From the angry Hum Gods. Time to sacrifice a cable to appease them. Whoochabooga. Whoochabooga!

Maybe you broke a phono lead while installing the new cartridge?

This may get me in hot water, and don't try it unless someone else says it's OK, but here's what I do:

1) Get a nice long piece of wire that can reach anywhere in your syetm.

2) Attach one end to a known ground that is independent of your system.

3) Then, start touching the other end of the wire to varoius places on your stereo - try touching it to your phono ground from the turntable, maybe to a screw on your phono-preamp, even to the motor assemply of your turntable. Just kind of roam around touching stuff and see what happens to the hum. I've even seen things happen when touching that sort of wire to the ground part of interconnects!

It sounds like you've created some sort of loop that is not immediately obvious.

***Don't touch anything "hot."***

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Next, or instead, go online and search for "star grounding," I think it makes a big difference.

There are some posts here about it, too, but I forget where.

OK, that's my idiot's way of checking grounds.

Don't get killed.

Rubber gloves, perhaps?

What tonearm is the cartridge mounted on and does the tonearm have an independent ground wire?

You could've done something to the phono preamp- I did once by not having the ground wire hooked up properly and a static charge went from cartridge to phono amp and fried a part. Plenty of hum came out... Just an idea.

Switch to CD?