You wont get a clean true AM signal unless you build the external ALC box and hook it to the icom. Then, You can turn the 7600's power up and then adjust the ALC knob to set your carrier. You can then get a fully modulated signal. The Device is cheap and easy to build.
I posted here about the external alc box
http://amfone.net/Amforum/index.php?topic=25323.0To give you an idea of the difference. I recently ran my Icom in the AM mode on the 15 meter band to W0VMC. I used a commander 2500E amplifier. Carrier was 200 and the most I could get was about 50% mod. I saw peaks of 450 or so on the Power master. With the box switched on and the same 200 watt carrier, PEP was now 1000. I had to back the audio down to get the scope right at 100%.
C