Firstly a warning: There are dangers in opening or building amplifiers. Unless they are battery powered, somewhere inside there are mains voltages that can KILL. Depending on your location these could be between 100V ac and 230V ac nominal. Only open an amplifier if you really know what you are doing. I  worked in Electronics until my retirement and was an IEng (Incorporated Engineer) Electronics, having studied to degree level. Despite that, I do  not know everything and have had to cram up on