All tubes are at least a little microphonic; if you don't believe it, try a Herbies tube damper on a supposedly non-microphonic tube like a Military grade 6SN7 or 12AX7.
For those who have bottom getters like the RCA smoked glass VT-231 and Sylvania 6SN7GT and are using Herbies tube dampers, you might try moving the damper all the way down the glass to where the damper just touches the base. I recently found that a second Herbies damper attached directly to the tube base was quite efficacious.
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