Amp build schematic of pre-amp and switching section.

[marshallguy]

I took one of my JCM800 2203 Marshall heads and over the years added mods to it. Here’s what I hope is the final schematic to the final mods. Page of 2. I still need to redraw the second page but the first page is completed.

Some builds use a relay to switch inputs between the clean and overdrive. I had an LDR between the O.D. Input and clean ch. tone stack without a coupling cap but decided on the .047 cap and removing the LDR. While scoping it on the O.D. Ch. output I noticed the trace signal mids would widen a little when I shorted across the LDR ( after installing the .047). I thought the clean ch. tone stack would effect the O.D. input more then it did. The .047 in series with the .022 cap reduces the total to .015 which helps reduce some lower mid bass response which in this case is a good thing. The harmonic content and sustain at low volumes is incredible. The carbon comps in the O.D. section might be a big contributor to that tone too.