Crybaby ---> Expression Pedal. Sounds simple, but that pot ain't right...

[Processaurus]

Howdy, I have 4 crybabies kicking around to make into TRS type voltage divider expression pedals, I've done one so far, but it was a treacherous undertaking.  Its the pot you see, the taper is... strange.  I wanted a linear taper, like the Boss ev-5.  Replacing the pot is tricky, because a) it doesn't have the pinion gear on it, b) it needs to have about 1/2" of thread to be able to tighten down on the thick metal bracket that suspends it in space, c) have a travel of about 270 degrees to get a full transition from lug to lug (most normal pots are more like about 320 degrees, seems like).


Its possible to make one of these, I did it by starting with a big CTS (I used 100K) pot with a long shaft, and filing down the side of the shaft so the D shaped pinion gear would fit on but leaving part of the shaft round so the gear won't slide all the way down the shaft to the threads, and hacksawing a groove at the end of the shaft for the circlip, so the gear wouldn't fall off the outer end.  I had to take it apart once because I forgot to put the nut, washer, and lockwasher on before the gear and circlip    Next I took apart the pot bottom and painted the resistive wafer with this conductive paint from work, to the point on the pot's travel that was the limit of how far the gear mechanism makes it go (about 270 degrees) to the end of the wafer, to electrically short out that unused section.   Last I cooked the paint on with the heat gun.  So now its about 84K but has the full travel from heel to toe position.   Works great now, but as you can tell it turned out to be an involved mechanical process to come up with this now very special pot.

An alternative way to put a gear on that doesn't have a D shaped hole on a pot is to file a little flat spot on the side of the shaft, and drill a little hole in it.  Then stick a little finish nail through the nylon pinion gear in a spot where it won't interfere with the rack gear, into the hole, and out through the other side of the gear.  Maybe bend or mutilate the nail ends if it looks like it can wiggle loose.

I'm wondering if anyone here has tried this kind of crybaby conversion with different techniques, either successfully or not?  Is there a better pot commercially available?  any weird S tapers that would be fairly linear in the middle, but change resistance very little on the ends?  With a long enough shaft and plenty of thread? 

[Processaurus]

With a similarly customized dual gang pot one could make an expression pedal that could always be hooked up to two different effects via two seperate jacks, could be a hit with the pedalboard real estate agents.

[scumbag]

To the first post - I have been thinking about doing this for some time. I had not considered the difficulties you mention however. I am glad I read this before I started hacking into my crybaby.

Have you considered some of the replacement wah pots available (Hotz Potz etc.)?

To the second post - I was thinking about this idea just recently. I'm in favour of anything like this, that saves a bit of real estate on the board.

[Processaurus]

I haven't tried checking the taper on the hot potz, the one I replaced was an older pot that looked like big ol 'merican CTS pots with the shiny metal shell and brown plastic.

A lot of the time spent on the first one was in figuring out what to do, I bet someone could knock one out about 1/2 an hour or less if you had the right tools (big flat file, vise with an indent for grabbing round stuff, hacksaw, two pairs of needle nose pliers or sharps for getting the circlip on and off, conductive paint) and notion of what to do.

Hah, yesterday when I tested it out I thought I knew actually nothing about electronics because the resistance from the wiper to the ground lug goes from 0 ohms to 35K in the middle, and then back to 0 ohms at the other end, and I'm staring at the simplest circuit in the world, I take it apart and it works great, wiper goes from 0 ohms to 84K, back together and its weird again.  What the hell!?  Then it hit me, the conductive paint glurped off the side of the resistive strip and was sometimes touching the pots bottom metal shell, making continuity through the chassis to the jack, back to the other side of the pot.  I thought the good Lord hated my project enough to bend the laws of electricity, and that the expression baby was an abomination of nature.

To the second post - I was thinking about this idea just recently. I'm in favour of anything like this, that saves a bit of real estate on the board.

If the multiple pedals needing expression pedals are the same manufacturer (and/or use the same control voltages, ie 0v-3.3v, 0v-5v, 0v-15v) it would be possible to just hook up the ring connection (connected in the expression pedal to the non ground side outer lug of the pot) V+ source to just one of the effects, but then send the wiper voltage to two jacks tip connections, and leave the V+ source of the other effect floating.  No need for a dual pot in that case.

[Paul Marossy]

I like to use the file a flat spot and drill a hole method when modifying a pot to use in a wah pedal. The harder part seems to be finding a pot with a long enough threaded section on it.