squeal-proofing a Si Fuzz Face: resistor, cap, or both!?

[Snufkinoob]

A while ago I was tinkering with a Silicon Fuzz Face build and found that the two common methods for taming the squeal that may occur with the fuzz pot maxed out are:

1) 50-500pf caps between C and B of either transistor
2) Adding a 10-20ohm resistor before the 1k fuzz pot

I tried the resistor method, and it worked fine so I'd prefer to stick to that method for my current build, but is there still a possibility of it squealing in certain situations? Would best practice be to use both resistor and capacitor methods to be absolutely sure? Has anyone done one method and then found they had to add the other? Is one more reliable than the other?

Also, I'll be omitting the fuzz pot, and using a fixed 1k value. But if I'm sticking to the resistor method, but to avoid using the extra 10-20ohm, would a single 1k1 have the same effect, or that extra 100ohm in there be too much?

Ta!

[GibsonGM]

I would do the minimum that takes the squeal away.  And I would understand that any high-gain pedal will not be quiet when I'm not playing!  Other 'helpers' can be routing the wires correctly, and using shielded wire for input, and maybe also output, with the shield grounded only at one end.
A lot of squeal (oscillation) is caused by wires/parts coupling to each other...care in wire routing and shielded wire can make a massive difference!

I would not worry about a 'missing 100 ohm' or anything.   The thing is - with the gain pot maxxed, there is NO resistance there, and then you squeal.  You say you'll always have 1k there (don't need 1k1, just whatever works), so you are all set. 

If you don't like the 1k and change back to a pot, you'd want that minimum resistance back...

[thermionix]

Like this?



That's not the same as increasing the 1k.  Keep in mind that a 1k pot likely doesn't read exactly 1k.

What I did with my Ge FF, since I didn't want a Fuzz control either, was use a 470r fixed resitor in series with a 1k trimmer, both bypassed by the cap in parallel.  So I adjust the bias with Re instead of Rc, and still always have the fuzz maxed out.  That way I maintain the original voltage divider values  for the output, if that makes any sense.

[Snufkinoob]

Like this?



That's not the same as increasing the 1k. Keep in mind that a 1k pot likely doesn't read exactly 1k.

That's right. Not sure what I was thinking with the 'combined' 1k1 idea.

[Gus]

You do not need the added parts if you have a good layout(perf, vero or PCB) and good wiring

[GibsonGM]

You do not need the added parts if you have a good layout(perf, vero or PCB) and good wiring

^ This

[anotherjim]

The originals had layout advantages.
The big round case spreads things apart.
The Bulgin switch also has better separation than the switches we use now.

[Snufkinoob]

Interesting. I wouldn't have thought that just the wiring/spacing was the main issue in contributing to the problem. I've seen Si FF/FF derived circuits with neat, spacious wiring but still incorporating the cap/resistor.

I'm aiming for high gain (500-600) and maybe increasing the volume with a 1k at 9v so that might increase the changes of oscillation?

[amptramp]

It is possible to get feedback through the power supply / battery leads when the output impedance of the power supply or battery increases.  An electrolytic capacitor across the power leads may alleviate some of the symptoms.

[midwayfair]

Good power filtering and shielded wire.

Both of the fixes kinda bite ... the 10R reduces your max gain and the cap darkens an already dark sounding effect. In other words, the main reason the 10R worked in one pedal is that you simply can't turn the gain knob all the way up any more, which is probably where it was squealing. You're just masking the problem at the moment instead of solving it.

Without seeing your exact build, layout, and schematic (if there were any changes), it's hard to get more specific about the exact steps you could take.

[PRR]

> I wouldn't have thought that just the wiring/spacing was the main issue

When simple circuits squeal, wiring/spacing is the first suspect.