i have a marshal valvestate 8200 bi chorus and in the last few mounths its been really quiet with a really crappy thin treble'y tone. Every now and again it will spring back into life and i can get the sound and volume im used to but after about an hour it wil cut out again. i've replaced the valve but that hasn't helped i'm thinking it may be the speaker cable but im not sure.
Before extensive investigation and expensive replacements get hold of a Switch Cleaning spray and give the control pots a squirt (Potentiometers - Variable Resistors)
Pots get dirt and dust and play up and often show the symptoms you describe - Switch Cleaner spray is relatively cheap and a squirt takes only a few moments.
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