McDonalds at Thaphae Gate
Outside the moat (same side as Starbucks). Burger King and Black Canyon are on the inside of the moat.
Please check the hare line page to see what time the bus leaves McDonalds, and make sure to get there a bit early 
Songtaews should take you to Thapahe Gate from anywhere inside the old city for 20 Baht.