HT fuse placement

[baudouin0]

A fuse has a much harder time rupturing when the current is DC (as in a filtered B+ feed), as compared to when the fuse is passing pulses of current where the current and voltage periodically reach zero (as in the CT connection, or a diode anode connection).

Ubiquitous 250VAC rated fuses are not rated for DC applications.

Fusing the B+ feed, say to an output transformer, may cause a high voltage transient on the output transformer winding if such a fuse ruptured, due to the DC current flowing in such a high inductance path.

Hence the diode.

 

[showdown]

The amp is idling at 215mA measured across the current sensing resistor. There’s not much of an inrush current that I can see with my Fluke, probably thanks to the NTCs. How much more current should I expect to see if I crank it? It powers up fine with 315mA fuses now, but should I go any lower?

 

[Kay Pirinha]

A class A amplifier doesn't vary much in power consumption. Ideally, it doesn't at all. Hence, your 315 mA choice seems reasonable.
Best regards!