Jerry,
Those broadband couplers will be expensive. The last I heard, Nick Tusa still makes them.
http://www.ce-multiphase.com/Email Nick:
nick.tusa@ce-multiphase.comI might just make a tuned amplifier myself instead of a broadband one if starting from scratch. The 600L runs class AB2, grid driven. The Retro75 at 12V driving the 600L (with an extra 75 ohm load across the 600L input - important) will drive the 600L to ~ 70 watts AM carrier output with headroom for > 100% positive modulation. The 813 runs a dull red, and you need an exhaust muffin fan on the top to draw out the hot air.
Good luck with your project!
Jim
WD5JKO