Here is the Mod' which uses OA4's not OB3's but I suspect any of the series could be used. It looks quite simple, it sure would be a better setup then it now has. Seems to me there is one of these regulation tubes that is a two in one, with it you could eliminate one of the tubes. http://members.cox.net/n4jk/viking_ii.htm Kenneth A. Crips Station W7ITC, Cheyenne, Wyoming.