I'm back working on my friend's 807 tube transmitter. I've had some previous posts on this and I'd like to thank all the people who responded for your help!
I bought an oscilloscope and a dummy load and almost instantly figured out the power problem. It's the 807 screen grids. With the current schematic, the 807 screen grids run at about 100 V. Power output calculated across the dummy load is 4 W RMS. If I increase the screen grid voltage to 250 V, the output jumps to more than 10 W RMS. The tube manual for dual 807s, class A, calls for 600 V plates (which I have) and 300 V screens.
Here's the problem: When I increase the screen grid voltage on the 807s, the modulation disappears. In the diagram, you'll notice the switch that controls the 807 screen grid voltage, also controls the EL84 audio amplifier plate voltage. What is the correlation here? What can I do to fix it? Do I need more peak-to-peak voltage on my input? I'm currently using an old cell phone with a tone generator for testing. Input voltage to the jack and 12AX7 is about 1.5 Vp-p.
Any help would be appreciated! Schematic is below.