You take the first fuse to come out and that is the broken one, then you put the second fuse to come out in the place the first one was and push the holder all the way in again. The contacts are just inside the fuse holder plastic, so you don’t need to open the box. In fact opening the box will achieve nothing at all!
In a one box amplifier it’s easy to interrupt the mains with a thermal trip near the heatsinks if things get too hot.
With a two box amp like the 300 it’s a bit more complex as the heat source & mains power are isolated from each other.
The 300 uses three tier thermal protection - semiconductors track the temperature of the heatsinks & run the fan at an appropriate speed. This is normal operation.
If the fan cannot bring the heatsink temperature down, 70 degree trips on the heatsinks cause an imbalance on the incoming supply rails. These rails are monitored by the power supply & a resistor strapped to a further trip within the PS will heat up & interrupt the mains. This process takes about 80 seconds & power will be restored once the resistor cools.
A further 90 degree trip on the amplifier chassis will place a direct short circuit across the supply rails & blow PCB fuses inside the PS.
So, in summary, I think that the double press of the switch caused an imbalance in the supply rails & it triggered the resistor to heat up. That’s why it played for a while & then shut down.
Thanks a lot for a thorough reply It sounds reasonable for the situation I entered with a double press. I’ve had tube amps in the past that blew up when shut off and on to fast and I know there are same warnings with transistor amps and I wouldn’t normally do it but this time the on off button didn’t stay on and my immediate reaction was to hit it again when I probably should have waited 30s or so. I’m happy the protection and amp design solved it