the basic concept is that the whole circuit,preamp and output is referenced to the center tap of the transformer secondary,and probably connected to chassis at some point,maybe the input,or the power amp,sometimes through a loop breaker,that can be a resistor,with a combination of cap and diodes.
In the schematic this arrangement doesn't appear,but i guess there should be some kind of connection.
In the schematic this arrangement doesn't appear,but i guess there should be some kind of connection.