Marshall Model 2203 JCM800’s are notorious for having a very bright normal channel. This is due to an input series resistance which is very different from the Classic Marshall circuit. (470k Ohms vs. 68k Ohm). The following mod details a method of warming up the tone on these amps.
1. Replace the 470k Ohm resistor that is tied to the input tube grid with either a 68k Ohm, 34k Ohm or 10k Ohm resistor. The lower the resistance, the darker the tone, but 68k is the “classic” value. (Don’t be tempted to remove this resistor altogether). It has to be there to attenuate the grid circuit and prevent excessive overload conditions.
2. Remove the 470pF cap which bridges the 470k Ohm resistor you just removed.
The combination of these two quick mods should return the normal channel of your JCM800 to a more classic and warmer Marshall tone.