The following mod will give your Solid State Rectifier Marshall the “creamy” sag of a rectifier tube as well as that wonderful “compressed” sound of cranked/overdriven power tubes.
1. Wire a .022 25 volt ceramic disk cap on to a mini toggle switch (on-off).
2. Ground the switch and run the hot wire of the switch to the “In” (or “wiper”) of the Presence pot.
NOTE: When this mod is “on” or “active” it will render the presence knob inactive. When the mod is off the Presence knob is restored to it’s original operating specs.
thanks to schematic heaven for the above mod