time, the Op-Amp's  output will drift with it. While responding to the music signal, the Cathode Follower's output voltage and current will vary, but variation is far too fast to register at the input of the OP Amp and will be ignored. (If the Op-Amp's is extended to include the source of the MOSFET, then the Op-Amp would respond to the change in current through the Cathode Follower. Not a good idea.)
    The 150k resistor that connects from the B+ voltage to the output is there to give the compliant current source a current path in the absence of a tube in its socket or at startup when the tube has yet to conduct any current."

   The basis of the compliant current source is a non-inverting integrator, which is nested in an inverting integrator. (If your head is beginning to hurt, jump to the next section.)  The MOSFET is configured in a grounded source topology that inverts the output; the tube, a cathode follower topology that doesn't invert the output phase. Thus the overall effect of this circuit is to apply a DC servo loop to the cathode follower output and to match the idle current of the cathode follower with a AC current source.
    This circuit eliminates the need for a coupling capacitor. If feat seems trivial, then maybe the math involved should be reviewed. The coupling capacitor's value is set by:
      C = 1/(2piFR),
where C is in farads and F is the lowest frequency and R is the resistance of the headphone; rewriting the formula for microfarads yields:
      C = 159155/F/Resistor.
For example, given a low frequency cutoff of 20