modding a Champion

[JC@]

Where would it be the best place to cut the signal path to add the simplest effects loop in a Champion 600?
Before R14 or before R17?
Thank you.

[GibsonLover]

For typical Boss FX, I'd put it before R14, right after the Volume pot.
The bubble in the schematic identifies the level as 180mVAC feeding the volume with 25mVAC in. That's a voltage gain of 7.2. You might like a small divider before the FX loop, but it's a level that the FX input pot should handle.
The second one is WAY too strong for a stomp pedal. (7VAC before R17). I hadn't seen the schematic before today, but it's odd to see a Fender tone stack with no pots. I feel some experimentation coming on...

[JC@]

I did it before R14 but the amp oscillates like mad (red plating and all)
Low frequency I guess: lots of distortion and very little sound power in the speaker.

[JC@]

Still not working.
Now it oscillates in a high pitch squeal in all volume settings but zero.
What should I change?
Thanks

[tubeswell]

Does it oscillate etc when a FX unit is plugged in or not?

I could be wrong about this, but shouldn't you have a blocking cap between where you've tapped off your return circuitry and the grid? (to stop grid current and/or DC from the FX unit upsetting the grid bias of that following stage?). Also do you need (something like a 1M) Grid bias reference there (because you are now dealing with a return signal from outside the amps own circuit and you need to watch your impedance bridging?) JM2CW (These are just my off-the-cuff hunches)

[JC@]

Does it oscillate etc when a FX unit is plugged in or not?

I could be wrong about this, but shouldn't you have a blocking cap between where you've tapped off your return circuitry and the grid? (to stop grid current and/or DC from the FX unit upsetting the grid bias of that following stage?). Also do you need (something like a 1M) Grid bias reference there (because you are now dealing with a return signal from outside the amps own circuit and you need to watch your impedance bridging?) JM2CW (These are just my off-the-cuff hunches)

The amp squeals even with no FX plugged in. I've used shielded cable all over the circuit.

[Robert M. Martinelli]

Hi,
I could be wrong, but I'd try to lift R14 on one side to see if the oscillation stops.

Cheers

Bob

[JC@]

well...mmmmm... I had taken the "ground" for the effects loop from a tap of the speakerīs female jack. But from the wrong one, that is the tip.