Vox V847 Wah mods
Stuart Castledine's Guitars, Amps, and Effects site is, in my opinion, the definitive source for mods to the Vox V847 wah-wah. This page is intended as a companion to his detailed instructions and explanations. Please note that mods to a wah can be very subjective; this page is merely documentation of my preferred component values. These modifications were tested with my so-called pretty guitar, so the values that you like best may not correspond with mine. That's fine, this is just a starting point.
I don't consider the Fulltone replacement pot (available from Small Bear Electronics) as part of my wah mods since it was just a replacement for the stock pot, which had become scratchy. I didn't audition different manufacturers' pots, I just chose the Fulltone since it was about the same price as the OEM replacement.
Also, the positioning of the rack and pinion assembly is difficult to quantify. You'll need to find the best position by trial and error.
|"Q" resistor A
stock value: 33k
|"Q" resistor B and Mids
stock value: 1k5
|Bass and Gain
stock value: 510 ohms
stock value: 0.01µF
Below are short descriptions of each modified component's role in the effect.
For more information, read RG Keen's excellent Technology of Wah Pedals
|This resistor sets the sharpness of the peak, larger values can provide increased vocality. I've found that the oft-suggested 68k is too sharp for me.||This sets the midrange content. With the stock 1k5, the heel-down position sounded muffled and weak. 1k8 wasn't quite enough, 2k2 was too much, and 2k (two 1k in series) was just right. Rock on, Goldilocks!||Converting to true bypass (which I've done) can make the effected signal seem less than unity gain. The 390 ohm resistor adds a bit of gain and bass, which also helped out the heel-down sound.||The 0.0133µF cap is a 0.0033µF in parallel with the original sweep cap on a small daughter board as seen below. The slightly larger cap moves the sweep range down a bit.|
Single in-line pin sockets (also available from Small Bear) were used in place of the stock components, which allow inserting fixed resistors or leads of a pot wired as a variable resistor. They can be tricky to solder, but a bit of electrical tape keeps them in place. Be careful when using the soldering iron on the PCB traces, they can peel off if abused. The sweep cap socket is a partial TO-5 transistor socket. I decided to make a daughter board to accomodate parallel caps for non-standard values.
Listen to it!
A common complaint is the poor interaction of a wah-wah driving a Fuzz Face. The below buffer/impedance matcher stage (very similar to the gain stage of the modified EA Tremolo, also designed by Gez Paton), can be added to your wah to solve this well-documented problem.