The simplest way to do this is to add a series resistor to the mains supply live line. A normal power resisitor of a low enough value to run the power amp will certainly burn out with a faulty power amp. Filament lamps (incandescent bulbs) make excellent current limiters. They have a low resistance with a normal load (good amplifier) in series. If the load (amplifier) is faulty the resistance goes up rapidly as the filament heats, so limiting the current nicely. The lit bulb also gives a clear indication of the fault condition.
Googling will soon lead to suitable DIY bulb current limiters. One such is UK Vintage Radio Repair and Restoration – Series Lamp Limiter
I made a similar unit but used two 13A sockets. One led off to the limiter load which was a 60W ES bulb housed in a clip-on spotlight. The second mains outlet socket was for the test amplifier. I omitted the bypass switch as I knew that sooner or later I would leave it switched to the unsafe postion (no limit).
If you proceed to make this you must be sure that your skill is adequate to safely complete mains voltage wiring, which can be lethal if mistakes are made.
In use, expect the bulb to flash initially due to inrush current. After 1 second the bulb should extinguish or just be a slight glimmer. The amplifier will only deliver minimal power if this limiter is left connected. If the amplifier indicator lamp is lit and the limiter bulb is off, then proceed to connect the amplifier directly to the mains supply. If there is no flash at power up with the limiter suspect an open circuit mains fuse. Unplug the amplifier, replace the fuse and restest with the limiter connected. If the fuse value is too low or of the type fitted is a quick blow, the amplifier is likely to power up fine via the limiter, but blow the fuse when the amplifier is directly connected to the mains. Fuses for large transformers should always be the slow blow (time delay) type, usually labelled “T”.