This Week on the Breadboard: the Supro 1304 Fuzz (Repo Fuzz)

Chuck D. Bones

Circuit Wizard
The first time I breadboarded this, I was not impressed. Then I listened to Andy Martin demo it.
Nothing like Andy playing Black Sabbath to make a fuzz pedal sound good.

So I thought I'd give it another go. The Repro is a Fuzz Face with a Sziklai pair doing the job of the 2nd transistor. After that is a Fender tone stack and a buffer. The bottom-end is very heavy and that's because the low impedance of R1 and C1 allow the guitar to beat Q1 to death. I used a Blue Top MP38A for Q1. I noticed that the circuit is very temperature sensitive, no big surprise there because Q1's HFE, Vbe and leakage all strongly influence the bias point for Q1-Q3. I connected a DMM to Q3-E, put my finger on Q1 and watched the bias run away. First thing I did was swap Q1 & Q2, installed a 1.5K resistor between E & B on Q3 to absorb some of Q2's leakage and increased R1 & R4 about 7x. It was still pretty bottom-heavy, so I reduced C1 to 15nF. There is still plenty of bottom-end. C1 is a good place to install a FAT switch. Now it's sounding more to my liking. I also reduced R8 to 3.3K because I wanted to bias Q2 & Q3 a little hotter. I used a C100K pot with a 4.7K in series for the Bias trim. At the low-resistance end (full CW), the sound is very gated and sputtery. Down the dial a bit, there is a strong 2nd harmonic presence. Below that there is a sweet spot where the mid presence increases and clipping is very symmetric. This thing has plenty of gain, around 75dB at full tilt. BASS & TREBLE were very touchy at the bottom end of rotation, so I changed them to A250K. With a silicon transistor for Q1, the bias point is very stable. We still get plenty of germanium tone (whatever that is) from Q2. Q3 can be any silicon PNP. I tried 2N5087 and 2N3906, they sound the same. I ended up increasing C4 to 220pF to move the mid notch down a little; one could go even larger. I bumped R10 up to 6.8K because that's what Fender uses in the Deluxe Reverb tone stack. That 500K VOLUME control hurts my brain. What's the point of putting a buffer after the tone stack if you're going to make the output impedance so high with a 500K volume control? I'm running A100K presently, but anything from A10K on up would work. I increased C8 to 1uF. I wanted less loading on the tone stack, so I made Q4 a MOSFET (BS170) and increased R11 to 3.3M and R12 to 1M. A 2N7000, JFET or Darlington would also work, just adjust R11 to get 4.5V to 5V on the top of R13.

1628819661165.png
L to R: Volume, Bass, Treble, Bias, Gain. Output jack is between Treble & Bass, power jack is hiding under the Gain control. Lots of spare transistors scattered around at the bottom of the pic.

Repo breadboard 02.jpg
 
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Very cool!
Yeah, seems to me Silicon for Q1 and Ge for Q2 would make more sense. I like the concept and you'd think you'd see more stuff like this.

So don't shoot me, but when I was look into all this nonsense recently, I did come across a 'servo fuzz' fuzz face schematic to potentially deal with some of the temperature drift.

servofuzz.gif
 
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The Fuzz Face is already a DC servo. The problem with the execution is it uses Q1's properties as a reference. That servo circuit could work, but the parts count is a bit much. I used to work with people who designed circuits like that.

There is some amount of bias shift that occurs dynamically during the attack and decay of the notes. It's something akin to amplifier sag. The DC voltages on C1 and C3 will vary depending on how hard a note or chord is struck, which causes the bias to shift and the timbre to change. A bias servo can interfere with this process, depending on how fast it responds.
 
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Love the writeup chuck, I'd definitely be interested in trying out some of these mods, as this is one of my favorite pedals, and I've got 1 1304 left in my stash after having already built 2 of these.
 
Love the writeup chuck, I'd definitely be interested in trying out some of these mods, as this is one of my favorite pedals, and I've got 1 1304 left in my stash after having already built 2 of these.
Just about any NPN Germanium will work for Q2, doesn't have to be a 2N1304. You can save your 2N1304 for something else if you like.
 
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Sorry to revive a thread. I'm new to building pedals but really enjoying the learning process. I'm about to build a Repro Fuzz and I have some questions about your suggestions, Chuck:

First, about swapping Q1 and Q2 (thus Q1 is now the Si 5088 and Q2 the Ge 1304)... why do we do this? What is the benefit?


Also, if swapping Q1 and Q2, must we also--
(install) a 1.5K resistor between E & B on Q3 to absorb some of Q2's leakage and increased R1 & R4 about 7x.
I have exact parts for this build, so I would not be able to change R1 & R4 values or add the 1.5k resistor... or can we leave these values as listed in the schematic and JUST swap Q1 and Q2 with no other changes?


Further,
(the) low impedance of R1 and C1 allow the guitar to beat Q1 to death
If this is true, can we change R1 and C1 to different values to improve this "low" input impedance? Alternatively, I noticed you changed the values of R1 and R4 to "7x". Are these two ideas related? Does this mean R1 becomes 23k-ish and R4 becomes 700k-ish?

Thanks!
 
Hi, welcome to the Forums.

If you're new to pedal building, then I recommend building in accordance with the build docs. Your success rate will be much higher and if you do get into trouble, there are bunch of people here who will help. The Repro Fuzz is a perfectly good pedal in stock form.

You ask some good questions, and I believe I addressed most of them above. I should have included my as-built schematic, here it is.

Repo Fuzz v1.1.png

There is a build report here: https://forum.pedalpcb.com/threads/repo-fuzz.8850/

About my mods...
I share them freely, but they come with limited support. If you build one of my mods faithfully and have trouble, I'll help. If you deviate from one of my mods, then that's out of scope and you're on your own. I don't know your background, how much you know about electronics, skill level, etc. Some of the mods presented in these forums are very easy to implement. Other mods are quite complicated; they are intended for experienced builders.

My advice to new folks on these forums is to peruse the Build Reports to see what good quality builds look like and Troubleshooting to see what we do when shit goes sideways. There is a very good thread in General Questions on build techniques: https://forum.pedalpcb.com/threads/basic-workflow-tips-for-building-a-pedalpcb.1165/

Have Fun!
 
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