I just took delivery of a new to me Redplate RP40 and have a couple of problems with it. 1. A power tube (6L6) has a glassy rattle. If I press on it the rattle stops. Switched positions between the power tubes and tried re-seating them and it's still there. 2. If I tap on the amp or knobs I can hear it through the speaker. It's like a metallic echo kind of sound. It's a cathode bias amp with (2) 6L6GC Please tell me this is just as simple as replacing the tubes.