Whenever I turn the level knob to about 1.5 I hear a scratching noise like a dirty pot and the overload light comes on and I get no sound. The overload light will stay on as long as the level knob is at 1.5 or above, if I move it below 1.5 it goes off. It does this whether I have a bass plugged in or not, I've also tried different wall outlets. Any idea what it could be? The amp shop doesn't open until monday so I'm hoping it's something that can be easily fixed.