Hum that wasn't there before

[Bama Slamma]

My amp developed a hum in both channels. I bet I know where it came from. I put some chassis mount 1/4" phone sockets in it for speaker outputs. With most sources the hum is very small and is drowned out with music when the amp is playing. But when I plug a mixer into the amp the hum becomes very noticeable even with music playing. I can vary the volume of the hum with the amp's volume controls. Guess I made a ground loop somewhere. Any advice on how to fix it? The amp has a three wire 120 VAC mains cord. The mains ground is attached to the chassis about three inches from where the power cord enters the chassis. The amp doesn't have any star grounding in it.

[Bama Slamma]

Here is the internal layout and wiring. this is before the phone jacks were added.