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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
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5/27/1998 6:28 PM
I took at look at Duncan's schematic, and it looks like the return goes directly to the grid of V102b, pin 7. What seems odd to me is that there doesn't seem to be a grid resistor on this triode. Unless I have the wrong schematic, all I see going to pin 7 is C123 and C301.  
The grid should be tied to ground through some kind of resistance, otherwise a charge will build up on the grid, causing the triode to go into saturation. This would explain why it takes a few minutes for the signal to break up.  
Try plugging your guitar directly into the effects return. The pickups should provide a path to ground for the grid, and the amp should work fine. If this is the case, add a 1M resistor from pin 7 to ground.
5/29/1998 8:44 PM
Steve A.

    The loop you used was borrowed from Duncan's Blues-112 schematic? And you used a TL072 ic for the buffered output with a +12v/-12v power supply? So how did you get the +/-12v- a separate transformer or by tapping off the existing PT? (I was wondering if there was a simpler design that would run off a single +~12v supply...)  
    Yeah, moocow's right on about the schematic missing a grid load resistor (probably 1 meg).  
Steve Ahola
6/1/1998 7:06 AM

Hi Steve,  
I would have been using a single transformer with multiple secondaries, but for the fact that I couldn't find a transformer kit big enough at the time. As it is I have two transformers fulfilling the role of the Trace Elliots used in Duncan's Blues 112 - i.e. HT, 24V and 6.3V. Bit bulky and heavy, but it's all tucked into a nice compact (11" x 13" x 9") combo.  
- Paul

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