Marshall Artiste reverb channel sounding muddy

[Stratfordade]

Will do thanks. So that would be more in line with the ‘bright’ channel of the Bassman that this Marshall design is based on anyway?

[Helmholtz]

Sorry I made a mistake.
Use a 100nF (= 0.1µF) instead. That will raise response above 600Hz.
Not like a bassman.

[Stratfordade]

OK thanks. I also found video of the amp which shows the cap present that’s missing on mine. It’s an electrolytic so maybe it is a 250uF on schematic? I’ll try your suggestion though given there’s no cap at all.

[Helmholtz]

The 250µ cap would boost all frequencies by about 5dB.
The fact that it's missing allows to boost the highs only.
If the 100nF gives too much mid boost, lower the value to 47nF or even 22nF.

[Stratfordade]

Just to wrap this up, in the end I did nothing to the amp except adjust bias which was a little cool. The owner loves it - he cross connects the channels for best tone. Looking at schematic the normal and reverb channels have very different tone controls so I expect the normal channel was intended to be a bit brighter. The ‘missing’ bypass cap has no space for it on PCB so I think Marshall changed design slightly between 1971 (schematic date) and late 1974 (quality (presence and treble wound up on Reverb) control sticker). It’s a lovely clean sound on either channel, and the owner tells me it breaks up nicely near max output.