Turning a Crybaby into an expression pedal - Problems

D

dk_ace

Experienced
So I decided to try an turn a Crybaby into an expression pedal. The pot is 100K already, so I picked up a TRS jack, and got to work. I wired it up the way that seemed right based on the diagrams I could fine, but when I plugged it in I had two problems:

1: It works backwards. Toe down is a value of 1 as opposed to 127.

2: When it is in the off position (should be heel down, but currently toe down), it turns the volume off. I lose all signal, and I don't know why. I believe it is somehow controlling the volume block as well. Both my volume and expression are going through my GCP sending on midi channel 2 and 10 respectively. This wah pedal is controlling them both somehow??? When I plug my Roland back in, everything works fine. This problem is being caused by the modded crybaby expression pedal.

Any help would be appreciated.

D
 
1. If you have a TRS jack you should have 3 pins on it. Connect CW on the pot to the sleeve on the jack. Connect CCW to ring, and SL to tip. If this doesn't work, switch tip and ring so that CCW is connected to tip and SL to ring. Make sure you use a TRS cable as well to connect the pedal to the Axe or MIDI controller.
2. The 100k pot in a standard Crybaby is the wrong value for most expression pedal inputs including the Axe. Axe is expecting a potentiometer resistance of 10 - 50kOhm. A 100k Ohm pot may work but you'll likely find that small pedal movements will make a bigger changes in value so control won't be as even. Some equipment is more tolerant of the 'wrong' pot values than others depending on how the buffer circuits are designed.

James
 
I got a 10K Linear pot at RadioShack. It's the standard long shaft re-branded Alpha pot. It was cheap and I thought I might be able to make it work. The pinion gear came off the Dunlop pot without too much trouble. I had to drill it out some to fit on the RS pot.

The threads on the pot aren't quite long enough to bolt it on like the stock pot. I'm not sure how I'll address that yet. I may file the mount in the casing thinner to accomadate it, or I may use a thicker nut loctited on and epoxy it into the casing unless I come up with a better idea. I got the wah for $25 and the pot for $2, so I figure I don't have much to lose either way.

D
 
Yeah, the threaded part of the RS pot isn't long enough to bolt to the mount on the Crybaby is it? The way I have seen this done before is to use a small L shaped bracket. You can probably get one at the hardware store. Drill a hole through one leg of the L to mount the pot. Drill two small holes in the other leg of the L to mount it to the chassis then you don't need to use the existing mount. You can use two small bolts and nuts or self tapping screws to fix the assembly to the chassis. Filing down the existing mounts would probably work. Remember that when you use the pedal you are putting a lot of weight and stress on these parts so they really do need to be secure if this is going to work. Also, since you drilled out the gear and the RS pot probably has a smooth shaft you have lost the meshing teeth that keep the gear and shaft together on the Dunlop part. You'll have to make sure to glue it good and tight to stop the gear turning on the shaft when you use the pedal.

James
 
I guess any value from 10k to 50k is fine for expression pedals.

I came across this 20k pot that would work perfectly, but I don't know if it's linear or logarithmic.

Would it make much difference one way or the other?
 
A true log pot will sweep more slowly at first becoming quicker as you move through the sweep. The principle use of log pots is in audio amplifier volume controls where the logarithmic nature of the pot compensates for the logarithmic nature of human hearing. Many 'log' or 'audio taper' pots are really just stepped though, rather than true log. Log pots are more expensive to make. A log pot will work of sorts. A linear taper pot will mostly give you a more consistent sweep through the range of the pedal and is usually recommended for expression pedals.

If you are up late and night and desperate for something to surf on :) try this http://www.geofex.com/article_folders/p ... tscret.htm

James
 
Actually, can't I just wire a static resistor in parallel to get the desired resistance?

EDIT: nevermind, the math doesn't work out the way I want

100k||25k = 20k
90k||25k = 19.565217391304348k
80k||25k = 19.047619047619047k
70k||25k = 18.42105263157895k
60k||25k = 17.64705882352941k
50k||25k = 16.666666666666668k
40k||25k = 15.384615384615383k
30k||25k = 13.636363636363637k
20k||25k = 11.11111111111111k
10k||25k = 7.142857142857142k
5k||25k = 4.166666666666666k
 
Sorry I missed all the discussion here.

The RS pot is not flat and the threads aren't long enough when you buy it. I fixed that with a Dremel tool. I ground a little off of the mounting bracket in the shell and then ground the useless metal tab off the pot. I also took some of the extra metal off of the top of the pot.

I did that yesterday. Mounted it in place and it works great mechanically. I'll try to wire it up today and test it tomorrow night. I left the Axe-FX for a rehearsal tomorrow.

As far as the resistor thing, I have no real electronics experience, so I don't know if it would work or not. I would have liked to try it before I went through this much trouble though. It really wasn't too bad I guess.

Log pots would be a nightmare for expression work. I tailored a curve to work okay for volume work, but it wouldn't sound right with a wah.

D
 
Ben J said:
I guess any value from 10k to 50k is fine for expression pedals.

I came across this 20k pot that would work perfectly, but I don't know if it's linear or logarithmic.

Would it make much difference one way or the other?
Click to expand...

IMO it is WAY more important to use a linear pot for expression purposes that to worry about the exact value.
I'm using 250K pots and they work just fine even though they're quite a bit outside the recommended range.

If you have a choice go with something between 10 and 50K but if the only thing you can find is 100K or 250K don't worry about it.

S.R.
 
IT'S ALIVE!!!!!!! :twisted:

I finally got it working. I had to rewire it twice (the RS pot isn't labeled, so I had to guess which lug went to each function). The end result was a wah that feels right and sounds right. I was surprised to find that the pot actually went through the full 0 to 127 range. I thought it wouldn't be quite right and I would have to tweak the modifiers some, but that wasn't the case.

D
 
dk_ace said:
IT'S ALIVE!!!!!!! :twisted:

I finally got it working. I had to rewire it twice (the RS pot isn't labeled, so I had to guess which lug went to each function). The end result was a wah that feels right and sounds right. I was surprised to find that the pot actually went through the full 0 to 127 range. I thought it wouldn't be quite right and I would have to tweak the modifiers some, but that wasn't the case.

D
Click to expand...

Awesome! I was gonna go to Radio Shack tomorrow to grab the pot and a couple trs jacks. Would you mind terribly taking a picture of the inside of your pedal, or at least describe the wiring? If you do, I'll buy you an e-beer. ;)
 
Ben J said:
[quote="dk_ace":lnv9qtbc]IT'S ALIVE!!!!!!! :twisted:

I finally got it working. I had to rewire it twice (the RS pot isn't labeled, so I had to guess which lug went to each function). The end result was a wah that feels right and sounds right. I was surprised to find that the pot actually went through the full 0 to 127 range. I thought it wouldn't be quite right and I would have to tweak the modifiers some, but that wasn't the case.

D
Click to expand...

Awesome! I was gonna go to Radio Shack tomorrow to grab the pot and a couple trs jacks. Would you mind terribly taking a picture of the inside of your pedal, or at least describe the wiring? If you do, I'll buy you an e-beer. ;)[/quote:lnv9qtbc]

No problem. I left the pedal at a gig, so it will be Wed before I have time to pick it up. I'll try to take a pic then. The wiring is really simple if you know which terminal goes to the wiper, CW, and CCW (which you will once you see the pic). The difficult part is getting the pot to fit, but if you're patient it isn't too bad. I ruined the threads on one pot because I was trying to force it to work without shaving enough metal off. The second attempt I made was smooth sailing though.

D
 
I'm interested in doing this as well so I wanted to say thanks, in advance, for the pictures and help!
 
Another thing I'm going to try based on the bypass modifier thread in the General forum:

I'm going to install the switch back in the wah casing. It won't be connected to anything, but it will be there. I'm going to try and set the bypass modifier so that it activates/deactivates the wah block when the pedal gets pushed into that last little bit of travel from pushing the switch.

I fight the auto engage because I like to use the heel down side of the wah a lot, and the autoengage makes it cut off a lot of times when I'm using it. If I set it long, the wah stays on too long after I step off of the pedal. If this works, both problems will be fixed when I want them to be and the pedal will effictively function just like a real wah.

D
 
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