I just can't seem to get away from these 807s. Although I am to the point where I might replace them with 6146s. I built this transmitter with a 6AG7 oscillator, and 2x 807 final amplifier. The schematic is a well known diagram and many people have built it. The oscillator works great. No problems there. The issue is when I measure the plates of the 807s, they are on a totally different frequency than the fundamental. I have experimented with different plate chokes, no change. On the 807 control grids, the frequency going in is dead on, 1000kHz. On the plates of the 807s, it's somewhere around 4000kHz (roughly.) I know 807s are prone to parasitics, so I shortened my leads from the 6AG7 plate. I shortened and re-routed the 807 plate B+ supply. No change. It's almost like the 807s are self-resonating. I've checked my high and low B+, they are clean. There is filtering on both. I have the parasitic suppressors on my 807 plates as shown in the diagram. There are a few things I have done differently than the diagram. I have my 807 plate current meter in the B+ line with a .01uF capacitor across the meter. (The diagram has the meter in the 807 cathodes.) I do not have a 750v transformer. Mine is 650v for the high B+. The crystal I am using is a HC-6 crystal. The diagram uses a FT-243. I do not have the grid current meter, 6800ohm, 270ohm, or 10k pot installed. I am doing all of my testing into a dummy load. Any ideas? Things I could test?
It's years since I've played with valve amps, but if you have parasitic oscillation, I'd look at Grid Stopper resistors.
Obviously the values referenced in the link are for an audio amp, but the analysis remains valid, and should allow you to select a size that suits your cut-off 1MHz frequency.
For example, here is a link to an 807 transmitter with grid resistors at 47R.