Sounds like you have a microphonic 12au7 linestage tube. I had the same problem in my pv11 a couple of years ago. I tried nos 12au7 tubes (tele,mullard,amperex) and found them pleasant sounding but lacking in "liveness" and a bit opaque when compared with the tubes supplied by c-j. I recommend sticking with them. They test and burn in their tubes, and, IMO, are quite reasonable in their pricing.
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