The mains fuse (the one supplying power to the power transformer) has to be able to allow a high current draw for a little while (1/2 a second?) while the power supply caps charge up, so it needs to be a "slow blow" fuse. Further to that, using the standby switch allows an inrush of current that may even exceed what you'd get if the tubes were starting from cold. The whole "supplying power to cold tubes damages them" has been pretty well debunked, so here's what I'd do: 1. Replace the mains fuse with the correct value of slo-blo fuse (I believe it'll be 1/2 the value of the American fuse). 2. Pull the tubes, turn OFF the standby (in other words, have it in the playing position) 3. Turn on the amp. If the fuse holds, then the transformers and power section are good. If it blows, you're going to have to send the amp in for repair/replacement. 4. Replace the tubes, leave the standby off. 5. Turn on the amp, if the fuse holds then you've solved your problem. If it blows you have a bad power tube. 6. If the amp is working OK, and you don't mind sacrificing another fuse, try turning the standby into the standby position, turn on the amp, then turn the standby off. If the fuse blows, you know you just need to leave the darn standby switch alone!