ESR Graphic Fuzz Power Supply

[Rob Strand]

That makes perfect sense. The volume pot therefore shouldn't be affecting the sound in the original circuit, other than what I've experienced with the original volume pot wiring right behind the opamp. Don't know why this is missing at the schematic I posted at the beginning.

When the pcb pics were put up a number of people had a stab at tracing it.   There seems to be three schematics on the web but only the jerms schematic is correct.

Now I connected the 100n input cap to a 470 ohm resistor and then to gnd.     

Yep, you got it.  Just like the output circuit on the output.

Oscillation is gone!     It stays unhearable until about 100R, then comes in gradually (I'm not stacked with every value). There might be a small effect on the frequency response, but really nothing I could define exactly. I didn't change the load, but with the 10k in the real original circuit it should always be 10k minimum.
Great work and a big thanks for your effort on this one.

That's good news.   Finally something that makes sense since adding that resistor is the right thing to do from a stability perspective.

I look at that circuit now and think how did they get to production without finding oscillation problems.  Also your breadboard didn't oscillate.  The circuit must be hanging on by a thread.

While we're at it. When my guitars volume is down completely, there's a lot going on for an undefinable time. Crackles and that kind of stuff. Not super loud, but you can hear it. That's on both, the breadboard and the boxed one.

The way the circuit is at the moment the input terminals are DC coupled to the input socket, that can cause some crackles.   So the way to fix that is to add a cap between current circuit input (the point where pin 2, the cap and the pot meet) and the input socket.  As to the value you might need a low value like 1uF or 2uF, listen for any bass cut.   It would also be wise to add say 100ohm to 220 ohm in series with that cap as well (to fend off obscure oscillation issues involving the input cable.)

[Andrekp]

I actually also own an original of this.  Bought in a store when they were current.  As I was in high school at the time, it is most likely that it was selected because it was cheap as hell (as compared to, say, a Big Muff), so it's likely it was a pedal primarily "designed" to get out the door quickly.  I couldn't have afforded anything decent at the time, so I wouldn't expect much more.

I made a version off the commonly accepted schematic a few years back (for reasons I'll get to momentarily).  It also behaved differently whenever it was in the mood.  I had to keep pulling it out of the box to change slightly to keep it from screaming bloody murder whenever it felt like it.  My recollection is that I redid the power supply to be modern-friendly and then ran a small cap across that feedback loop to keep the screams down.  I didn't want to tame it any further because I figured that was just part of its "charm."  Works great for that very non-muffy extreme sort of ragged fuzz that is mostly uncontrollable.  That can be a good thing at times, but I think if you try to tame it too far, you are just making a new pedal, not "fixing" the Graphic Fuzz.

The reason I made my own version is because I tried my original one day and it simply didn't want to make a sound.  I could find nothing actually wrong.  It just made no sound.  (using my 18watt clone).  At some point I plugged it into a modern SS amp and it worked.    (I think that was the sequence) after some further investigation, I found that there was significant DC on the output and for some reason that amp didn't like that.  Probably biasing it badly in some way.  The SS amp seemed to filter that out.  Input cap or something.  My re-made version did not have that problem.  (I'm sort of surprised nobody in various modern threads about this pedal has ever encountered that issue.).

Anyway, I am just posting this here for the record, FWIW, and to voice the opinion that as maddening as this pedal can be, I think that's where we should leave it.  If you want a BETTER pedal using a 741 to make fuzz, just make one.  Some things should just be left in their wild state.

[PRR]

> significant DC on the output and for some reason that amp didn't like that.

Most tube amp input stages are normally biased AT ground, and assuming no DC from the source, they can omit input caps. Many transistor amp inputs are biased off-ground, thus "must" have an input cap, which incidentally protects from most external DC abuse also. (Many exceptions in both camps.)

[Rob Strand]

Thanks for the background.

I made a version off the commonly accepted schematic a few years back (for reasons I'll get to momentarily).  It also behaved differently whenever it was in the mood.  I had to keep pulling it out of the box to change slightly to keep it from screaming bloody murder whenever it felt like it.  My recollection is that I redid the power supply to be modern-friendly and then ran a small cap across that feedback loop to keep the screams down.

I'm quite confident the jerms circuit I posted in reply #35 is correct.   However, I'm also quite confident the design has some inherent stability issues.   

I didn't want to tame it any further because I figured that was just part of its "charm."  Works great for that very non-muffy extreme sort of ragged fuzz that is mostly uncontrollable.  That can be a good thing at times, but I think if you try to tame it too far, you are just making a new pedal, not "fixing" the Graphic Fuzz.

Very much the case.   It's easy to fix it by changing it to something conventional.  The trick is to fix it without changing the character.   Adding the resistor in series with the cap is the correct fix from technical perspective and it shouldn't significantly change the character of the pedal.

This thread was a tricky thread.

[Goodrat]

I updated that schematic btw. I forgot the 10K on the output.

[Goodrat]

When I worked there, they did oscillate a lot. Tony (the other tech in customer repair) made a slight change to the circuit. I can't remember what that was. Maybe increasing the two 0.1 caps in the power supply to 1uf (although I doubt they would increase cost with a cap change) or increase the two 10K that split the power supply, I just don't know for sure.
If I could remember his last name (Italian) maybe I'll find him. Wow, 43 years ago.
I had one, gutted out the box and made a keyboard sustain pedal with it or something. Stuff long gone.

[Rob Strand]

When I worked there, they did oscillate a lot.

Interesting to know.