1951 Zenith 16" Porthole

[dieseljeep]

Originally Posted by Electronic M

Now this tells us something. Some sets used the audio output tube as part of the B+ voltage divider (instead of a dropping resistor they used a tube to put that energy to work). On sets with that form of stacked B+ if the audio output stage is miss-biased or otherwise not working correctly it will reduce B+ to other stages that it is the B+ dropper for. I recommend voltage checks in the audio out stage with the volume low and high...You may also want to scope the lower B+ rail that comes from the audio output cathode to see if it is not filtering well.

I honestly, don't remember Zenith using stacked B+. I'll have to consult my Rider's CD on the older models.
RCA used it on their lower-end models in the mid-50's, but not as much as some of the other makers.