Darkglass b3k clone

[TomVyborny]

Hi guys, I was trying to make my darkglass b3k clone based on the schematic that is included. I think that the schematic should be ok because it is from b3k diy kit so - called guma drive. It's acting strange, the good distorted sound comes only when the drive and volume pot is at 100% but with the drive pot at 100% there also comes some strange oscillations. It is fine when the blend pot is more to the clean sound but I guess that it's not how it should be working. Only thing I tried to swap was the J-Fet's but with no result. I wondered if some of you guys tried to built that to so if you have any tips what worked for you I would be really grateful. I've already spent few days by that but still have no idea what could help. Thank you.

[r080]

Welcome to the forum!

Can you clarify whether this link is the project you are doing?

https://schalltechnik04.de/en/instructions/guma-drive

Can you describe the oscillation? Does the frequency change with any of the other knobs? A high gain circuit oscillating could be caused by the wrong capacitor somewhere.

[TomVyborny]

The schema is from this page: http://peperspedals.blogspot.com/2017/03/b3k-clone-update.html
Today I found that the oscillations stops when I connect 220n capacitor to the volume pot to ground. But another problem that I can't figure out is when the grunt switch is in position where the 220p cap is connected to ground, the volume drops down. It's weird because I can still see that signal with scope.

[idy]

Kind of a hard schematic to read. Doesn't show pots, doesn't show (PCBguitar mania schematic does) that unused CMOS inputs should be grounded.

Often a cap to ground will kill oscillation. Almost always, somewhere, but it also tends to kill signal. Make sure unused CMOS investor inputs are grounded (check the PCB) and then try cap to ground at progressively "earlier" points in the circuit.

[idy]

And welcome. Save yourself headaches, find a real schematic. "Guma Drive" doc apparently don't include one. Bad on them.

[marcelomd]

PedalPCB has a kit with a verified schematic here: https://www.pedalpcb.com/product/obsidius/

[anotherjim]

^ You really must tie those unused inverter input pins. Also...
The circuit can have a lot of gain and it's non-inverting so you can get oscillation due to positive feedback if the input and output wiring is too close. Look at the wiring diagram on page4 of the PedalPCB build doc. That's the best way to run the wires and connect the 3PDT footswitch.

[TomVyborny]

Thank you all for your help. For some reason, the oscillations went off. Only problem that I'm now trying to figure out is why the signal goes off when the grunt switch is in the position where the 220p is grounded and in the position where it is connected back to the R14. I only get the sound when is pull the strings really hard. In the middle position everything is alright.