I played with the circuit of a commercial adjustable regulated power supply using pass tubes, for use as a series modulator to feed the plate of a low power final amp. It's very linear, and very adjustable. It also maintains a well-regulated average DC voltage. It can handle a class C stage which uses 150VDC as the unmodulated plate voltage. Although an 807 is much larger for an RF amp than required for this use, it worked very well. I am sure there is room for improvement, and the principle can be upscaled. Please have a look:
http://www.montagar.com/~patj/amxmtr_dc_classamod.html
