“Rab Fuzz” Project

Here’s something a little different from the other projects documented so far – a project designed for you to complete.  This thread will be devoted to building a “Rab Fuzz” or Kay Fuzztone clone from an old wah pedal enclosure.   The Rab Fuzz is used by  The Edge for the song “Elevation.”  You hear it mostly during the intro riff to that song.  It’s so named (designated as RABFUZ on Edge’s pedalboard) because as the story goes, one of U2’s technicians, “electronic wizard” Colm ‘Rab’ McCallister, took an old Kay Fuzztone pedal and rehoused the circuit in a sturdier enclosure for The Edge.  (Original Kay Fuzztones are made of rather flimsy orange plastic.)  The Kay Fuzztone was introduced to The Edge by Daniel Lanois during the recording sessions of All That You Can’t Leave Behind.  “Elevation” was the song U2 wrote around a riff played by The Edge using the effect.

This circuit replaces some hard-to-find components (transistors and diodes) with easier-to-find modern replacements, but the “Elevation” sound is all in there…

First, here’s a partial schematic of the circuit board (not yet showing complete wiring of the jacks and switch, etc.):

 

 

Changes to the original circuit include using 2N5088 transistors for Q1 and Q2 instead of 2SC644S, using 2N3904 transistors for Q3 and Q4 instead of 2SC828R, using 1N914 diodes for D1 and D2 instead of OA90, and changing the value of the potentiometer to 100k instead of 50k.

Edit:  I would like to add as and aside, that after testing several different diodes in this circuit, I personally think Germanium diodes are the way to go rather than the (Silicon) 1N914.  The 1N914s sounds good, but you get a bit of a volume boost since they clip at a higher threshold than the Ge diodes.  (This may or may not be to your liking.)