Fuzz Face oscillation problem (and introduction)

[Johnny Guitar]

Hello,

I have recently built a GGG etched board with Small Bear germaniums (and the resistor changes Steve recommends).

I have it on my test bench with aligator clips while I check the component values and operation. (BTW, I'm not happy with the 500K audio taper output level control -- all the gain happens during the final 20% of rotation -- I want to change this for sure). It definitely *sounds* like a Fuzz Face, BUT it does exhibit an oscillation problem: when the "fuzz level" is turned up approaching maximum an underlying oscillation becomes noticble (it sounds like it's about the same freq as the low "E" string. This oscillation can also surface becoming a scream as the guitar volume and tone controls are turned *DOWN* (??!!!) -- counterintuitive!

It could be merely a product of using aligator clips, or it may be a common problem with certain clones, or just a building error on my part. I hope there is someone here who can help me. I can tell there are a great number of people here who know way more than I do about electronics and stompboxes.

About me: I have a fair bit of experience building kits (maybe 50 to 100 -- mostly analog synth kits) from MOTM, Blacet, PAiA and the like. I've had a good bit of luck building PCBs and rounding up the parts. I've had mixed luck building things on perf boards.

I'm not very knowledgeable electronics or troubleshooting (though I have had a fair bit of luck repairing and modifying my synths and stompboxes).

This looks like a great forum!

Thanks for any help!
John

[Chris R]

I assume you built the negative ground version.  I had the same trouble untill i switched mine to positive ground.

C

[Johnny Guitar]

DAMN! :wink:

You're right Chris!

I *REALLY* wanted to use it with negative ground for what I wanted to do with it. I hope someone can offer a solution.

Thanks a lot for the update though.
J

[theaxeman]

Hi!

My ff is built from the same parts and schem as yours and mine dosnt oscilate, but it does make the schreeching noise when the volume pot on the guitar is turned right down. I think it's just luck when it dosnt oscilate.

[petemoore]

asuming it's biased.
I have no troubles with negative ground PNP Fuzz Faces.[ ]>
A smal cap from input to ground may help< another good place for one is q two"s collector to base>another way is>>turn the fuzz control down a hair< or preset it for lower gain< s

[petemoore]

edit to above post: set for lower mazximum gain at highest setting<

[R.G.]

(repeated here for emphasis...)

I tell you three times: you cannot rely on a negative ground version of a nominally positive ground circuit to not oscillate.

One may possibly get away with it, and some people do, but it's not good engineering practice to count on getting lucky when the assembly is done.

Of course, the lucky ones will swear that they have never had any problem, and then extrapolate to the idea that no one should ever have a problem.

[Johnny Guitar]

OK R G (and again Chris). I get the message :wink: .

I'm surprised, given the number of shematics of the negative ground version of the circuit that one sees, this problem isn't noted alongside of those circuits.

I'm now thinking that I may one day build this into my "uber rack" thingy which would almost certainly have a bipolar supply. Then, at least, I could run it off the -9V side and the "positive ground" would at least be the same ground that everything else saw.

Thanks again!
J