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Axis Wah

A Great Sounding Wah...On A Budget
Over the years, the wah pedal has become one of the most popular and influential of all guitar effects. Many famous guitar players have used this piece of equipment in their songs...who can forget the opening of "VooDoo Chile (Slight Return)" or the awesome wah pedal parts of "Machine Gun" both by Jimi Hendrix. All of these famous guitarists have created a huge demand for the wah pedal...but unfortuntely all is not well in the world of mass produced wah pedals. The sound of these pedals tends to be...not so great. But fear not, ye tone seekers, for there is a remedy to your wah pedal ailments...and it won't break your bank account, either!

A couple of years ago I exposed the guts of the famous Vox Clyde McCoy wah pedal for all to see, and even built a clone of the wonderful pedal, but the problem is that it required the purchase of a replica Halo inductor and a replica of the ICAR-taper wah pot, which can end up being quite pricey. As a result, I've been working on modifying a Vox V847, that I just received a few days ago, into an excellent sounding pedal...minus the new inductor and wah pot! That's right! For this project we're using the original Vox inductor and the original Vox wah pot! You can even use the original wah pedal circuit board for this project too...but I etched a new one just so I would have a fresh start, and a fresh look. I'll include the PCB and Layout files for the replacement board if you decide to take that route.

There are really only two major changes being made to the pedal...the addition of true bypass switching and the addition of an output buffer circuit board, which allows the wah pedal to work in series before a fuzz pedal. The other changes are all quick and simple resistor and transistor changes, which can be made quickly and easily by using a desoldering braid.

True Bypass Switching
Okay, so the first thing we need to do is to install the DPDT switch for true bypass. If you have a Vox V847 wah, then follow this diagram, and if you have a new Dunlop Crybaby, then you will need to follow the directions on this page since the operation for the Crybaby is a little more complicated with the PCB-mounted jacks. For the Crybaby, I would recommend that you follow the second set of intructions..."Eliminating Input Buffer." The input buffer will no longer be needed with true bypass switching. Both of those great true bypass conversions are located on Stuart Castledine's website, so be sure and check it out! Make a note that if you wire the bypass switches as shown in those great diagrams, then you may omit the 1M resistor in the input of the circuit that's shown in the schematic that's shown down the page a little.

Adding an Output Buffer
The second operation that we'll perform is adding the output buffer circuit to the wah pedal. This step is optional, but I would certainly add it if you plan to use a fuzz pedal (like a Fuzz Face) in series after the wah pedal. A common problem is that the wah pedal simply won't wah when put in series before a fuzz pedal. Unfortunately, this is the way it sounds the best to most people. This problem can be solved by adding an output buffer to the Axis Wah, which won't alter the tone of the pedal. This is a simple JFET buffer that's based on the Wah Wah project at Tonepad, and can be added to any wah pedal that doesn't have an output buffer, namely the Vox V847s and the Dunlop Crybabys. The input impedance of this buffer is set by the 1M resistor from the Gate of the JFET to ground. Since this buffer can give a slight vme boost, so a 100K trimpot is on the output of the buffer to act as a volume pot so you can keep the volume of the wah pedal at the same level as when it's off. The PCB and Layout for the output buffer are in the "Project Files" section at the bottom of this page.


Resistor Changeups
  • First, if you're using the true bypass wiring instructions on Stuart's website, then you can just forget about the 1M input pulldown resistor, but if you're wiring the switch like this, then you'll need to solder the 1M resistor to the solder side of the circuit board, from the input end of the input resistor to ground. This resistor will prevent a loud "pop" when the pedal is switched on and off.
  • Next we'll replace the 68K input resistor with a 47K resistor. This will give a little volume boost to go along with the true bypass switching.
  • To enhance the mids of the pedal, and to smooth out the bass to treble transition, we'll replace the 1K5 resistor on the base of Q1 with a 2K2 resistor.
  • The next resistor to change is the 470 (or 390 or 510) resistor on the emitter of Q1. Using a lower value will increase the gain of the pedal, and it will also enhance bass response, which is sometimes a problem with some pickup configurations. For this change I chose a 330 resistor, which isn't too big and it isn't too small. Using one that's too small could result in distortion, which we don't want.
  • The final resistor change that we'll make to the circuit is the "Q" resistor...the 33K resistor that parallels the inductor. To help give the wah pedal a more vocal quality, I've chosen a 47K resistor, which isn't quite as drastic as the 68K that some people use. It's very nice sounding and certainly an improvement over the original 33K.
Changing the Transistors
The transistors that come in the Vox V847 and Dunlop Crybaby pedals are the high gain, general purpose MPSA18. I personally think that these transistors are way too high gain to make a good sounding wah pedal. The toe-down position is quite ear-piercing and unpleasent. To help make a more mellow wah pedal, I've chosen to use a pair of nice BC109C transistors that I ordered from Steve Daniels of Small Bear Electronics a while back. For Q1 I chose a unit with a gain of 400, and for Q2 I chose one with a gain of 388. The end result was a very nice, a bit more mellow sounding wah that's not as ear-piercing when toe-down, and the heel-down is still nice and fat...the bass response actually made the springs in my Twin Amp's tube shields rattle!

Below is a schematic of the Axis Wah with all modifications mentioned above.


Axis Wah Schematic

Lastly, here are some pictures of the Axis Wah. Click the thumbnails for the full-sized image! Use your browser's "Back" button to return to this page.

Axis Wah

Axis Wah

Axis Wah

Axis Wah

Project Files
Note: These files are NOT to be used in a pedal that you are building for profit. The PCB file is copyrighted artwork and is subject to a licensing fee.

Axis Wah PCB
Here
Axis Wah Layout
Here
Axis Wah Parts List
Here
Build Difficulty: Moderate
Moderate

Output Buffer PCB
Here
Output Buffer Layout
Here
Outut Buffer Parts List
Here
Build Difficulty: Easy
Easy

The PCB and Layout
I've included a PCB and Layout for the wah pedal that's the same size as the original Vox V847, but like I said earlier...you don't need to make a new PCB. All the modifications can be performed on the original board. I just made this new board mainly so I could make sure that it was the right size and fit since I've been meaning to make a full-size Vox wah board for a while. This layout will accept the original Vox/Dunlop inductors with 0.3" lead spacing and it will also accept Stuart Castledine's Halo replicas, with 0.4" lead spacing. However, you will need to make a small PCB for the output buffer. I mounted mine at the "toe" of the pedal, like in my Clyde McCoy clone. There are several other locations inside the pedal that you could mount it though.

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Site last updated on May 20, 2007.

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