Why will too many electrical devices operating at one time often blow a fuse?

Too many electrical devices operating at one time often blow a fuse because too much current flow can lead to excessive heating of the wire. Fuses have the essential component which is a metal wire or strip that melts when too much current flows. For this reason, a fuse will blow out if too much current flows through it. Fuses blow quickly for massive overloads, like AC shorts to a grounded chassis. They blow with a some time lag - maybe only a second or two, up to minutes, as the overcurrent gets closer to the fuse's actual rating. Anything that uses enough power to cause the AC line current to exceed the rating of the fuse will eventually cause it to blow. It is important to remember that a fuse NEVER blows without something else being wrong. It could be that: fuse is the wrong rating - replace it with the correct rating power tube shorted rectifier tube shorted power supply filter cap failing Carbon trails on the output tube sockets between the plate lug and the other electrodes, especially the heater electrodes. power tubes have lost bias or biased incorrectly power section of the amp is oscillating at too high a frequency to hear there is an ac wiring short or high leakage power transformer is faulty choke (if present) is shorted/leaking to chassis