I had a problem after I insstalled a new sink faucet. I had a small drip coming from the cold water shut off valve below the sink. No matter how I tightened or loosened it, the screw in front and the knob itself, it would still drip slowly. So I figuered I would just replace the valve. I bought a new valve and tried to unscrew the large nut against the wall to replace the valve. Ok there doesn't seem to be threads, and it won't come off. I believe there is copper attached to it. My house is 27 years old. So now what, how do I remove the valve, to replace it with a new one. Did I damage anything turning the large nut/

Your old valve apparently is a sweat fitting that is soldered to a copper pipe. Do not worry about that nut, you are closed to throwing it out. Shut off your main and open a faucet to blow off pressure. Disconnect the suplly line going up to the sink faucet and open the old shut off valve, stop cock. Heat the valve to melt the old solder and pull out while still hot. Wipe the molten solder off the pipe with an old rag. Take the old stop to the hardware store to get a perfect match, thgen sweat the new fitting onto the old pipe.

If the end is a hex, likely it is pipe thread or compression fitting. One will turn one way, and the other, the other way. If you can't see any thread between the valve and wall, likely it is a compression fitting and you turn the top of it to the right to loosen.

By the way, where is it leaking? Maybe you can fix it.

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