For a simple and very effective modulator for rigs in this power class that doesn't require increasingly hard to find modulation iron, let me recommend a Heising circuit originally described in the April 1955 issue of Radio and Television News and revisited in ER #91, Nov. 1996. The scheme utilizes a clever bias shifting scheme to squeeze 30 - 40 watts of audio out of a single 807 modulator tube. No transformer required, only a 10 H filter choke. Won't modulate 100% in the negative direction (in common with other Heising circuits) but comes so close you can't tell the difference.
The original author called it a "Reference Shift Modulator." Can provide more information if anyone's interested. Dennis D. W7QHO Glendale, CA
