The P-780 on eBay (AM only) uses 6 "D" cells... I suspect that the AM-FM version (P-865) uses the same.
The very first GE transistor radio (ca 1955) uses an Eveready #239, which is a 13.5V with a 9V tap...Here "batterymaker" describes the special battery that he made for a GE #677:
http://www.youtube.com/watch?v=fwZFu...78B7578A1F537B
jr