Fuzz Factory clone: short oscillation at zero drive

[zemauno]

Hi guys,
Epic respect for you all!
These is my first post, i'm a noob and i need tour help.
I've built two of these Fuzz Factory:
https://3.bp.blogspot.com/-4Px8WQV5YD4/UQ0I73d9oxI/AAAAAAAAEuw/Z9plaUkyW-c/s1600/ZVex+Fuzz+Factory+-+compact.png

They works fine but both have the same strange problem.
I get a short and strange oscillation with these setting:
Gate=0
Comp=0
Drive=0
Stab=0 (yes, stab is reverse).
Whit these setting i should have a soft fuzz, but when i hit hard low notes, i get a short of "filtering like" oscillation. These short oscillation disappear if i turn on stab a little or if i turn up drive. Please, help.

Thank you very much!
zemauno

[BetterOffShred]

Q2 and Q3 are germanium... What are the hfe and leakage of the devices you used?

[zemauno]

Q1 near 80hfe and Q2 near 120hfe.
I also try with other Ac128 and 2n3909 (silicon) with the same result.
I really font understand why i get these sort of static filtered oscillation only when i hit hard the low notes.

[GibsonGM]

A guess on the oscillation:  the circuit is near oscillation when at rest.  Your low note energy is just enough to push it over the edge for a moment, then it returns to a form of stability.  It may in fact BE oscillating at some super-audible frequency already and you just can't hear it.  An O-scope might show this. 

You may be able to dress the leads enough to make the osc. stop, or use some carefully-placed capacitance to swamp it...it is worth trying anyway.

[zemauno]

Thanky you very much, interesting answer.
The problem i get i really dont think is a defect, is more like a "possibility "...a 1 more sound. Of course the original doesnt have this behaviour. So, how can i dress the leads? (The pedal is already in box). Maybe for the future next one, you suggest me different leads (now i'm swing monocore)?
Thank you very much guys!
zmn

[GibsonGM]

It may or may not help, but you could use shielded leads for the input wire, and output if it passes anywhere near the Q's.  You only ground one end of the shield.

Routing wiring well away from gain stages can help, and using short leads on components.  The art of reducing noise, coupling, oscillation - is a field of study all on its own!     There should be posts you can find here all about the topic.    I'm not entirely sure this is the cause of your problem, but it's one possibility.   And of course, if there is coupling between components on the board itself, this won't do much.   Though proper wire dress can definitely reduce noise, so there is no negative to it!

Another possible answer is some form of blocking distortion, but that could be a reach...!

[duck_arse]

is this one of those circuits that runs ge transistors upside-down with negative ground? because we like to see photos, can we see photos of what you have built, pleeze? and because no-one else has yet said welcome, welcome to the forum, zemauno.

[idy]

The fuzz factory is one of those circuits that runs PNP transistors "upside down" with negative ground, with an LPB attached to the front and pots replacing many of the fixed resistors. Oscillation is considered a feature. It is inherently and intentionally unstable. Extreme settings on several pots bring this out.

[zemauno]

Thank you all for the reply and for the welcome!
Yes, is negative G with upside down transistors.
I know is unstable but at the setting i said, thereare nooscillation...imo is not a big problem, just curious ti learn something new, as Mike explain.
Here some pics...be good with me, is only my third work!

https://i25.photobucket.com/albums/c55/zemauno/Diy/20191124_172427.jpg

https://i25.photobucket.com/albums/c55/zemauno/Diy/20191124_172317.jpg

https://i25.photobucket.com/albums/c55/zemauno/Diy/20191124_1722580.jpg

Thank you again!
zmn

[duck_arse]

see, now, this is why we always ask for both photos and the circuit diagram built to. on your board, you seem to be missing a 220k resistor below the 5k1 and right of the 47k. is this correct? is this correct to the circuit?

[idy]

Missing resistor is after the tone pot on the original, absence would make tone control pretty weak, and who knows what else. We would like to see the solder side of the board.... though one can't always tell if a solder joint is good just by looking....

and this is exactly the kind of high gain circuit where lots of lose wires wold lead to undesirable interaction.

[willienillie]

There's no Tone control on a Fuzz Factory.

[zemauno]

What a shame!!!
Thank you for the correction! I didnt realize the missing R becouse this pedal sound exactly like the other one with all the resistor...now is fixed but the short oscillation remain in both...no bridge in the solder side, i always use a cutter to be' shure...some people say me can be also a socket problem...is better to sold transistor directly to vero...i really dont know becouse both pedals sound great but both have this strange behavior. Maybe i try to do some audio samples...iyo,  can a 47uf, positive to 9v, negative ti round solve this problem? Is the last possibility i found...if not i'll try to do a veroboard with pot soldered directly and i use multicolore cable..

[idy]

Sorry, no tone control on original, some variants put one there. The 220k is just a pull down. Would not affect operation much, slight reduction in output and reduced popping on bypass.

A lot of times solder joints may look good but be loose inside.

Still most likely problem is all those wires to the pots. A veroboard layout of this with pots soldered to board, tricky... Sabrotone has a layout like that, la Revolution Deux did one too. These look pretty neat.

[zemauno]

SOLVED: without 100R i dont get the short oscillation...guys, i dont know why but i take a bridge instead of 100R and now it work properly!
Any suggestion?
By the way, thank you very much for your reply guys!
Respect,
zmn

[iainpunk]

Thanky you very much, interesting answer.
The problem i get i really dont think is a defect, is more like a "possibility "...a 1 more sound. Of course the original doesnt have this behaviour. So, how can i dress the leads? (The pedal is already in box). Maybe for the future next one, you suggest me different leads (now i'm swing monocore)?
Thank you very much guys!
zmn

the original ZVEX one i have tried (and didn´t buy because the price was just crazy) did that as well, a short oscillation when strumming hard chords, its just in the nature of the schematic.