Marshall DSL100H in @shroprockdoctor after the owner had changed the valves, I assume he meant power output valves that failed to solve the problem.
My investigation began at the front end checking voltages on
This DSL100H has a digital reverb which I removed to eliminate it from the issue.
Following on by checking the dual rail +/- 12v supply it became apparent there was no minus 12v output from mc7912ct regulator. Turns out it was missing its supply from a pair of 1n4007 diodes D13 & D15. Replaced both & she is good as new.