# Quarantine Fuzz -> Viral Mutation



## Chuck D. Bones (Jun 2, 2020)

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.


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## Robert (Jun 2, 2020)

You _know_ I'm building one of these.....


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## Chuck D. Bones (Jun 2, 2020)

A brief note on transistor selection and biasing.  Use whatever transistors you like.  Because the gate threshold on JFETs & MOSFETs vary considerably from part-to-part, plan on checking and adjusting their biases during the build, even if you use the same part numbers I did.  The only restriction on Q2 is it's Vp has to be less than D3's Vf.  Ge transistor HFE and leakage are highly variable, so plan on trying different ones and/or tweaking the bias on Q3.  R11, R12 & R14 stabilize Q3's bias, so once you dial it in, it will stay put.  Q3 and Q4 are DC-coupled, therefore Q3's bias affects Q4's bias.  Set Q3 first.  Because this circuit does not have the usual limiting resistors that you find in the BMP, the drive to each stage is asymmetrical.  Each stage can drive the next stage into cutoff more easily than it can drive it into saturation.  For that reason, I biased Q3 and Q4 on the hot side to improve headroom.

The other configuration I liked was JFET for Q1 and Si BJT for Q2.  The advantage is you can put this pedal anywhere in the chain; it doesn't mind being driven by buffered-bypass pedals.  The disadvantage is that it's not nearly as sensitive to the guitar's volume control when you put it first in the chain.


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## Bobbyd67 (Jun 2, 2020)

Sometimes I get the tin foil hat out and start making wild theory's about Chuck... Starting to think he's a undercover owner/designer of a pedal company who just never gave up the diy community xD


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## Chuck D. Bones (Jun 2, 2020)

Indeed.  Sometimes I think I missed my calling.  Spent the last 30-odd years designing electronics that go on satellites.


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## Bobbyd67 (Jun 2, 2020)

Chuck D. Bones said:


> Indeed.  Sometimes I think I missed my calling.  Spent the last 30-odd years designing electronics that go on satellites.


Nice cover story Chuck ^^
As for me I wish I knew more about electronics... T_T I tried to pick a few books for dummy's but with the job, kids , and addiction to building pedals, ain't much time or energy left :/! Having known earlier how fun these electronic thingamajig are I would have maybe tried to study in that field !


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## Chuck D. Bones (Jun 3, 2020)

Most of my electronics knowledge that I apply to modding and designing pedals I learned on my own, studying the work of others and experimenting.  I definitely apply some of the stuff I learned in college and at work, but the foundation came from doing electronics as a hobby. I'm still learning.


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## HamishR (Jun 3, 2020)

There are times when I read what Chuck writes and think I can actually follow some of it.  But that's only because he uses words similar to those we use. You see English words so your mind thinks "I know these words!" and it's true - I know just about all of the words Chuck uses. It's just how he uses them...

I read and read, thinking I know what he is saying. Then realising that he is on another level. Still, I think he is using his powers for good. Pretty sure.


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## gri (Jun 6, 2020)

i think i just blew my mind reading this. im populating all the flat components right now. i have my share of muffs, hoofs. etc. but thinking i might socket and play with this one. as its the quarantine pcb. wouldnt mind a sick sputtery or velcro odd sound. want to try different Q's. i have some NOS germanium in post from russia also..


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## Chuck D. Bones (Jun 6, 2020)

This circuit doesn't do velcro, sputter or gating.  You _might _be able to get there by severely skewing the bias on Q2 or Q3.


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## cooder (Jun 6, 2020)

HamishR said:


> There are times when I read what Chuck writes and think I can actually follow some of it.  But that's only because he uses words similar to those we use. You see English words so your mind thinks "I know these words!" and it's true - I know just about all of the words Chuck uses. It's just how he uses them...
> 
> I read and read, thinking I know what he is saying. Then realising that he is on another level. Still, I think he is using his powers for good. Pretty sure.


This.

Thanks for sharing your insights and knowledge Sir Chuck and taking the time to write it up for us mortals vegetating in the dungeons of stompbox building.

Question: Could you post a modified schematic for this option to make sure I get that right?
_"The other configuration I liked was JFET for Q1 and Si BJT for Q2.  The advantage is you can put this pedal anywhere in the chain; it doesn't mind being driven by buffered-bypass pedals. "_
That might be the version I'd like to try as I would have it not first in chain. Will a J201 as jfet of 2N5457 be suitable?

Also:
_"Ge transistor HFE and leakage are highly variable, so plan on trying different ones and/or tweaking the bias on Q3."_
What bias voltage would I be looking for recommended for GE trannie?


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## Chuck D. Bones (Jun 6, 2020)

First the bias question. Refer to the schematic ABOVE.  Collector (or drain) voltages, measured just now on my breadboard:
Q1 = 4.83V
Q2 = 5.34V
Q3 = 4.73V
Q4 = 5.93V
Get anywhere within ±0.5V of these numbers and it will sound good.  Biasing outside this range might also sound good, I haven't tried it.

Here is the alternate schematic with a JFET 1st stage.  I did not spend a lot of time listening to this version.  It works, but some tweaking might be in order. You are on your own with this one.  As with most mods, all component values are negotiable. The value of R5 depends on the JFET you use.  You will have to determine it experimentally.  I'd shoot for a drain voltage between 5V and 6V.  Cx determines how much bass gets thru. Bigger for more bass, small for less bass. There are no pads on the board for Cx, you have to piggy-back it on R5. I highly recommend breadboarding this before soldering anything.


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## cooder (Jun 6, 2020)

Awesome Chuck, will report back when I get a chance to mess with this!


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