@PTS all original output transistors from factory! If you can find some original transistors that would be great, the same transistors were used in something like sansui 771 so you can find them, I actually managed to find original matched pairs just incase I blew an output while rebuilding and testing. As dBel84 said you can find other newer modern transistors or ones close to original specs perhaps or modify what you have now but why do that? If you can keep it as original as possible do that instead, I had to replace my filter caps they were shot and replacements were very expensive and didn't fit safely or properly, I did subtle changes not to interfere with it but to slightly improve what I already had, these amps will stand the test of time but you have to service it, change parts, check connections etc. Currently mine is burning in and powering a set of refurbished quad ELS 57s being fed by a Krell PAM 5 preamplifier and its bloody wonderful!