Wah/Expression mode mod

[HamSandwich]

Hey all, new member here. I'm thinking of a new project to convert my old cry-baby gbc95 so it has 2 "modes" instead of on/off. I'm thinking this is just a wiring/switch problem essentially, but not sure where to start on how to isolate the 100k pot properly and if my idea would result in any "tone suck" or lack of function. The expression pedal conversions I've seen basically take out all the guts and hook up a TRS plug to the pot - simple as that. Just thought I'd ask y'all if something like this is possible as I just don't know enough about electronics. There must be some type of switch one could install that could perform a function like this, leaving me with an expression pedal when wah is NOT in use. LMK if this is just craziness

Goals :

1)Footswitch becomes a toggle that goes between exp pedal/wah "modes".
2)Bypass circuit of the wah must be preserved so guitar signal still goes through and functions normally.
3)It'd be cool to have an LED to indicate either "mode"
4)TRS plug installed

Thanks for taking a look, happy new year!

[HamSandwich]

Here's a crude schematic of above idea.

[UnexplodedCow]

I did something like this a couple years ago to a Vox wah. It was cheaper than buying an expression pedal, and sturdier.

I took a ham-fisted approach to it.

The switch is a 4PDT. Half the switch converted the wah to true bypass.
I drilled a hole and added a stereo jack.
The other half of the switch disconnects the wah pot from the wah circuit and puts it as an in/out for the stereo jack, much like a passive volume control. It works just fine that way for me.

The one issue I noticed is a very slight *pop* when turning the wah effect on or off, and I could probably fix that with a capacitor or two (or maybe a buffered bypass?).

I will try to hunt down the crudely drawn wiring schematic for what I did. If I can't find it, I can draw a new one that might be a little cleaner.

Another suggestion; there is plenty of space in that GCB95, and they benefit from some mods (I'm not a fan of their stock sound). I ended up adding a Q control to the Vox as a final touch. It helps the wah, but it's still second best to the BBE wah I happened upon (stock, and I absolutely love the sound of it).

[UnexplodedCow]

I revisited this one as I wasn't quite happy with the parameter control, and the original way I did it surprised me in that it worked at all.

I removed the 4PDT switch (it broke yesterday while in use), and swapped in a Carling DPDT for switching the wah section off true bypass style (no longer clicking, either).

Next that's required is a 3PDT switch (rotary, toggle, doesn't matter).

This is assuming the stock wah pot is a 100k (which is the problematic part).

I noticed the wah pot is wired in reverse compared to what I see for expression pedals. More specifically, the "positive" and "negative" side of the pot are reversed to what the expression pedal input needs (probably why mine worked oddly).

So we have to do two things here; swap the polarity of the pot, as well as cut its resistance to 10k. Some walk-throughs I've seen show adding a 1K resistor to the wiper.

What I changed is to use 2/3 of the 3PDT to swap the pot polarity, and the third is used to cut the parallel (between positive and negative legs) pot resistor in and out.

I left the wiper in place with the stock wah circuit, attached a 1K resistor to the tip of the expression jack, and that pretty much did it

The parallel pot resistor is around 11k25, or 11k should be close enough to bring the pot down. Of course, measuring the actual resistance of the pot in your wah will help calculate a closer resistor to add in.

I have not tested it yet, as I'm pretty tired, but I thought of posting back here about it. Hopefully it gives a mental image of how things work. I've not drawn anything up for it yet (really should this time).

It might be easier to use electronic switching that interacts with the main footswitch, but it would need to be at least 5PDP, with 6P probably working better (disconnecting the wiper from the wah circuit, essentially removing it entirely from the wah). I don't know how to do that, although I've seen some interesting threads on it.

And, if this turns out to be successful, it might be a worthwhile mod for guys who want a dual (or triple) purpose pedal. I know I'd be up for additional input on this if it hasn't been done already (and I haven't seen it posted, nor have I seen it all on this board).

[HamSandwich]

Hey glad someone took a look! I finally got mine to work few weeks back. Took a little wrangling, the effect I'm mating it with (univibe type) was designed with the Hot Potz II pot in particular for speed control, but I still had to confer back with the maker about the wiring as his diagram was not clear.  I got all sorts of weird artifacts when I had it wired wrong.

There's still one small issue I developed a workaround for, but I've basically taken 3 poles of the 4PDT switch, and isolated the pot itself (wiper,C, & CCW), then used the last pole to run the wah simply as originally wired (no true-bypass). Its a bit different than my orig idea of somehow "tapping" into the pot. After reading up on the basics of impedance and voltage, my brain cell began to lag considerably, so I decided to go the total isolation route :)

I'll try to describe as best I can, but I think there's a grounding problem.  If the TRS jack is in contact with the case of the wah, I get a totally different sweep.  It's as if too much voltage runs to the speed control of the vibe, and a tiny movement of the treadle (1/4"?) produces 0->full, making it totally unusable.  I found as long as I've got the TRS jack insulated from the case it works fine - it took a while to figure that out, I thought I had a bum pot and wound up buying a new one, but it produced the same result. I just used the rubber grommets that came with the jack and it works fine. It's not a perfect solution because I found that if anything grounded contacts the metal parts of the TRS cable I made, it does the same thing. I seem to be good to go however, if everything is rubber coated, haven't had any other probs besides that. Is that some kind of ground issue? I'm just not well versed enough electronically to know one way or another.............

I re-assembled using the brand new pot, so the wah is totally quiet. I don't really get a loud pop when I kick into wah mode. I use a compressor at the very end of my effects chain, I noticed that placement significantly reduced scratchiness of the old pot - a nice additional use for compressors in general BTW.  Maybe it's taking care of the pop as well. I'm sure there's an elegant/proper way to take deal with these things, but the way I've got it works well enough :)  

Another small problem is that when I kick on the wah, the vibe pedal is no longer getting input from the TRS cable - so it effectively changes the speed to zero if the vibe is on while I engage wah. I might install a micro footswitch into the TRS cable to disengage, so its as if the cable has been unplugged - returning the speed of the effect to wherever its manual dial is set. I don't really use wah & vibe at the same time anyway, but it'd be cool to have it as an added feature I guess............

I'd really like to get an LED working on it, but with my limited knowledge, I can only figure one would need a 5PDT switch to achieve. I wasn't able to find one made in the heavy footswitch style. 4PDT was as big as they come. It sounds like you were able to tap into the pot using only 2 poles of the switch, I'd be interested to know exactly how that's accomplished - then I'd be freed up to install LED's.

Unfortunately, there's really no standard for expression pedals in terms of pot styles (logarithmic/linear) and resistance, so you've effectively got to tailor your exp pedal to each effect its going to be mated to. I found two diff guys on-line trying to bridge the gap of course, kinda interesting............the 'expressionator' is a really cool concept :)

mission engineering expression pedals: http://missionengineering.com/product-category/expression-pedals/
Thru-tone ernie ball vp mods: http://www.thru-tone.com/Ernie_Ball_Exp.html

Thanks UnexplodedCow for the input and ideas. The whole motivation was just to avoid another large gas-pedal on my board, they take up tons of space. It's def a bit of a mind-bender when you consider having the function of the wah needing to be preserved & bypassed normally, but still use as exp pedal. I think I read somewhere in my 'net-travels that the HotPotzII pot is actually somewhere in between being a linear and logarithmic design, which throws an added wrench in there if you need particular response curve. From what I gather, your latest variation will now have an additional switch to engage exp mode? (side-mounted?)

Here's a pic of the finished project - works like a charm :P

thanks for the reply!

[joegagan]

cool project!

kudos for getting it to work. not sure if full understand the switching, but it is possible that millenium2 bypass would allow an LED without added poles n the switch. look it up.

the hotpotz2 ( and hotpotz1 , and thomas crybaby pots prior to that ) are a special taper that is not related to lin in any way. a better descrip would be 'condensed log' because they put a log sweep in approx 30 - 40 % of the rotation, with near horizontal ramps on either end ( treble end longer flat due to switch runout area needed.)

in the graph below, the upper left diagram shows 'G" which is what the hotpotz looks like, if you consider the top of the graph bass and bottom treble: