[Arran]

There is common negative, which is connected to the neutral side of the power line in an AC/DC set like this one, and then there is chassis ground which is connected to common negative through that funky capacitor with the coil wrapped around it. Common negative, or B-, and chassis ground are two different things, the resistor you speak of may have it's case riveted to the chassis, but the resistor itself is insulated from the chassis. and is actually connected in series with the hot side of the power line along with the tube filaments. On some schematics they will use two different symbols to differentiate between chassis ground and B-, one looks like an arrow made up of lines, the other looks like a garden rake, they will sometimes do this to simplify the schematic drawing, an arrow means it goes to B-, rake goes to the chassis, this set actually has the B- or common negative drawn out as a line so the arrow means chassis ground in this case.
Regards
Arran