Turntable hum

If I hook up the grounding wire from my turntable to my amp the hum gets noticeably louder. What gives?

what brand of turntable?  I guess it hums even without the ground wire?  Is the turntable a new addition?  Does it hum with the motor on or off?
It does have some hum with the motor on, louder when ground is connected. I have not checked what the ground wire hum situation is with the motor off. It is a Gates CB 77 otherwise wonderful. 
Is everything plugged into the same outlet/circuit? 
Yes Miller, good thought but everything is.
I wouldn’t have thought of that one! But no, it is a stereo cart, Denon 103. 
Turntable ground is not always tonearm ground. The tonearm needs to be independently grounded to the phono stage. If the turntable power supply and or motor ground is tied to the tonearm, it will cause a ground loop and hence the hum. 
I took a look at the manual for that turntable, which apparently dates back to around 1960 or thereabouts.

One thing I would try, assuming that its two-prong power plug is non-polarized, is reversing the orientation with which it is plugged into the outlet.

Also, I see in the schematic that it has a "filter assembly," which appears to be placed across the AC line "after" the power switch, and which appears likely to incorporate an electrolytic capacitor. If that capacitor has not been replaced in modern times I’m thinking that the problem may be related to electrical leakage through the capacitor.

There is also a "motor start capacitor," stated to be 2 uF for 60 Hz countries and 3.75 uF for 50 Hz countries. Presumably that is also an electrolytic, and although it’s hard to tell from the schematic I’m thinking it might also be responsible for the problem if it has not been replaced in recent times.


-- Al
Wow gold star and an apple, Al! When starter caps go bad the motor does make a pretty obvious humming sound, but that only happens on start-up and if bad enough the motor would just make the awful sound without even moving. I've seen this on larger motors and it really is an awful sound!

Older stuff like this they used to let us flip the plugs around eliminating hum. Thanks for the reminder just how far down the nanny state you can't be trusted to tie your own shoes slippery slope we have gone. 

But hey- 1960's turntable, still plenty valuable and useful. Imagine saying that about anything digital. 

Okay. Okay. You can stop laughing now. Seriously. Yes digital is a joke. But come on. Knock it off. Digital, useful, and valuable. All in the same sentence. Dang now I can't stop laughing,...
What preamp or phono amp are you using? Does the hum change as you mov the tone arm across the record?

Thanks so much everyone for your thoughts!  I will check these out tomorrow when I’m at work as I brought this turntable in to work to replace a Thorens 135 that stopped working.  It is going through the phono input of an Ampex integrated amp into powered Genelec speakers. 
Oh and the hum does not change in relation to the tone arm. 
Sounds like an electrical issue as @almarg  suggested.
Btw, polarized plugs are not a “nanny state” encumbrance. They ensure that equipment which has a “grounding resistor” from neutral to chassis, usually several megohms, doesn’t inadvertently have the chassis become energized to hot lead potential by mistake. The whole concept is flawed of course.  ULc, CSA and UL allows this terrible method of false economy for a manufacturer to avoid cost of a proper grounded power cord. 
Run a wire from a screw on the bottom of your turntable and hook it up to the ground right along with the grounding wire.  Completely fixed the hum problem on my Pioneer PL 630.