Move the end of the cap from R7 to pin 6 of the tube. That means you create a junction at the cap, R7 and op amp input (schematically keep the junction and switch places of the lead at pin 6 with the cap at R7, look at C4 in the L1-schematic in post #15 for reference). That way the cap blocks the DC feeding the tube plate from reaching the op amp input. You will only want the AC portion of the circuit reaching the op amp input.
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