Hi folks, Having an issue with my Classic 50. I get a crackling/static sound intermittently - I have noticed it always happens if I play an open E and let it ring for a moment, and a bit on an open A. It's never with a note attack, either, it's always as it decays. I did recently retube the whole thing, both preamp and power - it happened before that as well (I was ready to retube anyway). I've sprayed the jacks thoroughly with Radio Shack contact cleaner, input as well as effects loop. Since it doesn't happen when any pots are adjusted, I don't think it's a scratchy pot. Ideas?