Fortunately, you don't need to be an engineer to repair, service, mod, or even design a basic amplifier. Though as time goes on, you will find yourself getting deeper and deeper in a quest for understanding. That's what happened to me, anyway.
RCA receiving manual- basic tube function and design. Required reading for tube heads.
Radio Shack or other book on basic electronics- info on basic components and their functions, and basic electronics theory.
Radio Shack or other book on soldering. Good soldering is pretty much the most important manual skill to have. Understanding why & how soldering works helps you to realize how critical good joints are.
Some basic books on guitar amp repair and servicing. Learning how to service amps is a great way to understand them.
Here is a link to Magic parts catalog. Click on books. I recommend Gerald Weber's book for good background and mods, and Dan Torres, Dave Funk, and Tom Mitchell's books as a starting point. The Brosnac book is good, especially for building, and the Torres book has step-by-step walkthroughs of a small amp project. It took me reading a few different books to get the basics. You find the guy you can understand. Be warned- some are not cheap. You may find a better source somewhere else besides Magic. Get an old Fender tube Champ- pretty much as simple as it gets. Power supply, preamp, output section. Take it apart, look at it, understand it. All amps are basically derived from Fenders, on some level.
READ AND FOLLOW ALL SAFETY GUIDELINES. Tube amps, even small ones, can carry lethal voltages.
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