[Mondial]

Steve, where are you measuring the AVC voltage? Is it the actual AVC voltage across the detector diode load (the volume control), or are you referring to the voltage across the shadowmeter coil? The AVC voltage at the detector should be on the order of negative 10 to 12 volts with a strong signal.

The screen voltage is not really significant, in that the AVC action will compensate for the increased gain caused by the high screen voltage. The negative bias generated by the AVC circuit will lower the tube amplification in opposition to the increased amplification of the higher screen voltage. In the end, the IF tube plate current which activates the shadowmeter will remain about the same with a given received signal strength.

You might check those two 110 pF caps at the detector output. They filter the IF freq signal from the detected audio. If they are not attenuating the IF signal, it can cause the raspy audio you describe.

You could also have an IF oscillation, which may cause the poor audio. Does the radio tune in a station symmetrically from above and below the center carrier freq? That is, as you tune to the station from below its frequency, does it sound the same as when you tune down to it from a higher freq on the dial?