I have a question about the center-tapped AC transformer used in the attached figure and its bias voltage generation. Mainly, my question is how to generate DC supply voltage for the 12AZ7 (V4) plate (ORN - RED/WHT - ORN/WHT inductor).
From the figure, the plate DC voltage of 12AZ7 should be defined by the voltage at the 'RED/WHT' tap of the 'ORN - RED/WHT - ORN/WHT' inductor. But this center tap is connected to another center tap of an AC power transformer shown at the bottom (RED/YEL tap of M-215).
Initially, I thought the current determined the plate voltage of 12AZ7 through a 36 kΩ resistor (18 kΩ + 18 kΩ).
This is because 12AZ7 is used as a cathode follower, its grid voltage is biased at -57 V, and the cathode voltage shall be close to -57 V. Since the DC voltage at the other side of the 36 kΩ resistor is biased at -150 V, the voltage difference between the 36 kΩ resistor determines the current, which determines the plate voltage.
But I thought this argument did not make sense because the plate-cathode current should be the function of anode voltage, not the other way around.
In short, I cannot see how the M-215 transformer (shown in the bottom) center tap (RED/YEL) is creating 205 V.
I would greatly appreciate it if anyone could help me understand this circuit better.