try flipping the plug if you can some old amps had a reverse polarity problem also some times it may be your guitar cord or guitar connection that is causing i have found that it it is most of the time a bad ground that causae buzzing been playing since the 1970 s and own both fender and marshall amps also newer crate amps but the older (tube) amps tend to be more subject to buzzing also have had some guitars that caused it with bad wiring (ground also )