Fuses, especially the anti-surge mains fuses will eventually fail, even if there is nothing wrong with the amp. You can get the correct fuses from Maplins (it's always useful to have some spares!!), or online from Rapid Electronics.
Replace the fuse and turn the amp on. Does the fuse blow? If it does turn the amp off, install a new fuse and remove the power valves.
Turn the amp on.
If the fuse doesn't blow, then you almost certainly have a shorted power valve.
If the fuse still blows you mostly likely have a damaged power transformer and a trip to the tech is in order.
^ +1
some sound advice from JPF, exactly what I'd do too.
Valves can short out without any warning. I've installed brand new and tested valves into amps and had them go down in a shower of sparks as soon as the HT is applied!
I went through a bunch of JJ EL34s that did this and marked the end of my relationship with JJ output valves.
We've had good results with JJ valves, which we have gone back to using now that they have sorted the pins out. I've had a couple of JJ-6L6s flash over in amps with B+ > 500V. E34Ls have all be fine. We have probably installed 150-200 of 6L6s and E34Ls in amps over the last couple of years.
What do you use now? I really like =C=, but we can't get them in bulk (ie at least 50 pieces) at a sensible price.
I don't like EH EL34s (either from a sound or reliability point of view).
The bog standard 5881-WXT are fairly bullet proof (if a little dull sounding), but again staring to get expensive due to the collapse of the pound vs the dollar. Not really tried any other of the EH 6L6 style valves, although I do have a pair in my main amp.
S-logo Svets are good, but aren't really suppose to be sold in the UK. We got a very good deal on some 6L6s a couple of years ago but since then haven't been able to get them reliably.
My experience would suggest that there is considerable batch variation between valves. We had one order of 50 JJ-EL34s that all matched to within a 8 ma range for current draw (I matched these). The next lot of 50 seemed to be in two groups — a low current draw group and a high current draw group, suggesting that they came from different batches (or that someone had pre-selected out valves that had the correct current draw to work correctly in their unadjustable fixed bias amps, not mentioning any names.......).
