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If the power tubes are relatively new or very old, or if the amp has been roughly handled when it was hot, a power tube may have an internal short. These shorts are rare in tubes past infant mortality and not yet worn out.
The whole trick here is to separate truly defective shorted output tubes from other flaws that will kill new, good tubes that you put into the amp.
Remove the power tubes, replace the fuse, and see if the fuse still blows when powered up with the power tubes out. If it still blows, the tubes are not your problem.
If the fuse does not blow with the power tubes out, check to see that the screen resistors are not open or burned, then, for fixed bias amps, ensure that there is an adequately negative voltage on the control grid socket contact with a voltmeter. In cathode biased amps, the grids will be held at ground, but for a fixed bias amp you should have -20 to -60 volts here, depending on what kind of output tubes the amp uses. In fixed bias amps, if you can't find a negative voltage on the control grid socket contact, YOU WILL PROBABLY KILL ANY NEW TUBE YOU STICK IN THERE. Check lost bias or incorrect bias to save killing a new pair of tubes, and if no trouble is found there, then suspect that the tube itself shorted.
Replace the output tubes with new ones, and place the amplifier chassis on a stable, uncluttered surface in a room you can darken. Be certain that you can turn the amp and room lights on and off without touching anything inside the amplifier for safety's sake. Turn the amp on and the room lights out, and watch the new output tubes like a hawk for signs of overheating. If the fuse does not blow again and there is no sign of red or orange glow other than the normal filament glow inside the power tubes after the amp heats up, the old tubes were the problem.
If you've had an output tube die, always be suspicious of a bad screen grid resistor and a bad input grid resistor; check the value and appearance of these every time you have a power tube fail. Particularly Fender's carbon comps go high resistance or cracked
Be sure to rebias for the new tubes.
If the fuse now blows again, or the new tubes glow red-orange on the plates, shut it down immediately and inspect all of the components and wiring around the tubes to find the problem that is keeping the tubes from being biased properly.