Multi-Option Dunlop GCB-95 Modification

This is a writeup on the extensive modifications I have made to my GCB-95 in order to provide a wide variety of sounds beyond the stock pedal.

While I heavily utilize IK Multimedia’s AmpliTube for my guitar rig there are times where I want to use outboard gear as well. After completing my Amplitube MIDI Controller I was literally left with a shell of a Dunlop Cry Baby GCB-95 pedal. I used the gear mechanism from the pedal to prototype the mechanics of my MIDI board then ended up stealing the bypass switch for a power button on a clean amp project.

After a few other projects this (aside from the circuit board and potentiometer not pictured) was all that was left:

When I started this project the intent was simply to get the OEM replacement parts and put the pedal back to stock shipping operations. As I looked at the replacement parts it started to become apparent that the existing circuit board could accept alterations such as replacement inductors and after some more digging online I found countless circuit board modifications to the GCB-95. Most all of the mods appeared to be one-way where the only way to ‘roll back’ to the original stock features was to replace all the parts. In addition, I didn’t see too many people trying to increase options for the inductors so I ultimately decided to combine many of the upgrades into one pedal. I had three primary goals for this project:

  •     I wanted the ability to switch between stock and mod settings
  •     All components needed to fit in the pedal casing
  •     The battery compartment could not be used for additional component storage

It took a few days but I was able to take on the project one phase at a time and ultimately get exactly what I was looking for. As always keep in mind that I am not a road-warrior musician so my projects have silly characteristics like switches sticking out of the side of a pedal that could easily be sheared right off by overzealous stage antics. Before I jump into the how-to I’ll go ahead and give you a look at the final product with a description of the features.

Left side of the pedal with a simple toggle to switch the voicing of the wah sweep. This basically toggles between the stock capacitor and a replacement for a more defined lower to midrange sweep.

Right side of the pedal with three toggles for midrange boost, gain boost, and vocal shaping. The three way switch near the heel provides access to the stock inductor, yellow vintage Fasel, and modern red Fasel inductors.

Two more notes before we dig in. The decision to include an inductor switch was made later on in the build after alot of the circuitry had already been reinstalled. You may want to do all the casing modifications first but if not, be aware you could end up with aluminum dust or shavings in the circuitry. Simply shop-vac or compressed air blast the casing clean in either case to avoid any shorting out of the circuitry. Additionally this mod does NOT send any of the signal to a buffering resistor on switching of the added features so it will most definitely ‘pop’ on switching as the capacitor discharges. You’ll want to throw the pedal into bypass or put down a volume/mute switch on the amp side of the pedal before toggling options… especially if you’re on a live and fully loaded amp.

If you just want to know the changes made, here is the baseline schematic:

Start by removing the wiring harness at top, 1/4 inch jack screws on the outside body, and mounting screw on the lower right of the board. Then remove the board and de-solder each of the following components from the stock circuit board.

L1 – The stock inductor. Save this component. IMPORTANT! … mark the component in some manner which helps you recall the position BEFORE you remove it. For instance you cannot see it here but I put mark on the side of the inductor near the area of the board that has L1 printed on it. This told me where the bottom right corner of the inductor should go. This will help you know how to correctly wire the inductor later.
R5 – Resistor controlling vocal shaping. Toss this.
R1 – Resistor controlling midrange. Toss this.
R9 – Resister controlling gain. Toss this.
C5 – Capacitor controlling the ‘sweep’ of the wah sound. Toss this.

This leaves us with a clean board:

Next we’ll work on the casing a little to hold 4 DPDT (Double Pole, Double Throw) miniature toggle switches.

The smaller the better when it comes to these switches because there isn’t alot of space in the casing.

The right side of the pedal is best for multiple switches given that when the potentiometer is placed in the housing there isn’t too much clearance on the left side. You’ll want to be ultra aware of the switch body size and proximity to the 1/4 jack mount flange (shown left) and rubber foot mount (shown right). Aesthetically your best bet is to mark the mounting position of those two holes first, then simply split the distance between them for the middle switch. The pedal base is cast aluminum which is very easy to drill. I recommend using a punch to set the center for drilling and starting with a smaller pilot hole to ensure the larger bit doesn’t travel when you start drilling the final mounting holes. For the sweep capacitor switch on the left side, simply match the position of the hole closest to the 1/4 inch jack to keep clear of the potentiometer base.

Outside shot of the housing showing the mounting holes running parallel with the top of the pedal base.

A quick fit-test to ensure the switches are not going to give us any issues.

Note the ample clearance between the circuit board and how the switch leads are just inside of the cut out area (rounded rectangle) for the gear assembly. Smaller switches that stay inside that area won’t interfere with the potentiometer.

Now remove each switch. Solder 2 leads about 6 inches long to the center terminals on each switch then getting the leads on the resistors and capacitors as close to the external terminals on the switch as possible, create the following switch combinations:

  • Switch One (Midrange): 2 six inch center leads with a 2.2kOhm (R-R-R-Gold) resistor across the terminals on one side of the switch and a 1.5kOhm (B-G-R-Gold) resistor across the opposite terminals of the switch.
  • Switch Two (Vocal Shaping): 2 six inch center leads with a 47kOhm (Y-V-O-Gold) resistor across the terminals on one side of the switch and a 33kOhm (O-O-O-Gold) resistor across the opposite terminals of the switch.
  • Switch Three (Gain): 2 six inch center leads with a 270Ohm (R-V-B-Gold) resistor across the terminals on one side of the switch and a 390Ohm (O-W-B-Gold) resistor across the opposite terminals of the switch.
  • Switch Four (Sweep): 2 six inch center leads with a 0.01uF capacitor across the terminals on one side of the switch and a 0.022uF capacitor across the opposite terminals of the switch.

Remember to mount the components as close as humanly possible to the terminals (note the image above) because you need to ensure they will not make contact with the mechanical components on the gear side of the potentiometer or the leads on the opposite side.

Now remount the toggle switches in the case and shape the leads to run as close to the casing walls as possible before falling between the 1/4 inch jacks and inductor space. I should point out here that I didn’t pay too much attention to the placement of the toggles with respect to the original resistor and capacitor values. It would probably be a good idea to put them all either towards the toe end or heel end of the pedal so when all switches point in the same direction you know you have reverted the pedal to ‘stock’ mode.

Cut another pair of 8 inch leads. Trim all toggle leads to as short a length as practical to keep wiring runs as short as you can and hook up the toggle leads and spare 8 inch leads as follows:

  • Switch One (Midrange): Solder leads to R1
  • Switch Two (Vocal Shaping): Solder leads to R5
  • Switch Three (Gain): Solder leads to R9
  • Switch Four (Sweep): Solder leads to C5
  • 8 Inch Leads: Referencing the image above solder the leads to the two holes on the left inside the inductor outline. Basically look to the right of R5 and connect to the two leads inside the larger circle but just to the left of the larger drilled hole in center of that circle. Leave the opposite ends loose for the time being.

Now remount the board to the casing with the single screw to the lower right.

Now it’s time to mount the inductors. Above you’ll see the stock black inductor and the aftermarket red and yellow Fasel inductors. It took me a little time to figure out how to jam these things in the pedal along with that massive switch but here is what I ultimately ended up doing.

The heel of the casing is practically the only place that all three inductors and an additional switch can be placed. After staring at this void for awhile I realized that just below that battery foam is a totally unused screw mount. This image was taken right after that discovery because there was also a much longer screw protruding from the nut just to the right of that mount point. Take a pair of pliers, grab that screw end and give it a couple of to and from tweaks. It will snap off right at the nut base.

Now find yourself some sort of non-conductive material and cut it to cover a majority of the void in the heel of the casing. In this case I used a semi thick sheet of plastic from some kind of binder. Those lightweight 1/2 inch vinyl binders with snap rings come to mind. The key is to just find something that a sharp wire or component lead will not easily puncture.

Next you are going to need a really short 6-32 screw… the same length as the one that holds the circuit board in place. If you can’t find one short enough just run a nut onto a 3/4 or longer 6-32 screw then using a hacksaw or bolt cutters, lop off the additional length. Running the nut back off of the screw will re-cut any damaged threading, making it easy to use the screw on the mounting hole.

Now find yourself a 1 3/4 inch square circuit board (set with copper solder points on the opposing side will help) and test fit the board using the mounting screw. This particular board came from Radio Shack and just happens to be a perfect fit all around… with the exception of that top notch which I will now explain.

As mentioned in the start of this post one of my requirements was to ensure I didn’t intrude on the existing pedal functionality… that included the battery compartment. The Dunlop Wah has this funky clip on the battery compartment that ends up taking up some real estate and just happens to run right along the line of that mounting screw. So using some nibblers and a file, I removed enough material to ensure the battery clip would still fit as expected.

You don’t need much clearance here… just enough to prevent the battery clip from pushing down the newly mounted circuit board.

And here is where things got tricky. You see the black inductor? It’s ever so slightly too tall to clear the bottom of the pedal… it sticks out if you look from the side so the bottom plate cannot be put on the pedal. If you look on the OEM circuit board between the 1/4 inch jacks you’ll see that there is a big hole drilled in the board.

The OEM inductor has a flange at the bottom. You’ll need to drill the circuit board to allow this flange to slide into the board. It will help get the clearance you need. Now for the next problem. The Fasel inductors mount vertically taking up even more room than the OEM inductor.

Using a pair of needle nose pliers gently (and I do mean GENTLY) bend the terminals right at the edge to move them into a 90 degree position. Do this for both the read and yellow Fasel inductors. You need to be super careful here because if you snap off the terminal you’re done and if you look on the opposite side of the plastic mount you’ll see just how thin and fragile the inductor wire really is.

If you did it right, the Fasel should now mount horizontally on the circuit board.

Temporarily Solder the inductors in the following configuration, keeping the OEM aligned the same as before. Remember I marked the lower right corner earlier which means I mounted this with the lower right corner mark towards the bottom right of the heel and picture in this photo. Now we move on to the switch. Notice from the previous picture how long the terminals are on the switch? Notice how close the lower mounting hole is to the rubber foot mount? This is going to be tight. Start by using wire cutters and trim off to the top half of the switch terminals. If you notice you’ll see additional mounting holes close to the body that we can use. We need that real estate or the battery will either not fit, or will shor the contacts on the switch.

Measure the height of the switch using a ruler or caliper.

Now pick a drill bit just slightly larger than that.

This step is a little tricky as you have to look at the inside and outside of the casing to get it just right but place the switch in the leftmost throw and set it on the body where if you were to drill through and place a nut on the backside, it would not interfere with that mounting post. The Dunlop is painted with powder coat so it’s easy to scratch. Scratch a mark at the outside edge of the switch then carefully move the slider to the far opposite throw point and make a mark on that outside edge as well.

I got a little carried away here but scratch a line parallel to the top of the pedal body then punch three holes at points where the drill bit will meet the left top, top, and top right edges of the mark from a dead center drill on the punch marks.

Drill pilot holes first with a 1/8 inch or smaller bit. This will help keep the larger bit from traveling too far on the final sizing run. After the pilot holes have been drilled, switch to your selected bit and open up the slot by drilling into each pilot hole.

You’ll need to use a smaller file at first to knock out the ‘points’ between the holes.

Once you have gained enough space, a larger file will fit and definitely speed up the job. Check your work often with both the caliper/ruler and actual switch to ensure there is clearance for the throw on all sides.

Once you are happy with the clearance place the switch on the outside of the body with the slider pointing in. Hold firmly while ensuring the slider moves effortlessly down the channel then make to registration marks in the mounting holes and drill for the mounting screws.

As you can see the final mounted switch with trimmed terminals and short screws/nuts will keep clear of the 9volt ‘zone’

Wire all inductors as indicated in this schematic:

The completed mod. Note the black wire ties used on the inductors just to help add stability and wire ties across all added wiring to keep things clear.

Everything works great so far. As mentioned earlier this mod does NOT send any of the signal to a buffering resistor on switching of the added features so it will most definitely ‘pop’ on switching as the capacitor discharges. You’ll want to throw the pedal into bypass or put down a volume/mute switch on the amp side of the of the pedal before toggling options… especially if you’re on a live and fully loaded amp.

In an upcoming post I’ll show the features of the mods with some sound samples, etc.

7 Comments

Leave a Comment

  1. Great Job! I am looking to do mods on my crybaby too.
    Instead of using switches for R5,R1,R9 can i use pots?
    As far the C5 cap i was thinking to use a 6pin rotary switch.5-6 caps soldered on the switch.

    Thanks.

    Like

    • Thanks… I don’t see any reason why the pots wouldn’t work. I’m not an expert on the boards but I would wonder a bit about reducing or increasing the voltages too much with the pot ranges. Switch wise… a switch is a switch.

      Like

  2. Thanks for the great report and over description. I have been looking into different mods for my cry baby. I have 3 of them and was looking into a different mod for each one, with this I can put all mods in one. That’s cool! You really layed out the instructions in an easy to understand way. I worked in avionics years ago so I am going to try to remember my electronics. Again thank you for your good work and for sharing.

    Like

    • Hey Tom… thanks. The only reason I went with the slide was to retain the stock option of using the 9 volt battery. It’s pretty tight in there but a switch or smaller rotary dial might work. Otherwise you could get rid of the battery and have plenty of real estate for whatever switch you wanted.

      Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s