[Ron Ramirez]

I think there is a Theory of Operation of the 37-690 in a 1936-37 issue of the Philco Serviceman, but this would not do you much good since the tweeters are driven by the output tubes in the 37-690.

I'm not aware of any such documentation for the 38-690. In fact, there does not seem to be a Service Bulletin for the 38-690; only the schematic, under-chassis drawings, and a (typewritten) parts list. Quite bare-bones.