[kiwi_steve]

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.

Sorry, yep I'm measuring across the shadowmeter... another casualty of my brain having been staring at this too long... I need some fresh air and sunlight

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.

Good point - although my concern with the high screen voltage was more that it was about 25% over the max rated running point of the tubes. This all happened in about a week on the bench... it wasn't that high when I started, but the big power dogbone that made up half of the voltage divider decided a change was as good as a holiday... and it needed a holiday

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.

I replaced these a few days back - although not with micas, just high-reading poly film 1nF caps. How critical are the values here? I'm assuming this is a PI filter arrangement. If I calculate the filter frequency of a 110pF and a 20k resistor it comes out to around 72kHz... I suspect I'm looking at this wrong though? I do have some micas coming by the way - just have none on the shelf at present... Also, I used a 22k resistor - if that value is critical I can throw a 220k resistor across it...

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?

It seems to, but I will check this later today...

Thanks for all the help - I'll leave it on the bench for now and see what I can find.