The second consideration when troubleshooting this problem: If the fuse keeps blowing, the output and rectifier tubes are the next suspects. Preamp tubes don’t often blow fuses when they fail. Start by replacing the rectifier tube(s) if your amp has any. If replacing rectifier tubes doesn’t help, the output tubes are the next suspects. Replace the outputs tubes with known-good substitutes.
If new output tubes or rectifier tubes don’t fix the issue, it’s time to get your amp into a shop for service.