This is all interesting.

So, apologies for the "noob" questions, but:

1) do you see any disadvantage in having an unregulated supply for B+ and a regulated supply for the filament?

2) When working toward a target voltage, would this math be right? Say I wanted 105 VDC output. To find the desired VAC input, is the following equation right?

(V -1.4) * √2 = 105V. Solve for V.

In this case, V = about 75.6 VAC.

Is that totally wrong? My reasoning is that 1.4v is dropped in the bridge rectifier, and after smoothing the DC voltage will be at or very near the peak AC voltage, hence the multiplication by the square root of 2.

Also, how do I account for the load resistance in the microphone itself... i.e. absent a regulator, how do I know the correct voltage without the microphone connected? Thanks for your patience. I have 2 music degrees but was never better than a "B" student in math.