I've got Marshall JCM800 100W Bass head. It's definately a copy. Outer look suggest cheap and kind of lousy construction, however when I look inside I see everything being handmande and FULL tube constructuction. -one tube rectifier -three tube preamp -four tube poweramp Anyway one problem occured recently: I can't get any sound from it beside strange buzzing coming from inside of the amp. My father suggested that it is transformator problem or damaged solder joint. Would You help me how and what to look for to repair it?