Electronic – Types of wire and applicability for guitar amps

For learning about op-amps and their problems, I'm tempted to suggest App Note 47; it's very long and focuses more on higher speed applications, but it has a lot of practical information on debugging this kind of system.

"How can you learn from experience if you can't identify the problem and just hope the circuit works the next time you attemt to build it?"

You have to localise the problem, even if that's just to one half of the circuit or another. You have to either build up from something known to work, or dismantle down to a definitely faulty core.

You need a model of the system, either a mental one or a simulation. Then you need to find out where the reality differs from the model. For audio stuff, this will involve poking around with an osciliscope a lot.

Have you checked the voltage at the outlet? Your guitar amp may be 120 volts AC only, while most computers use switch mode power supplies that can function with anywhere fron 100 volts AC to 240 volts AC at the input. If the outlet supplies significantly more then 125 volts AC the switch mode supplies in the computer gear may work fine but any device rated for 125 volts AC (or a bit less) will, at best, blow the AC line fuse.