Biasing a Marsh Fuzz?

[Mo1978]

Hi all, first post here. I stumbled across PedalPCB a few weeks ago and was very pleased to see that musikding here in germany has a good selection of PCBs including all the parts. So I ordered a marsh fuzz, because I saw mike hermans demo the BoG a few weeks ago on youtube and very much loved the sound.

It is not my first pedal build but I did my last soldering a few years ago, so I needed a little time to get used to it again. But everything seemed fine, I measured at a few points with my DMM and found a mistake with on of the capacitors in the power cleaning stage so I removed that, because I have no spare part with 220 uF here. But that should not be a problem if my electronic knowledge is not off by a mile.

So I put everything together and turned all knobs to noon (including the internal trimpot) and fired my amp up. Yes! Great sound, as I remembered, but a lot cleaner than I thought. So I turned up the (external) bias pot to around 2 o'clock. Sound completely gone. I thought that's why there is an internal trim pot, so I turned that up and down (even to full and to off) but no sound at all, just cracking. I could not get the pedal to sound normal (whats normal on a fuzz?) again.

Next step was to measure things, i.e. voltages across the board. Vcc was fine on all points and collector voltage at Q1 and Q4 around 6V.

But Q3 was at 1.3V and Q2 was at 2V and both were not changing when I adjusted the internal trim pot. So I replaced the trim pot (as I was thinking that it was broken) with a 1K resistor, because that is the resistance on the collector of Q1 and Q4. I still got those values of around 1 to 2V at Q2 and Q3.
I removed all transistors (I soldered them directly to the PCB) and instead used 4 of those pin-sockets. Inserted 4 new transistors and still, 1-2V on Q2 and Q3.

What am I doing wrong? Or is there nothing wrong and I should look for the fault elsewhere?

Kind regards,

mo

 

[Chuck D. Bones]

Check the voltage at each end of R102, one at a time. Should be close to +9V. +1.5V to +2V on Q2's collector is about right. Q2 & Q3 form the basic Fuzz Face circuit. It is normal for the first transistor to operate near saturation. The voltage on Q3's collector should definitely vary when turning the bias pots. Pin 3 of the FUZZ pot should be ~ 0.7V. Verify R13, R16 & R17 are the correct values. And inspect all of your solder joints. When I have trouble with a build, it's usually a bad or missing solder joint.

 

[Mo1978]

First of all, many thanks, it is working now properly.

I don't really know why... I measured 1.5V on PIN3 of the Fuzz Pot, so that was too much. Then I resoldered a few joints that looked not so fine (most of them...) and inserted a spare internal bias pot. I checked the resistor values and checked for continuity on most of the parts. Then I measured voltages: 0.65V on the FUZZ-3 and changing voltages on Q2 and Q3 when I adjusted the bias pots. Ready to try it out and it worked.
So I think I gave too much voltage to Q2 with my 1K-resistor replacement of the internal trim pot. If I understand transistor saturation correctly then Fuzz is when there is just enough voltage to set the transistor to a state in between saturation and active?! Is there a good source to read about fuzz circuits?

Thanks again,

mo

 

[Chuck D. Bones]

Electrosmash has a pretty good Fuzz Face analysis. They analyse quite a few other pedals too. Here's a quick overview:
Transistor-based distortion circuits like Fuzz Face, Tone Bender, Pink Purple Fuzz, Boss Tone, Harmonic Percolator, etc. exploit the non-linear behavior of transistors when they go into saturation, cutoff, or both. Saturation occurs when the collector-emitter voltage gets below the base-emitter voltage. The collector current has maxed out. Saturation clipping is abrupt and generates plenty of harmonics. Cutoff occurs when the collector current approaches zero. When the collector current gets close to zero, the transistor's gain drops. This makes cutoff clipping more gradual and the harmonics generated are not as harsh as with saturation. The gain drop also makes a natural compression. What usually happens in these pedals is at low signal amplitude (guitar volume dialed down and/or soft picking), the transistors don't saturate or cutoff. The signal is more-or-less clean. As the signal increases, one of the transistors will go into cutoff or saturation and we get asymmetrical clipping. Driven even harder, the transistors will both go non-linear; saturating, going into cutoff, or both. The clipping gets more symmetrical. As the symmetry changes, the harmonic content and the tone change. We can play with the bias and get a gating action where one of the transistors is cutoff when no signal is present. The pedal is very quiet when you're not playing and the note decay is sputtery.