I'm looking for plans for a simple AM antenna to connect to my stereo to
replace the little loop antenna that came with it. In the past I have
used long wires but they are not very directional.

"not very directional"
A loop will have figure 8 pattern with sharp nulls on axis (both sides)

Long wires aren not very directional.


I have found some fairly good plans online

How about posting the URL.

In no particular order. . .

http://www.mindspring.com/~loop_antenna/
http://www.dxzone.com/catalog/Antennas/Loop/
http://www.mtmscientific.com/loop.html
http://www.frontiernet.net/~jadale/Loop.htm
http://www.dxzone.com/catalog/Antennas/Shortwave/
http://www.schmarder.com/radios/misc-stuff/loops.htm



for what looks like wire wound around a box but all of them seem to have
potentiometer in the mix somewhere. It would be very little problem to
build an aimable 18 to even 48 inch frame.

"seem to have potentiometer in the mix somewhere."
I think you mean a variable capacitor.

One or the other depending on the plans


I only have two AM stations I'd like to come in clearer (690 KHz and 1440
KHz). Would it be possible to make a 'box' with one 'tap' for 690 and
one for 1440 then just connect one or the other the radio? How long
would be wires need to be and how would I feed it into the connections on
the radio (coax)?
A tap could be done but I think it would be difficult to find the proper
position.
A better way would be to use a switch with two different tuning
capacitors. You
could have the smaller value fixed in the circuit (for 1440) then close a
switch
to put the second larger value in parallel to tune 690 khz.
If the stations are 90 degrees from each other at your location, you
might find one station
in the null of you antenna.

I think a simple measurement would do it. For 690KHz you could tap it at
1425 feet or at any fraction thereof. For 1440KHz you just use 919 feet.

That was my original plan. To take a 4 foot square frame and put 22 winds
of wire on it. That would give me about 1/4 of a wavelenght of 690. I
could put a tap at the 14th winding to give me a 1/4 wavelenght for 1440.

Mike