[Conelrad]

Some circuits will jack up the center tap (read negative return) of the power transformer off chassis ground through a resistor network to get a bit of negative polarity bias voltage for final amp tubes, or sometimes the AVC function.

From your perspective, it is backwards to chassis as the voltage is 'positive ground'. Sometimes this is tricky to discern, and troubling to wrap your head around.

Take a closer look at your schematic and you will see the 'Ohms' I'm talking about.

Dennis