I have a control similar to this one. Attachment 306580 Connected to a 100~110 volt DC treadmill motor. The specs claimed up to 150 volt but I took that with a grain of salt. Probably should've been an economy size grain.

The fuse is 5 amp, 250 volt and if I turn the knob up past halfway the fuse starts to glow. That, is not good. :P Could I get away with a 7 amp slow blow or is there a circuit breaker that can snap in place of these 20x6mm round tube fuses?

Before I ever connected it to anything I noticed the transistors pressed against heat transfer pads under the PCB were not making full contact. A little careful bending and shimming between them and the board with small bits of thin yet dens packing foam achieved full contact. Also, the four standoffs and the screw holding the larger transistor to the heat sink were loose. Everything snugged up and making firm contact should help the heat dissipation. I also have it mounted to a good size piece of aluminum, which is mounted to the inside of the original steel electrical box on the back of the lathe headstock. I drilled vent holes and cut a flap so that air from the motor's flywheel/fan will blow through to keep things cool.