@OP, I'm not a tech and that's what makes my post worthwhile.
Knowing what I DO know is enough to say that rather than screw around with your old amp, I'd have it looked over by a reputable tech. If the manufacturer is still around... even better. I bet the old PS and coupling caps are dried out and need replacing. Other parts may need to be replaced too. Here's a quote from TLS49's article:
"You should probably consider replacing the capacitors on an old piece of equipment even if it doesn’t hum. Old filter caps will fail sooner or later, so you may save yourself some time and trouble later on."
Seems like good advice to me.
Knowing what I DO know is enough to say that rather than screw around with your old amp, I'd have it looked over by a reputable tech. If the manufacturer is still around... even better. I bet the old PS and coupling caps are dried out and need replacing. Other parts may need to be replaced too. Here's a quote from TLS49's article:
"You should probably consider replacing the capacitors on an old piece of equipment even if it doesn’t hum. Old filter caps will fail sooner or later, so you may save yourself some time and trouble later on."
Seems like good advice to me.