DOD FX69 Grunge Pedal Modifications

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WhiteWarlock
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DOD FX69 Grunge Pedal Modifications

Post by WhiteWarlock »

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DOD GRUNGE MOD

REMOVE C19. YOU DONxe2x80x99T NEED TO REPLACE IT WITH ANOTHER COMPONENT OR ADD A JUMPER.
THIS ADDS MIDS AND GIVES A FULLER TONE.

CHANGE R16 TO A 1K RESISTOR. THIS LETS THE PEDAL CLEAN UP WHEN THE GAIN IS TURNED DOWN.

CHANGE R19 TO A 22K RESISTOR.
THIS MAKES THE DISTORTION A BIT RICHER WHEN CRANKED.

NOW CHANGE C24 TO A .0047CAPACITOR. THIS MAKES THE xe2x80x98HIGHxe2x80x99 KNOB AS MORE LIKE A HIGH KNOB SHOULD.

THIS CAP IS POLYSTYRENE, WHICH IS THE xe2x80x98HOLY GRAILxe2x80x99 OF FILM CAPACITORS. EXTRA CARE SHOULD BE TAKEN WHEN SOLDERING AS THESE ARE MORE HEAT SENSITIVE THAN OTHERS.

CHANGE D2 TO THE SMALL YELLOW LED.

DOD GRUNGE MOD

REMOVE C19. YOU DONxe2x80x99T NEED TO REPLACE IT WITH ANOTHER COMPONENT OR ADD A JUMPER.
THIS ADDS MIDS AND GIVES A FULLER TONE.
CHANGE R16 TO A 1K RESISTOR. THIS LETS THE PEDAL CLEAN UP WHEN THE GAIN IS TURNED DOWN.
CHANGE R19 TO A 22K RESISTOR.
THIS MAKES THE DISTORTION A BIT RICHER WHEN CRANKED.
NOW CHANGE C24 TO A .0047CAPACITOR. THIS MAKES THE xe2x80x98HIGHxe2x80x99 KNOB AS MORE LIKE A HIGH KNOB SHOULD.
THIS CAP IS POLYSTYRENE, WHICH IS THE xe2x80x98HOLY GRAILxe2x80x99 OF FILM CAPACITORS. EXTRA CARE SHOULD BE TAKEN WHEN SOLDERING AS THESE ARE MORE HEAT SENSITIVE THAN OTHERS.
CHANGE D2 TO THE SMALL YELLOW LED.
THIS GIVES A MORE COMPLEX, ENHANCED DISTORTION.

Here are some details (and suppositions) about the purpose of the mods you did :

Removing C19 (the only 15nF cap in the schematic) takes out of the circuit one of the transistor-based gyrators. Probably a boost at a fixed frequency that turned out to be harsh or muddy.
R16 is the 10k resistor next to the gain pot, it sets a minimum gain. Lowering its value allows the control to reach milder overdrives, or even crunch tones.
R19 seems to be the series resistor after the clipping diodes. Reducing its value lets more current through.
The clipping diodes are the heart of the distortion. A pair of silicon diodes clip at about +/- 0.7V. The suggestion to replace one of them with a LED is to get asymmetrical clipping, and a moderate boost in volume. This is due to a higher threshold of the LED, above 1V and depending on color.

But what you did was more radical : by removing the diodes completely, you let the whole amplitude of the preamplified signal to attack the tone-shaping stages, and drive them into non-linear regions. No more a textbook distortion, but a heck of an overdrive. Now the volume pot deserves its name.
Adding back a low-threshold germanium diode (about 0.3V) clips the signal heavily again, but still asymmetrically : you get a lot of even-order harmonics. A nice addition would be a on-on-on switch to select various combinations of clipping diodes (or absence thereof).
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