Blown Fuse..?

So, I'm jamming at practice, and something doesn't seem right...the amp (RM100Combo) doesn't seem to have enough 'bite' to it...So during a break, I check it over, and the tube failure light on tube four is on, but the tube is clearly lit, not dim or anything. So I shut the amp down, and check the fuse, and it is BLACK. So, I obviously blew the fuse, and need a new one. My question is, since the fuse is shot, and the tube was still lit, what the heck is the fuse even doing? I used the amp after the fuse went out, so it must not be a big deal. And has anyone else had this happen? I'd like the comfort in knowing I'm not the only person who's done this :-D

I have heard if you blow a fuse or tube on this amp that it will just keep on playing with the other 3. It will cause a drop in power. I would think that it may be running at like 75 watts. Just replace the fuse and you should be fine. Pretty cool design though that it just pulls that tube out of the chain and allows you to keep on playing.