Crackling sound then nothing

I have a VAC PA100/100 amp. Tonight after turning the amp on, as it warmed up, there was a quick crackling sound. This has never happened before. I tryed to play an LP and nothing. I then tried to play a CD and everything sounded fine. Then I played an LP for a few minutes.

I noticed that two of the four bias lights were red so I re-biased to green. Everything was fine for about 10 minutes. Then a very brief crackling sound and the amp went dead. No power.

I pulled the fuse (Slo blow 4 amps) and it was toasted. Amp has always worked fine. All Tubes are about a year and a half old. Before I put in another fuse any opinions on what went wrong? Any damage that may result?
You have a bad output tube. Whichever ones you tweaked the bias for are the bad ones. Install new tubes and a fuse and you should be back in business. I use my PA100 about 3-4 hrs a day and after a year and a half it needs new tubes.
How many hrs. on the tubes? Checking them is the next logical step.