It's probably (and hopefully) a faulty output tube. If something inside the amp was shorted the fuse would likely blow instantly every time. I'm assuming that your speaker and/or cable is good as the speaker does not pop, crackle or cut out all together before the fuse blows, right? Try this: replace the 2a slow-blo fuse (make sure it is a slow-blo and not a fast acting) turn on the power and then the stand-by and firmly tap on the output tubes with your finger. If you see a lightning storm, and a terrible sound from the speaker, and the fuse blows again then you know which tube is shot. If you transport and gig the amp a lot sometimes the elements will rattle loose. Hope you find it. - Pat.