Three suggestions:
First hum...the ground wire may not be the cause of your hum it could be the interconnect between the pre amp and turntable. This can occur if the interconnect has been placed too close to a power cord of any kind.
Note placing the power cord or the interconnects at right angles to each other may not solve your hum problem. Only distance between the power cord and interconnect will make a difference.
Second...
Replace the 30 foot section of ground wire with a single contiguous piece of 20 to 24 gauge wire.
Third...
When your cartriage output is only 3 to 4 mv, 30 feet is a long way for the signal to travel without picking up noise. Can you use a step up transformer for the cartriage to boost the signal over this distance?
First hum...the ground wire may not be the cause of your hum it could be the interconnect between the pre amp and turntable. This can occur if the interconnect has been placed too close to a power cord of any kind.
Note placing the power cord or the interconnects at right angles to each other may not solve your hum problem. Only distance between the power cord and interconnect will make a difference.
Second...
Replace the 30 foot section of ground wire with a single contiguous piece of 20 to 24 gauge wire.
Third...
When your cartriage output is only 3 to 4 mv, 30 feet is a long way for the signal to travel without picking up noise. Can you use a step up transformer for the cartriage to boost the signal over this distance?