Wah Pot replacement - How to Substitute the Value ?

[Rectangular]

hey,  I recently completed this Ibanez wah, heres the schematic:  http://8bitsindgenug.net/ibanez_wh10.png

I had always intended on putting this circuit into a massive Fender rocker pedal I have. however, the Fender pot doesn't correspond with the 50kb that the Ibanez uses to sweep the filter. I wanted to know if its possible to modify this circuit to accept a 10kb pot, for the full "wah" frequency sweep that the 50kb is intended to provide...from what I understand, R18 and C9 are  likely the key to this ?

thanks for reading
-rec

[birt]

you can also replace the taper part of the fender pot with one of a 50k pot
or make the pot control a led/ldr

[Rectangular]

hey birt

its a dual gang pot, so disassembly isn't really an option. plus, its not a removable wafer inside, so there's nothing to replace. I've never actually seen a pot with a free-floating wafer, do they even manufacture those anymore ?

as for led/ldr control... probably a last resort, I'm skeptical about how accurate that method really is, at least with the LDRs I have available locally

[petemoore]

  Diddle.
  Two 10k's seriesed gets you 20k...whether that's enough, and you'll be using two wafers, twice the chance for funk in a wah pot.
  AFAICT we're looking at a VR1, 50k 'freq' pot., wired as V/R [variable resistance, two connections].
  Whether 20k does it or not depends, sticking between 0k - 20k there and trying it out [I know, wahs are a drag to try like this..but..you can probably get an idea if what it's doing..]..so, if that don't do it.
  The LDR idea..not really that hard to figure, since there's two wires for that variable.
  Figure out what you think the lowest resistance you can get from an LDR is, check the data sheets/availability, pick a resistor of that value, put that in series with a 50k or 100k pot, place it in the circuit..try it out, the available range..I think you can get an LDR goes to 5k or so...pretty good?
  The max resistance can of course be set by placing a parallel resistor across the LDR.
  You said big box...
  Then you need a light and light shield [morley], or LED w/brightness control to activate the LDR, haven't checked, but you can probably make a 10k pot a PS, and some few parts to do that.
  If you can get the sweep you want, scratchy pot sound will be essentially eliminated. I get the idea the dual ganged 10k is a part of the wah case.
  As far as circuit tweeks, I'd look for a resistor I could clip a parallel resistor to [lowering it's value] or some offboard wire [hook something in there], I'd try working a V/R in the circuit from...something, first.
  There of course are mods can be done to the circuit, but 'center targetting' the V/R would be the approach I'd take. 
 

[Rectangular]

hey pete. the dual gang is made up of a 10k lin, and a 1M log. the 1m portion is scratchy to death,  so that first option is out

but I like the idea of ignoring the circuit and finding some way to utilize the 10k pot to control a different  resistance sweep

so... that raises another option/question. what would be the  easiest way to go about getting a 10k pot to control a 50k sweep in resistance without using an optocoupler ? I remember reading some article about how you can make a voltage controlled variable resistor using an OTA, thats definitely something I'd consider. a CA3080 is still a lot cheaper than a 25 dollar wah pot

such a solution would have so many applications, so I'd be interested in general

-rec

[Rectangular]

ok, so I just tried this optocoupling thing anyway, mostly because I'm eager to actually wah the pedal with my foot

maybe I'm doing something wrong , but 10kb doesn't give much of a voltage sweep. I was able to get about 5k resistance from the LDR, the led just didn't have the light gradient needed for the full 50k

[birt]

check geofex technology of the wah. the led/ldr thingie is explained in that article.

what i meant by replacing the taper is removing the bottom of the dual ganged pot (that part has the taper of one of the pots) and replacing it with a bottom of a good pot. i've done this with a wah before.

[Rectangular]

hey birt

if you meant this article: http://www.geofex.com/Article_Folders/wahpedl/wahped.htm#multfb , its what I was working from  , I just spent some additional time toying with it, with poor results

also, in regards to replacing part of the Wah pot, theres nothing removable/swapable in terms of how its constructed, its completely rigid, so unfortunately there's no method for disassembly.  I've done similar things to what you're saying with joystick pots, but this one doesn't allow anything like that 


that anderton optocoupler circuit seems to have been built with a 100k pot in mind, because when I use a 10k, I get a response far from linear, it crawls though the first 5k, then rockets to over 2M. I've tried running some resistors in parallel to compensate for this semi-logarithmic response, but its still pretty sloppy (ie: unusable)