Alternative Modification For Dual Rectifier Loop

Posted: July 31, 2015 in Effects, Mesa Boogie Dual Rectifier, Modification, Tube Amps
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I e-mailed Mesa Engineering awhile back about the loop mod for the 3 Channel DR parallel fx loop to make it serial.  I received the same schematic that I now know everyone else gets.  I can see the appeal for someone who doesn’t want their signal to be “tainted” by a pair of jfets, but they aren’t amplifying, only opening to allow the signal through.  It’s no different than having any other quality part in the path.

More recently, I saw that a company is advertising their bench fee for a different mod and the loop retains some of the functionality afterward.  I thought to myself, what would do that?  One thing that came to mind while off on one tangent is the idea to pinch the dry channel off. Then the light bulb flashed and blew up over the idea I’m writing.  This is not only easier than adding a part or bypassing the jfets, it is entirely reversible.  Here’s how to mod the fx loop and gain a killswitch:

FX loop alternative

Bridging the pot with one wire is effective.

I’ll be doing this ASAP.

In case anyone is worried about the footswitch and doesn’t need a way to kill all the sound: when the assignment control is set to “On”, the footswitch function doesn’t work.  The only assignment that doesn’t really make sense to use after this mod is for an individual channel.  If a person only wants an effect for one channel, but not another, a Voodoo Lab Control Switcher is relatively inexpensive and works with any midi capable effects unit.  Alternatively many of the stompbox looping and switching pedals have features for simultaneous amp switching and midi control.

I’ll be following up after I do the mod.  Thanks for reading and let me know what you think.


Pictures of the job.

I got it completed and it was very easy.  IT IS TOTALLY AWESOME!!!!!  I love it.  The sound of the amp is totally improved (and wasn’t really “bad” to begin).  It’s much more full, punchy, and feels better.  I wish I’d done this a long time ago.  If you’re considering a mod to your loop, quit considering it and just do it, or have someone else do it.  It makes a BIG difference.

The killswitch works.  There is a tiny, tiny bit of leakage when the loop is muted.  I’m not sure why, since there’s no parallel connection, but I’m guessing there’s a small amount of capacitance in the loop circuit that allows some signal to literally cross over.  There is a lot of track and a lot of power.  Anyone with enough time on a breadboard will have experienced something similar, even at 9 to 24 volts.

The only thing I can add is: just tape the wires off and tuck them under.  They are much too short to tie off somewhere.

  1. […] Alternative Modifica… on Idea: Switching a Parallel… […]


  2. Andrew says:

    Thanks for putting this up. It worked great easy to do and I’m terrible at soldering.

    Liked by 1 person

  3. Andrew Freeman says:

    Hi – Nice Post. I know it is a while since you posted this originally, but I’ve only recently picked up my first Dual Rec.

    Would it be possible / practical to make a mod to the parallel FX loop to add in a 3PDT Toggle Switch routing the three existing wires that you have taped off (in your mod) to the common of each pole on the toggle switch, with them wired back to the pads from throw position 1 outputs, then shorting two of the outputs of throw position 2 (pads two and three) so that you can have either a parallel or serial FX loop at the flick of a switch?

    Just a thought – but as you have experience of accessing / modifying the circuitry, I thought I’d ask?


    • Warpedpig#1 says:

      It might be. I haven’t thought about it before. Wires would need to be run from the tracks on the board to the appropriate center lugs on the switch.

      One side of the switch would be to the pot’s available wires. This would enable parallel operation.

      The other side of the switch would just be the wet track jumpered to the V4b input. That would provide serial operation.


      • Andrew Freeman says:

        Yes – I realised after I’d posted that I’d made a mistake in that the three wires you taped off should go to the throw position 1 outputs, and a set of new wires added from the pads to the common terminals of each pole. Throw position 2 would have the pads two & three jumpered.

        I’ll probably give it a go sometime and report back (though it probably won’t be for a while).

        Thanks once again.


  4. Steve Hunter says:

    I am attempting this on a Recto Recording preamp which has a parallel loop with two returns. No schematic available and I’m not an amp tech but I think I’ve got it and will post pics either in triumph or desperation soon.


  5. bresan79 says:

    Hi, I’m a little confused on what this actually does besides make the loop serial. Will this still allow the footswitch to control the loop on/off? What all does this effect? What do you mean by killswitch? On a certain setting it will completely mute the signal? Thanks.


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