Zonk Machine mk1 reducing artifacts and noise?

[Reg18]

I've built a couple of these with OC81 transistors (around 70-100 hfe) and I've had a bit of trouble tuning them. Always get a lot of noise and artifacts in the sustaining notes.
I managed to reduce this by adding a 5nf pull down cap on the input and flipping Q2 around (swapping the emitter and collector pins) this in turn reduced the gain a bit so I altered the 470k to 240k and the 2.2k going to the gain pot to 10k which raised the gain up again. I didn't know this would work it was just trial and error.
Any other ways to reduce the noise and artifacts in a Zonk?

[iainpunk]

large capacitor between the FUZZ pot and ground might help? it looks odd that the fuzz control changes the bias as well...

cheers

[bmsiddall]

Howdy.  What collector voltages do you see?

Cheers,
Brett

[Keppy]

It looks like that circuit is a slightly modified Tonebender Mk1. Search for threads on that and you should find a lot of info.

[Reg18]

It looks like that circuit is a slightly modified Tonebender Mk1. Search for threads on that and you should find a lot of info.

Yeah the Zonk Machine by all accounts is a mk1 variant, they can put 1965 so just after the tone Bender which makes it highly like they cloned it. They do sound a bit different though and can be finicky to bias right.

[Reg18]

Still having trouble building this circuit, tried again today and get the same noises. I've now got a good selection of OC81, Russian GT402 and npn GT404 transistors to swap around.
I have managed to build a few mk1 Tone Benders recently but still can't get a good working Zonk.
After doing some more research I have discovered leaky transistors could be important to getting this build right, I see it is possible to add a resistor (say 500k-1m) across C-B to simulate leakage, will this work in this circuit?

[antonis]

IMHO, it will not..!!

By simulating B-C leakage this way, you intervene in bias configuration dramatically..
e.g. Q1 Base will sit at -4.5V and also alter Q1 stage gain due to NFB loop 1M resistor set..

But, as usually, you have nothing to loose by trying it..