For a number of years, I was busy modifying wah pedals for friends and guitarists, mainly in and around Sweden. Lately, though, it has started becoming a bit of a chore, so in an effort to lighten the load I’ve decided to share that mod with the public. Most of the info is already shared on the other wah mod pages, of course, but here are the exact values I use for my wah mods.

The pedal

I start out with a regular Dunlop GCB-95 Crybaby, but can also work with the JH-1 Jimi Hendrix wah (both the older ones and the new ”signature” model), Zakk Wylde wah or GCB-100 (the old bass wah with a mechanical switch). However, with those you will need to change sweep capacitor. Basically, any Dunlop wah with the GCB-95 circuit board (Revision F or higher) will work.

The modifications

The first part of the mod need not be repeated, as it is the basic true bypass mod as described in another post on this site. I always remove the buffer stage, so do that to yours as well 🙂

Then there’s the circuit mods. I make three main changes (four if the pedal is one of the other ones mentioned earlier). The component locations are marked in the picture (click to enlarge it).

  • Replace the 390ohm ”gain resistor” with 330ohms.
  • Replace the 1.5K ”Mid/Q” resistor for a 1.7 or 1.8K.
  • Change the 33K resistor in parallel with the inductor for a 68K (the so-called ”vocal mod”). This only makes a slight difference, but the new value is correct for a vintage Vox, so I usually do this mod even so.
  • If the ”sweep cap” isn’t the stock 0.01uF (103) value – for instance, a JH-1 wah has a 0.022uF (223) here instead – replace it.

Now, I prefer to use trimpots for the three resistor changes, as they let me fine-tune the pedal’s response to a particular rig. But I always set them to the above mentioned values, as a starting point. And I find that I rarely touch them… So chances are you’ll be just fine with fixed value resistors. If you want to use trimpots, here are the ones to get. All trimpots are the 10mm horizontal type, and the values etc are:

  • Gain/bass: 470ohm. Clip the CCW leg (leaving only the center and right legs in place), so the trimpot value decreases as you turn it clockwise.
  • Mid/Q: 4.7Kohm. Here, we clip the CW leg instead, making the value increase as you turn it up.

Note that a transistor needs to be stood up to make room for the gain/bass trimpot, and that the mid/Q trimpot has to be installed facing the other way to the others (again to make it fit the component holes). Also, in the pic to the right, the 68K input resistor has been replaced with a trimpot (this time a 100K). It is set up the same way as the gain/bass trimpot, so it too decreases in value as you turn it up. But I find that I rarely change that one out of its 68K starting position, so it’s not terribly important to fit one.

Finally, I also adjust the pedal’s mechanical sweep range (as described here). From the stock position (assuming it hasn’t been changed since it left the factory) I usually end up backing the pot up one notch. I always test it through an amp first, though, to see if I actually need to do this. And I also test after this change, to see that I haven’t gone too far 🙂

Parts list

For the true bypass mod, you need:

  • A quality DPDT switch (Carling 316PP recommended)
  • Hookup wire

Then the components:

  • 1/4w (or 0.6w) resistors (carbon film or metal film) in the following values: 330ohms, 1.7 – 1.8K, 68K.
  • A 0.01uF (103) film capacitor. Voltage is irrelevant, but make sure the capacitor legs will fit the space between the holes in the circuit board (regular 5mm spacing works). Note: this part is only needed if you have a JH-1 or other model with a lower range. It is not needed for a regular GCB-95.

If you want to use trimpots instead of fixed resistors, you need:

  • 10mm horizontal-type trimpots, in the following values: 470ohms, 4.7K (100K for the input gain trimpot, if you want to fit one).

Vox V847-A and V845
Adding an LED to the GCB-95 Crybaby