Hi gang, I just bought a used Ampeg SVT 6PRO from G.C. I received it about a week ago (a day after having a knee replaced). just got a chance to plug it in, and its making a sound like shaking an incandescent lightbulb (metallic SSSHHHHH coming through the cab. Anyone have an amp do this? This is my first amp with tubes can it need a new tube set?