Effects loop mod

[fumbi]

Hi all. I own a marshall VS 30. I have a zoom g7.1ut and i want to use it with this amp. My question is, is there a way that i can bypass the pre-amp in the marshal and use my zoom as the preamp? Basically use my amp as a power amp. Any help is appreciated .

[teemuk]

I guess no FX loop in that one ( ? ) so you really would have to modify the circuit to ignore the pre. Anyway, most Marshalls have nearly linear frequency response when middle control is maxed and bass and treble controls turned to zero. Dialing that in and using the clean channel should minimize the effect of the pre. Perhaps not the best way to achieve what you are after but unless you want to start tweaking with the amp probably the only way.

If you want to start tweaking, locate the wire/trace between preamp and power amp sections and cut it. Insert an effects loop circuit (or something similar) between the cut sections. Make sure all impedances in the added and original circuitry match. Easier said than done, though but this is practically all there is to do. You can use this schematic as an example of one possible configuration: http://www.drtube.com/schematics/marshall/8040.gif