Wah mod question

[Viral Rock]

Hi all - first post, but I've learned a lot from this forum. 

I'm just starting to get into wahs, and am looking into some Crybaby wah mods.  Can someone explain the difference between (1) manually adjusting the tone pot and (2) swapping out the sweep cap?  When I say adjusting the tone pot, I'm referring to rolling back the pot a few notches (no soldering or desoldering) so that the entire sweep range of the pot is moved up or down.  It seems like they both can effect the amount of high and low end content.  Is the primary difference that changing the cap will change the center frequency, while adjusting the tone pot will not? 

I'm looking to get more low end out of my wah, and these two mods look like the first steps.

[rocket8810]

by moving the position of the wah pot will alter the sweep frequency range. what that will do is change frequencies that are being altered only when you move the treadle, not the overall amount of treble and bass. changing sweep cap will alter the overall frequencies that can possibly be affected by the treadle.

i would suggest a .068uF cap for the sweep cap if you're using a bass or want a lot of low end out of it. i think that's what you typically find a a bass wah.

if you want more low end you overall you can increase the input and output caps, and i would suggest lowering the resistor from Emitter to ground off Q1. the lower the resistor it should increase the gain and bass.

[Rob Strand]

It seems like they both can effect the amount of high and low end content.  Is the primary difference that changing the cap will change the center frequency, while adjusting the tone pot will not? 

It's a bit different because the shape of the frequency response (ie. EQ profile) isn't the same for the two cases.

[Viral Rock]

Thanks for the replies.  I'm just starting to get my head around the standard wah circuit.  Seems like manually adjusting the wah pot will give the impression of a more bassy wah because the resonant peak will move lower in frequencies (assuming you adjust the pot in that direction), even if the overall bass and treble passing through the circuit aren't altered.

I'm working with a Crybaby Classic, and am using it with guitar, but I like a lot of bass response from all my pedals.  I looked at the schematic Rick Viola traced, and am planning to try to add a cap in parallel with C7 (schematic here: http://www.diystompboxes.com/smfforum/index.php?topic=98281.0).  I know it's SMD, but haven't worked with SMD boards before.  Anyone have any luck modding the Crybaby Classic this way?

[Transmogrifox]

I think this was a pretty good article:
http://www.electrosmash.com/vox-v847-analysis

If you can take the time to wrap your mind around it then that will be well worth it. 

In short, limiting the pot causes you to focus more finely on a "sweet" range within what is available with this circuit.

Changing the feedback cap shifts that whole range up or down the spectrum, but keeps the same ratio from low to high sweep.

If you want more lows, then there are a lot of different ways to do it:
1)  If you like where the sweep range is set, but you just want more low-end content, then change the 1.5k resistor in the feedback (at first BJT input) to a larger value, and/or make the input cap value larger.
2)  If you want it to sweep deeper, but also able to go as high as it already goes, then change the Q1 emitter resistor to something smaller (GCB95 uses 390 ohms here, you could change to 300 ohms, or smaller).  The resonance keeps increasing on the low end as you increase this resistor so you might want to make the parallel 33k resistor smaller to compensate.
3) If you want it to sweep deeper, but don't need as much high-end "wacka wacka", then change the feedback cap to larger values to shift the whole range down.

I would save modifications to the pot sweep range until last.  This is something where I would make mods if I found myself always hovering over a certain range of motion on the pedal and never went full forward or full back.

I'm like you, I really like a wah to be more heel-heavy so I don't find the top 3/4 range of motion to be very useful most of the time.

[Viral Rock]

That Electrosmash page was super helpful.  But now I'm back to thinking both these changes will have a similar effect - moving the whole range of the sweep - even if the end result isn't identical. 

I was a bit skeptical of being able to do any mod due to SMD construction, but changing the sweep cap value was super easy.  There are unused holes on the board where you can easily solder a cap to be in parallel with the existing cap.  These are labeled JP-1 and JP-3 in the Rick Viola schematic for those that are interested.  I added a .01 cap and the difference was dramatic.  The toe down position was much less trebly and the sweep felt more comfortable for me because I wasn't trying to avoid the end of the wah travel.  I'll wait to try it at full rehearsal volume, but that may be all the mods I need for now.  One of the easiest mods I've done and totally worth it.

Thanks for all the suggestions.