It might have been the preamp if that was a PAT-4 . I built one. It seemed to work fine but I hated the sound of it. So I sold it and built another. It sounded exactly the same. So I sold that one and bought a SWTP 198/A That sounded better but it was still awful and a certain combo of tone controls would oscillate. So I gave that one away and bought a MacIntosh C-20 which I was very happy with for a long time.I once fully recapped an st-70 with orange drops and wasn’t happy with the outcome. Not sure if it was the cap choice, preamp (dynaco 4 pre) or speakers (bozak symphonies), but all the hype I’d read about the st-70 was not there for me….the amp and rest of the gear were moved on promptly.