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Effects loop

5/26/1998 7:43 AM
Effects loop
I have built a Marshall 50w amp/master volume preamp based on the schematics on this site. I have also taken the effects loop part of the circuit on Duncan's amp pages and almost (but not quite) got a very satisfactory rig.  
The problem is this: I have the TL072 buffered output between the 2nd and 3rd triode stages. With a jack in the return socket, the effects pedal output is fed straight onto the gate of the next stage (no shunt resistors or caps) and the loop is an in-line circuit.  
The output to the effects works fine - I can listen to my pedal on headphones and it sounds really good. However, when I complete the loop mback to the amp, I get a good soud for a couple of minutes, then it starts breaking up and falls to silence. It comes back only after I remove and reinsert the effects return plug.  
Can anyone tell what is going on and how to fix it?  
- Paz

moocow I took at look at Duncan's schemati... -- 5/27/1998 6:28 PM
Steve A. Paz:    ... -- 5/29/1998 8:44 PM