I've been working on a St-70 and have got it to work nicely with new quad cap,
tubes, etc. The last issue I was trying to address is 60 hz hum that was a little higher than it should be. This evening I disconnected my PAS3 from the amp and then the amp was dead quiet. I reconnected the preamp and the hum returned. So the hum seems to be from the PAS3. The hum increases with volume level on the preamp. The PAS3 is mostly stock. I replaced the boards with vintage electron clone boards. The only real mod is a SDS cap board. I also added a grounded power cord with the ground attached to the power trans case. FWIW I live about 2 blocks from a very powerful FM radio tower which I can also hear slightly through the system.
Just looking for any ideas on how to reduce the hum.