I've got some preamp tubes that are the cat's meow in one particular rock amp, but they are slightly microphonic and can begin to self-resonate. Their microphonic behavior is part of what makes them great in this amp - very edgy with a borderline psychosis when the tube itself begins to participate in the sound in this way. I'm wondering whether someone make a doughnut to fit over the glass and dampen the tube resonances. I know you can use heat-shrink tubing, but these need more inertial mass to keep them in bounds, but still exhibit the right amount of "controlled" microphonics. Who makes such dampeners and where do I get them?