Need to purchase a separate ground cable - from my TT to my Phono Preamp

Any bit of metal wire will work - light cord or mains - outside the audio loop but eliminates hum if there’s an earthing issue 

But make sure it's been cryogenically treated, reviewed by at least two fan mags (or the equivalent online vlog), and the lugs are rhodium-plated unobtanium!

GOT IT 

Electrons have become very fashion conscious lately...conductors must be silver plated, have at least 6 Nines, clad in PTFE jackets, with gluten-free packaging.

In other words, standard 20 ga. hookup wire with or without spade lugs crimped on should get the hum out.

My TT has no separate ground wire and it is dead silent. 
even with a MC cart.