First off, don't take this the wrong way but when dealing with big tube amps, you should really know what you're doing due to the voltage that is involved. You've made a mistake and that's fine. Mistakes are how we learn valuable lessons. However, DEATH is not a valuable lesson.
Feel free to change your fuse but DO NOT fool around inside your amp in an attempt to fix it. No offense... But... You have no business in there. So you know, there are capacitors within that have stored up electricity that WILL KILL YOU upon your first false move.
So, here's some info that might help you out.
1. The aforementioned output transformer is the last component of your output stage. You said you didn't know what this was. Perhaps I can explain it. Your output tubes produce separate, high voltage signals for the + and - phases of your guitar signal. The Output Transformer or "OT", as it is sometimes called, puts these together and reduces the voltage while doing some impedance magic. If you don't understand this, just suffice it to say that what comes out of your tubes can't make a speaker work. After it goes through the OT, it can.
2. You mentioned that you don't know what a "Retube" is. Here's what it is: Modern tubes don't last as long as tubes did in the old days. Every year or so, you need to get new tubes. You can make them last even less time if you use them all the time or, in your case, subject them to undue stress like plugging your speaker output into a guitar port. When you put in new tubes, it's called a "Retube". Mesa amps are cool because if you just use Mesa tubes, you'll never have to Bias the amp. I do not recomend that you do this yourself.
One other thing. The whole idea of the guitar port is that you don't need to use an amp to record. Of course, you can do what you want but that thing is designed for line level or less signals. Don't plug speaker juice in to it. If it's not allready melted, use it in good health by taking my advice.
Good luck. I hope your fuse is the only problem.
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