Why my overdrive is becoming a tremolo at high gain?

[jackwithoneye]

Hi,

I've built a Jan-ray, timmy-like overdive, but i have a problem when pushing the gain up. It's starts to act like a tremolo, the volume is pumping up and down 2 times/second.
My vref is getting this oscillation, pumping from 3 to 4,5v giving this weird effect.
I resolder every solder joint, without success.

schematic i based my circuit on :
http://4.bp.blogspot.com/-VdnteeLbY6E/UVrWxgpLOYI/AAAAAAAAB1I/Hsk2E4zMULE/s1600/JanRay-2.png

Any ideas?
I dont know if it can be related but, i have read stuffs about running seperate grounding for power and audio grounds, but I have a lack of understanding how to do that when designing pcb.

Thanks a lot for your time

[amz-fx]

Sounds like it might be self-oscillating. This would probably be related to the layout that was used. In/Out should be separated by some space on the layout, for example.

You could also try a 100n in parallel with C12 just be sure that you have a good low impedance AC path to ground for the Vref.

If you have a ground plane an the pcb, it should be good enough at audio frequencies

Best regards, Jack

[Chillums]

Did you notice that the schematic is missing the net label for the vref in the power supply?  If you forgot that net label like on the schematic that may be it.  Although I doubt it would work at all if that was the case.  Also with the resistor values in the schematic you should be getting around 3.7v on vref, not 4.5v.  That's if your powering it with 9v. 

[jackwithoneye]

Thanks Jack,

In/out traces are far, except where the pads where it connects to the jacks (during testings i dont connect any 3pdt bypass), the pas are side by side:



i've tried the 100n cap in paralelle with c12, it doesn't help with the level oscillation/pumping

The oscillation almost go away when i play, and progressively comes back to a "full depth tremolo" as long as the guitar level sustain diminish (plucking a chord, audio pump coming back after 2 seconds, and raise to full trem utnil the end of the sustain)

"If you have a ground plane an the pcb, it should be good enough at audio frequencies"
what do you mean? sorry i dont get it (english isn't my native language). I have a ground plane, on one side of the whole pcb.


-------------
Thanks Chillum
you're right, about voltages and Vref spot, the schematic i posted is what i used to draw, but i modified a bit, with VRef where you put it, and grouding C5 and C4, instead of Vref, considering it'd help to keep a cleaner Vref, and wont change anything as the capacitor don't let DC pass through.

these minor mods i made :

[Vivek]

Try increasing R6 and R7 to 47k each

[antonis]

My vref is getting this oscillation, pumping from 3 to 4,5v giving this weird effect.

Is your power supply also getting this oscillation..??

[jackwithoneye]

My vref is getting this oscillation, pumping from 3 to 4,5v giving this weird effect.

Is your power supply also getting this oscillation..??

The power supply doesnt doesn't get the oscillation.

Try increasing R6 and R7 to 47k each

Mate, you nailed it. Thank you so much, Big up to you Vivek! After changing these two 3k3 to 47K, no more volume oscillation.
Could you tell me how did you come to this solution?

Again, big thanks to all of you for spending some time on your day to give me a hand with my problem!
I love this community :heart:

[amz-fx]

Try increasing R6 and R7 to 47k each

Good one! 

Best regards, Jack