MOSFET based ripple filter for tube amp
The MOSFET is acting as a source follower and this generally means that whatever signal is placed on the gate, is also seen on the source AND it is quite resiliant to ripple or movements on the drain. 

So the source is the output and the gate is at DC level of 82% of the drain voltage BUT it's filtered (by R1 and C1) to remove any ripple seen on the drain.


  R2, R3, R1, C1, R5, D1 and M1 work as something called "Capacitance
  multiplier"


It is generally called a gyrator circuit and in this configuration, the MOSFET acts as a series component resembling an inductor.


  B+ Out takes circa 110 Seconds to reach its maximum


This is dictated by R1 and C1 - they have a time constant (RC) of 22 seconds. The rule of thumb is that it takes about 5 RC to reach the final value hence 5 x 22 = 110 seconds there or thereabouts.