Tube transmitter tuning
I think this is normal operation for a screen modulated AM linear amplifier.  The output of the amp for an unmodulated carrier should be 1/2 power.  Applying full amplitude audio should cause the 807s power to cover the range of zero power to full power.  The average power is still half the maximum power.

By being a class-A amplifier, a resonant cutcuit on the output isn't needed.