According to the Garrett buyers guide the coils available for purchase for the ace 350 and the at pro have different product numbers and prices: ie AT Pro 8.5x11 DD coil is product 2222400 $159.95 and the Ace 8.5x11 DD coil is product 2222000 $139.95. The Ace series operates at frequencies (depends on the model); 8.25khz to 6.5khz while the AT Pros (depends on gold series or not) are 18khz to 15khz. If you can use an ace series coil on a at pro it might impact your performance. Perhaps one of the Garrett dealers can chime in as my information is based on the published specs only.
Oh and probably more important here is the fact that the AT Pro is waterproof to 10 ft so the coil connector has an "o" ring that once seated makes a waterproof connection. This fact is probably the major difference in the coils; ace vs at pro.
Alrighty thanx much for the bad news i was hoping to upgrade to a at pro and keep my nel without buying a new 1 but i guess it should be worth something when i do sell it
