muff tone control after fuzz face problem

[kismet78]

I'm having oscillation at the high freq end of the tone pot when adding a muff/biscuit tone control after a fuzz face circuit. The fuzz face part of the circuit doesn't have an output cap. Do I need to add a coupling cap between the two? Or am I looking at a different problem with these two playing well together?

[kismet78]

Here's the relevant part of the schem:



Both stages are switchable via 3pdt toggles and you'll see that there is an output cap after the switches so it works with whichever stage is switched in. The input cap on the fuzz face stage also serves as coupling cap for when the other preceding stages (not shown) are switched in.

[kismet78]

Is this one of those "duh" questions?

[kismet78]

bump

[ubersam]

...Do I need to add a coupling cap between the two?...

That would be my first inclination. If you haven't already, just try it and see if it cures your ills.

[Dragonfly]

Cap it.

[kismet78]

Cap it.

Is that going to change the response of the following rc filters?

[Skreddy]

Cap it.

Is that going to change the response of the following rc filters?

No; it'll just roll off low frequencies if it's on the small side.  You need a cap, either on the output of your ff stage or at the input of your tone stack stage.  Otherwise, you have a dc leakage (will throw off the bias of your ff) through the 33k resistor, the 100k pot, and finally through the 22k resistor to ground.

Basically it's best to have all of your output stages their own output caps.

[kismet78]

Thanks everybody. That's exactly what I needed to know.

[petemoore]

  i would also move the left output cap to inside the switch, for true bypass.
  It looked to me like no matter how the switches were set, the inputs and outputs all had the necessary DC blocking.

[kismet78]

  i would also move the left output cap to inside the switch, for true bypass.
  It looked to me like no matter how the switches were set, the inputs and outputs all had the necessary DC blocking.

The footswitch is after the last output cap, so the cap is completely switched out when bypassed. Each of the pedal's 4 stages are true bypassed individually from the circuit with dpdt toggles and then there's the separate output cap (so that no matter what combination of stages are stacked it's always there). All of that lives within the greater true bypass umbrella of the footswitch. Wow, that was hard to explain. And it's a headache to wire up.