| Will, according to your first post, you state that the electrolytics are not connected to the chassis. I take this to mean that neither of the 8mfd caps are connected to chassis. However, the schematic shows that one of the C13 caps installs to chassis ground, making it a conventional power supply. If the negative side of this cap is connected anywhere else, then things just ain’t gonna work like they should. Maybe this is the cause of your strange voltage readings. Perhaps the speaker field may be confusing you somewhat. As with many radios of this time period, the speaker field also serves as a choke for the power supply filter. This choke does the same thing, regardless if it is installed between the caps in the B+ or in the negative center tap of the power transformer. Manufacturers did them both ways. In the case of your Zenith, this part of the circuit supplies the bias or negative voltage to the output tube grid. Therefore, this voltage should be negative when measured from chassis ground. You did not state where you are measuring your DC chassis voltages from, but all B+ should be measured from chassis ground.
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Will Hodges 
TonyC 