Hi guys,   i haven't used the amp for a month and when i turned it back on i found that i have an unpleasant harsh sound which is present on both channels.  i have changed the preamp tubes which did not make it go away.  it is like distortion but  seems to show a very short decay when you play the strings.  sorry if this isn't clear but i doing my best.  Can anyone suggest what has gone wrong?  BTW it absolutely is not me being unable to tweak my amp it is a problem !!!! thanks, laurie