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I’m an absolute novice at this but decided I’d like a holiday project. I’ve built up a Zonk Machine MK11 (I think) with 2 x silicon transistors but I have been getting some oscillation at full gain. I have tried a 81ohm resistor in line with the fuzz knob and I’ve also attached a 10nf cap between the C & B of Q1 as per suggestions I found online. The capacitor as worked the best but it still has some funny noises and some random gating.
Can anyone help?

I was just stoked to get a working pedal first time tbh but I’d like it to be usable as well.

Pics would help.

What qs did you use? The bias is determined by the hfe of the transistor and they're all over the place.

You can add the Miller cap across both transistors to cut down runaway high frequencies (like the big muff adds a 470pf to each) which should tame it but switching a milder transistor will help or looking at the 47k resistor which would ideally be a trimpot.

Is it only at full rotation that you get squeals? Could be a feature

NippleWrestler wrote: ↑Sat Sep 26, 2020 5:50 pm Pics would help.

What qs did you use? The bias is determined by the hfe of the transistor and they're all over the place.

You can add the Miller cap across both transistors to cut down runaway high frequencies (like the big muff adds a 470pf to each) which should tame it but switching a milder transistor will help or looking at the 47k resistor which would ideally be a trimpot.

I have 2 x BC147 and 2x BC148 to try, all parts in the build are just what’s available at Jaycar.

When you say Miller cap is that what I’ve done by adding a cap across the Q1? So are you saying trying it across both?

They're both pretty low gain.

And yes, one per each transistor. The big muff uses a 470pf. My fuzzboy uses 220pf. The bigger the value the more highs you cut (but anything above 4khz is pretty useless for guitar anyway).

I didn’t have any 220pf/470pf caps so I tried it with a couple of 4-10nf caps and an still getting the issue. Would the value make all the difference here or would the concept still work ok?
Here’s what I’m getting at full gain, any suggestions?

https://youtu.be/xFG18E_HwMc

I might be closing in on the issues, turns out the transistors under closer inspection are BC547 and BC548, I swapped out the 548 for 547 and the squealing went away! Yah! However the attack and string definition went away when the noise did too!
I preferred the sound of the other ones.
Might need to cycle through some transistors to find what sounds best. Any suggestions on silicon NPN transistors fit this circuit and where to buy them?

Any npn will fit. Jaycar will happily rip you off for a 3c part. Part of the fun of fuzz pedals is cycling through transistors. The bc548 is pretty generic, hfe around 300. I start with a 2n2222 and move around from there when it comes to transistors.

What value gain pot are you using? Should be a 2k. And throw up some photos of the actual board.

NippleWrestler wrote: ↑Sun Sep 27, 2020 2:25 pm Any npn will fit. Jaycar will happily rip you off for a 3c part. Part of the fun of fuzz pedals is cycling through transistors. The bc548 is pretty generic, hfe around 300. I start with a 2n2222 and move around from there when it comes to transistors.

What value gain pot are you using? Should be a 2k. And throw up some photos of the actual board.

I copied the diagram in original post which recommended 5k Lin for fuzz, if I put a 2.5k resistor over it would it reduce it 2.5k?

No it'd be a 5k.

I mention the 2k pot because it was mentioned in the comments.

If you like the sound of it and just want the gain control not to oscillate, then just add a resistor in series with the gain pot to effectively limit it's range - i.e. between the Q2 and the pot. You've tried 81ohm, try bigger: 470ohm then 1k.

NippleWrestler wrote: ↑Sun Sep 27, 2020 3:09 pm No it'd be a 5k.

2.5k and 5k in parallel becomes 1.6k

Well ideally you'd want a 3.3k to make the result 1.99k on the pot, but to test a theory it's easy to crocodile clip a resistor across lugs 1 and 3.

Did so many changes to the first board it was getting unreliable so I just built up a second one to experiment on.
It’s sounding good but the same oscillation.
I’ll try that resistor in series and see how it goes.
The pedal does seem sound best at full gain, most attack and definition. So hoping to solve the issue.
Didn’t have a trim pit so I’ve wired in a 100k pot in place of the 33k resistor, will that be ok? Or contributing to issues?

You need to show the track side, do a close up, just in case theres any bridging