Someone please explain to me...

Ah, so they’re designed to trip at a much lower amperage and will trip well before your breaker box breaker will. Didn't think of that. Makes perfect sense, thank you, Jerry!

The circuit breaker in your panel is designed to protect the wire in your wall, not the components plugged into it. If your wire is sized for 20 amps, your breaker will be 20 amps. The breaker will trip before the wire heats up from overcurrent and starts a fire.

Your amp is a much smaller load, perhaps 1 amp. So the OEM will size the fuse at perhaps 3 amps. (these are just typical numbers, your current and fuse may be different). This fuse is to protect equipment, not to prevent a fire in your wall.

So if you want your amp to be protected against an overcurrent transient--say a tube develops a short, you have a fuse. you hope that your fuse or breaker will blow before it takes out, let’s say, a transformer (it usually won’t). So the 3 amp fuse will blow long before the 20 amp circuit breaker trips. I put a wrong tube in an amp once and it was shorted across 2 pins that shouldn’t have been. a resistor acted as a fuse but nothing else blew.

So how can you move the protective device from the fuse holder at the back of the amp to the SDFB, 2 feet away? The answer is that electrically, these two locations are the same. the fuse will blow or the SDFB will trip providing the same level of protection.

Jerry