Hi list,
I have a nice U64 but since the last replacment of the nuvistor there is a microphone-effect when I knock (gently) on the corpus of the mic. It rings like a tiny bell. I checked the whole preamp and found an overaged 250MOhm-Resistor. When I replaced it, the Mic sounded better (and did not hum anymore), but the ringing refused to go away. I tried several other nuvistors and it seems, that the ones which are best in shape do ring the most. Now I wonder if it's just normal behaviour, and the real problem is the rubber mounting, which's gone to dust, long ago, as I can't find any other broken component...

So this is my question: has anyone an U64 and does it behave like mine? Or has anyone an explanation?