You may want to replace the power tubes too. What i would do is that when i replace the fuse, if i turn the amp on standby and see the tube(s) glow a color other than faint orange, immediately turn it off and remember which tube is bad. You'll probably want to replace the tube pair, 1-4 are paired, and 2-3 are paired. If everything looks ok, try the amp on and see if you have any type of noise that shouldn't be there or if any tubes start glowing red hot. If there are tubes that aren't behaving correctly in either of those steps then that is most likely the culprit. If instead things work fine then you may want to have an amp tech look at it, especially if the fuse blows again. That means there is some other problem in the inner working of the amp, maybe bad capacitors. You'd want to take it to a tech then because i presume that you don't have expertise in electronics, let alone amps.