STOCK GAIN and TREBLE pots — that would provide plenty of support for the PCB, fixing it in place — and offboard-wire the rest of the pots to accommodate the desired control layout.



What I'm calling "R5b", as a variable resistor wired as a rheostat, would act as your second "GAIN" pot. Still only two GAIN pots.

The STOCK GAIN pot feeds into R5; you will replace R5 with a wire that goes to a footswitch that has R5a stock value 470k...



Depicted is a 3PDT with centre pole for LED, but you'd use a 4PDT with the 4th pole swapping out the second VOL pot.
Note that GAIN2's lugs 2&1 are tied together = rheostat as variable resistor.

By using a smaller resistance for R5, you'd be increasing the gain hitting Q2. How much smaller is up to you to figure out, because it will be interactive with the STOCK GAIN, which notably is a large value of 1M.

Wherever you set STOCK GAIN will affect how much gain gets to R5b, when it's engaged instead of the stock 470k value. They'll be highly interactive, but if you keep it in your head that R5b is serving under STOCK GAIN when adjusting them, you'll be able to figure out something that works for you. Maybe the STOCK GAIN is the high-gain channel and you set it very high — you'd then want a large value for R5b to reduce the gain for the lower gain channel. Then wire it up maybe like this:



That inline R5a above frees up a pole! The pot's 2&1 is wired to the same lug as the blue-wire return to the PCB (the green wire off the pot's lug1&2 as depicted looks like it's going through 3DPT's lugs 7 and 5, but no, only connected to lug 3.)

You could replace R5 completely with a 500k pot on the switch. Use the A1M to feeding directly to Q2 as your high-gain channel and the R5-as-pot (VR5) to tame the signal a bit for the low-gain channel.



That would further simplify the build; you'll still have interaction, of course. Lower the A1M and you will likely need to reset VR5's value. Further complicating things is that tweaking the GAINS most probably means tweaking the 2nd Channel's VOL as well. Highly interactive. Breadboard it all and see.

Many ways to skin the Projectile.
You may still wish to leave R5 as is and simply add a second GAIN pot.
Breadboarding the circuit would be the best way to determine what works best for you.

 

[jwyles90]

Many ways to skin the Projectile.
You may still wish to leave R5 as is and simply add a second GAIN pot.
Breadboarding the circuit would be the best way to determine what works best for you.

Man, you rock!! Thank you for all of this. As I continue to get into my pedal building journey I'm realizing more and more that I'm a visual learner, so those diagrams are super helpful for me to wrap my head around what you're talking about haha.

I had a feeling variable resistor was the same thing as a potentiometer, just wanted to double check.

I've been meaning to bust out the breadboard I purchased a while back. This feels like a good way to dip my toe into that water, as the circuit itself doesn't seem to be too overly complicated. I put in an order for the PCB today, so I'll be sure to check back in once I start tinkering around with things to update you (and anyone else who happens to stumble here). I might end up just going the double pot route for simplicity's sake, but I'd definitely like to try the other option on a breadboard first just to see if it's worth making those additional changes for the final build.

 

[Feral Feline]

Man, you rock!! Thank you for all of this. As I continue to get into my pedal building journey I'm realizing more and more that I'm a visual learner, so those diagrams are super helpful for me to wrap my head around what you're talking about haha.

I had a feeling variable resistor was the same thing as a potentiometer, just wanted to double check.

I've been meaning to bust out the breadboard I purchased a while back. This feels like a good way to dip my toe into that water, as the circuit itself doesn't seem to be too overly complicated. I put in an order for the PCB today, so I'll be sure to check back in once I start tinkering around with things to update you (and anyone else who happens to stumble here). I might end up just going the double pot route for simplicity's sake, but I'd definitely like to try the other option on a breadboard first just to see if it's worth making those additional changes for the final build.

Suggesting you start with breadboarding a simple boost, since you want one in the grand-scheme-of-the-pedal anyway.
Try breadboarding a few different types of boosts, to find the ones you like, then compare and test with the main circuit once you've breadboarded that, too.

VIVA VISUAL LEARNING!

 

[jwyles90]

This is an excellent idea. I think a JFET boost is what I'm feeling, so I'll scrounge around for something simple and give that a go.