I've been in this field for 30 years designing all sort of transistor circuit, RF, low noise and high voltage circuits and even spent two years designing bipolar analog IC, never have I use a curve tracer nor find it necessary. More importantly, people here have a bad habit of pull parts out to test and put it back it. This is a very bad habit in my book. You study, learn the circuit, check out all the voltage to make sure it makes sense. Unless there is a problem, you don't pull things out just for the sake of pulling them out and check. If you suspect a problem, show me the symptom first, not pull the transistor and run curve tracing. I don't expect a lot of the cheap commercial amp use top quality pcb. I don't pull transistors out for no reason, when I pull any out, I intend to replace them with new ones no matter what. EDIT: Again, the best weapon of testing transistor is knowledge. Study how transistor works, spend the time to get the schematic of the amp you are using, study the schematic. Write out all the DC voltages in the circuit, check it against the real circuit and compare. If you cannot write out the DC voltages, go back to the transistor basic books and study how to. You waste time pulling out transistors, wasting time to fix if something gone south along the way during the pulling, your time is well spent to work and write out all the DC voltages. You can spot the problem 90% the time just by a simple volt meter and the knowledge. then if all else failed, then consider more drastic approaches like pulling, not from the get go, particular not if the amp is working and you pull it all just for the sake of pulling.