[62v8]

Is it OK to run a cable from its earth lead, into the earth pin of a mains plug?
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I gather you're talking about the earth lead, normally in the RCA plugs / audio cable bundle. Under no circumstances connect to the mains earth!
This is meant to be attached to the amp or pre-amp chassis or other device (eg sound card).

Go near the mains and you'll have more problems than leaving the unit with a floating earth. Mains earths can be noisy and let's not go near the subject of ground loops.

OK, if you don't have a ground post on the chassis that you want to attach to ....
The next best thing is to drill a hole in said chassis and wrap the turntable earth wire under a tightened screw etc.

If you just want to experiment .... try the turntable earth wire jammed in the RCA male plugs outer (which is common or earth) when you connect it to the RCA female socket on the amp / SD etc. On 99% of equipment the RCA outer will end on the ground plane of the chassis.

You now have your turntable buzzing and extraneous noise dissappear.

GT