Got a new to me 1978 JMP 2204 last week. Practiced with it all week, done a show with it Saturday. No issues all week but during the show had a subtle high pitch feed back the hole day. Even if I dropped guitar volume. Only time it would completely disappear is when I put in stand by. Sound guy said sounded like the pre amp tubes ( may as well do all of them) need replaced and rebiased. I guess it’s time I learned to do this shit myself.