[Mondial]

Greg, as i mentioned above, I believe there is an error in the schematic regarding the connection of the negative of caps 47 and 48. I don't think they should connect to chassis ground, but rather to the center tap of the power transformer. Connecting the negatives as shown on the schematic will cause all the ripple current to pass through the bias resistor, creating a hum.

Try disconnecting the negative lead of 47 and 48 from chassis and connect to the center tap and see if it cures the hum.