Substituting Pot values?

[bobbletrox]

I've been looking up parts for a Big Muff project, but the place I'm ordering all my parts from don't have 16mm 100k Log pots -they only have 1k, 10k, & 50k.  Is there a way to work around this?  I have to use 16mm pots so I can cram all the parts into a BB sized box.

[smoguzbenjamin]

If it were bigger, say 200k log then you could just add a 200k resistor in parallel to make it act like 100k... As far as I know there isn't really a way to work around this.... Unles you can get a dual ganged pot and wire them in series

[brett]

Hi.  In the Muff Pi, for both the "drive" and "volume" pots a 250k, 500k or 1M pot will work the same as a 100k.  

Also, I can't see any reason why the "tone" pot can't be larger than 100k (though I'm only 99.9% sure about this).  The tone pot definately needs to be a linear pot (it acts as a mixer), whereas you can get away with linear (instead of log) for the other pots if need be.  

Cheers

[Mark Hammer]

A lot of the time, pot values are intended to cover a wide range of settings, many of which you may not actually use.  Alternatively, they can sometimes cover LESS than the range of settings you would desire or don't go far enough at one end of the scale.

In the case of distortion units, often the pot value used to set the gain (and hence the distortion amount) is intended to range between some minimum and some maximum amount of gain.  In the case of something like the DOD250 or Distortion+, the pot goes between the inverting input and ground, with smaller pot resistance settings producing more gain.  If it is not your intention to ever USE less than a certain amount of gain then you can make do with a smaller value pot.  

In the case of the Big Muff, the 100k pot that sets the amount of distortion serves to attenuate the boosted output of the first stage.  The more you trim back on that level, the less the subsequent stages are able to produce clipping with that signal.  In some respects that control serves to allow you to match up the amount of attenuation to the output level of the guitar - some guitars need less boosting for the same amount of fuzz.  IN other respects, though, that control serves to produce a variety of distortion sounds.  Of course, if you never really got anything useful below a certain setting, the full range of that pot is not essential.  IN which case, you could easily replace the pot with a 50k unit and a 47k fixed resistor to ground, so that it would act like a 100k pot that can never be turned more than about 1/3 of the way down.  You will note, in fact that the control on a BMP already has a 1k resistor on the ground end of the pot to simulate a pot that can never be fully turned down all the way.  Tinkering with the value of the fixed resistor simply tweaks how much of the total attenuation the pot will be able to accomplish.

I suspect Brett's comments are probably correct, and a 250k pot will do fine.  Similarly, a 250k pot with a 120-150k resistor from the wiper to each outside lug will let it sub for the standard 100k tone pot.  A 50k pot should be fine for the output.