I also agree with Ron. I was the kid who was doing electronics and failed miserably as an EE in the 70's, but did manage an Associate's degree in Electronics Technology with a 4.0 GPA. Go figure. I also got a BAS degree in Engineering Technology and Technical Management. Unlike Ron, I also have substantial machine shop skills.
I have small engine skills because dad said, "You ran the mower without oil, your rebuilding it. I was probably 10-12 years old. I have car repair skills because my first car cost me $25 and it had run into a tree.
I have HVAC skills and now certified for automobile because I applied work skills (Hi vacuum vacuum systems) with my mechanical skills plus some schooling in thermo, building design etc. and put my AC in my car from a box. 14 hrs labor. It failed every 8 years with two blown hoses. Not a single leak otherwise. My parent's had a car in which oak leaves got into the evaporator and corroded the evaporator. I wasn't licenced then, but it was required to handle R-12. The service guy said it will cost you way too much money (about 8 hours labor) for him to fix it. We worked out a deal. He recycled the R-12 and I did the install and I made sure it held vacuum. He verified, charged the system and we had no problems.
I've repointed a chimney as my first brick project. I do plumbing too. Soldering skills I learned from Dad and then from machinists. I could silver solder better than anyone,
I had a short stint at glass blowing too, primarily quartz and sealing tubes under vacuum with a Hydrogen/Oxygen torch I then taught that skill to a new-hire. If I haven't done it in a while, I have to practice at least once to get the feel back.
As for learning how to repair amps, I recommend building this one:
Leach Amp Plans - Part 1 I did it in the early 80's. I accidently made a mirror image of the board. I can now repair high power amps in my sleep.
It's a really nice High Fidelity amp. I made some changes to the published design.
My first tube amp was a 50 Watt mono Fisher model 50A and I was probably 10-12 years old. I had to fix the thing without the proper equipment.
I had Heathkit and Eico for teachers. You built your own test equipment because you could not afford new stuff.
I was asked to fix a Mcintosh tube amp where someone connected the output of one to the input of another in an attempt to make it louder. It nonetheless, didn't work.
Douglas Self is an author of a few very good transistor amplifier design book. They are good reads.
You can get some decent tube servicing/design info from the net (particularly military) and even ebay.
Minimum amplifier servicing stuff is an Variable AC isolation transformer, oscillloscope, voltmeter, dummy loads.
When servicing, you need to be aware of types of resistors and types of capacitors and their application.
Finding a mentor that can help when your stuck can make all the difference.