Chuck D. Bones
Circuit Wizard
I thought I'd share what I did when I built my version of the Quarantine Fuzz. I had two goals in mind:
1) Make the cool board art as visible as possible.
2) See how much I could tweak the circuit without breaking it.
The first goal was purely a mechanical mod. I decided that the box needed a clear lid. When Music6000 pointed me to the clear Hammond boxes at Mouser, I went that way. I decided to populate the art side of the board, which is actually the solder side, and face that side up. This presented two challenges: The pots will be backwards and the board assembly is mirrored w.r.t. the assembly diagram in the Build Docs. I had to carefully check and recheck the component placement before soldering because assembling a board backwards significantly increases the likelihood of an assembly error. The fact that I introduced electrical mods made things even harder. To get around the backwards pots, I went with solder-lug pots and cross-wired them to the board. This provided the added benefit of allowing me to position the pots so they don't obscure the board. The pot placement is still reversed: SUSTAIN is on the left and VOLUME is on the right, but I can live with that. The switch wiring is mirrored so that the wires can come straight off of the board. As you can see from the photos in the build report, I crossed the wires leading from the switch to the IN and OUT jacks so that they are placed normally. I also had to shield those wires because the absence of a metal box caused unacceptable hum, even in bypass mode. The shields are grounded at both ends. The bosses for the cover screws are moved inboard compared to the metal 125B boxes, presumably to avoid cracking at the corners of the cover. This reduces the space available for top-mounted jacks. I might have been able to squeeze in one of those skinny power jacks, but I opted to go for a side-mounted "outie" instead.
I had one simple self-imposed rule for the electrical mods: keep it clean. That meant no cuts, no long jumpers, no hay-wired parts, no additional controls, no vero boards. The stock Quarantine Fuzz is the exact same circuit as the Bugg Raincoat. It is similar to a Big Muff, minus the clipping diodes, limiting resistors and mid-scoop tone control. It is a great platform for modifications. Mind you, the Raincoat is an awesome fuzz in its own right. Sounds nothing like a Muff. Without the clipping diodes, all of the distortion comes from the transistors being driven into cutoff, saturation, or both. First, I breadboarded the stock circuit so I could get a feel for how it sounded, how the controls behaved, interaction with the guitar, probe it with a scope and have a baseline for comparison as I made mods. Some of my favorite dirt pedals employ a variety of transistor types on one board: Fuchsia, Screw Driver, Ungula... So I thought I'd try four types here: silicon BJT, germanium BJT, JFET & MOSFET. I tried a few different transistor orders and landed on what you see in the schematic below. I kept the 1st stage pretty much stock, reducing C1 and C2 slightly. Because there is no series limiting resistor between the input and Q1, this pedal interacts strongly with the guitar's volume & tone controls if it's first in the chain, and may or may not play well with any pedals placed in front of it. The bottom-end of the SUSTAIN control was not usable, so I increased R6 at the last minute (which is why the board has flux residue around R6). 2nd stage is a JFET; it's low-noise, high-gain and doesn't load the SUSTAIN control or the 1st stage. The red LED (D3) in the source lead is a trick I learned from some fancy hi-fi tubes amps. It creates enough voltage drop to bias the JFET, but has a low impedance. Kinda works like a resistor with a fat cap in parallel. It flickers slightly when playing and I used a super-bright to magnify the effect. I adjusted R7 to get the desired bias in Q2. The 3rd stage was converted to germanium, which required changing the bias resistors. The transistor I used has an HFE around 180. I think more is better here. I adjusted R11 to set Q3's bias. Last stage is a MOSFET. Because the MOSFET's gate threshold is higher than a BJT, I was able to DC couple it to the 3rd stage. Instead of connecting to Vcc, R15 goes in place of C8. R15/R16 sets the MOSFET's bias. The tone control network is interesting because it heavily loads the last stage, increasing distortion. I was perplexed at first, but it turns out that this tone control works very well and is useful over its entire range. It provides treble cut at the bottom end, treble boost at the top end and is "flat-ish" near noon. I fiddled every part in the tone control network to see what it did and if I could make any improvements. It works damned well in stock trim and the best I could manage was to increase C9 a little bit to fatten up the sound slightly.
I'm very pleased with the result. It's tone is very amp-like and responsive to fingers and guitar controls. Chords come thru amazingly clear. Even with the SUSTAIN cranked, it's not thick like a Muff. End-to-end gain is just over 60dB with SUSTAIN dimed and TONE at noon. Internally, the gain maxes out at 80dB, but the tone network eats almost 20dB. Numbers aside, there is plenty of volume and sustain on tap.
1) Make the cool board art as visible as possible.
2) See how much I could tweak the circuit without breaking it.
The first goal was purely a mechanical mod. I decided that the box needed a clear lid. When Music6000 pointed me to the clear Hammond boxes at Mouser, I went that way. I decided to populate the art side of the board, which is actually the solder side, and face that side up. This presented two challenges: The pots will be backwards and the board assembly is mirrored w.r.t. the assembly diagram in the Build Docs. I had to carefully check and recheck the component placement before soldering because assembling a board backwards significantly increases the likelihood of an assembly error. The fact that I introduced electrical mods made things even harder. To get around the backwards pots, I went with solder-lug pots and cross-wired them to the board. This provided the added benefit of allowing me to position the pots so they don't obscure the board. The pot placement is still reversed: SUSTAIN is on the left and VOLUME is on the right, but I can live with that. The switch wiring is mirrored so that the wires can come straight off of the board. As you can see from the photos in the build report, I crossed the wires leading from the switch to the IN and OUT jacks so that they are placed normally. I also had to shield those wires because the absence of a metal box caused unacceptable hum, even in bypass mode. The shields are grounded at both ends. The bosses for the cover screws are moved inboard compared to the metal 125B boxes, presumably to avoid cracking at the corners of the cover. This reduces the space available for top-mounted jacks. I might have been able to squeeze in one of those skinny power jacks, but I opted to go for a side-mounted "outie" instead.
I had one simple self-imposed rule for the electrical mods: keep it clean. That meant no cuts, no long jumpers, no hay-wired parts, no additional controls, no vero boards. The stock Quarantine Fuzz is the exact same circuit as the Bugg Raincoat. It is similar to a Big Muff, minus the clipping diodes, limiting resistors and mid-scoop tone control. It is a great platform for modifications. Mind you, the Raincoat is an awesome fuzz in its own right. Sounds nothing like a Muff. Without the clipping diodes, all of the distortion comes from the transistors being driven into cutoff, saturation, or both. First, I breadboarded the stock circuit so I could get a feel for how it sounded, how the controls behaved, interaction with the guitar, probe it with a scope and have a baseline for comparison as I made mods. Some of my favorite dirt pedals employ a variety of transistor types on one board: Fuchsia, Screw Driver, Ungula... So I thought I'd try four types here: silicon BJT, germanium BJT, JFET & MOSFET. I tried a few different transistor orders and landed on what you see in the schematic below. I kept the 1st stage pretty much stock, reducing C1 and C2 slightly. Because there is no series limiting resistor between the input and Q1, this pedal interacts strongly with the guitar's volume & tone controls if it's first in the chain, and may or may not play well with any pedals placed in front of it. The bottom-end of the SUSTAIN control was not usable, so I increased R6 at the last minute (which is why the board has flux residue around R6). 2nd stage is a JFET; it's low-noise, high-gain and doesn't load the SUSTAIN control or the 1st stage. The red LED (D3) in the source lead is a trick I learned from some fancy hi-fi tubes amps. It creates enough voltage drop to bias the JFET, but has a low impedance. Kinda works like a resistor with a fat cap in parallel. It flickers slightly when playing and I used a super-bright to magnify the effect. I adjusted R7 to get the desired bias in Q2. The 3rd stage was converted to germanium, which required changing the bias resistors. The transistor I used has an HFE around 180. I think more is better here. I adjusted R11 to set Q3's bias. Last stage is a MOSFET. Because the MOSFET's gate threshold is higher than a BJT, I was able to DC couple it to the 3rd stage. Instead of connecting to Vcc, R15 goes in place of C8. R15/R16 sets the MOSFET's bias. The tone control network is interesting because it heavily loads the last stage, increasing distortion. I was perplexed at first, but it turns out that this tone control works very well and is useful over its entire range. It provides treble cut at the bottom end, treble boost at the top end and is "flat-ish" near noon. I fiddled every part in the tone control network to see what it did and if I could make any improvements. It works damned well in stock trim and the best I could manage was to increase C9 a little bit to fatten up the sound slightly.
I'm very pleased with the result. It's tone is very amp-like and responsive to fingers and guitar controls. Chords come thru amazingly clear. Even with the SUSTAIN cranked, it's not thick like a Muff. End-to-end gain is just over 60dB with SUSTAIN dimed and TONE at noon. Internally, the gain maxes out at 80dB, but the tone network eats almost 20dB. Numbers aside, there is plenty of volume and sustain on tap.
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