The Thirteenth Edition of the Bill Orr "Radio Handbook" (not the ARRL
handbook) has an article on a 500-watt, 813 modulator with photos,
parts list, and schematic. The next volume I have is the sixteenth
and it only has the schematic and parts list. So, you need something
around the 13th to 16th editions for the 813 modulator article. --Ed