1955 RCA 6-XD-5 Help Needed

[RJMiranda]

The idea behind the tapped OPT was to use a small filter condenser, and "cancel out" the hum in the transformer, because the residual ripple over the B+ flows in two directions in the primary: from the center tap UP to the 50C5 plate, and from the center tap DOWN to the other tubes´plates. If the 50C5 plate current is of the same order than the sum of the other tubes´plate currents, the flow in both sides of the primary would be about the same, and the two opposite flows of the hum components would cancel each other because they would create similar 60 Hz flows in the xfrmr iron but in opposite directions.
Somehow it is not working in your radio.
When you have hum in a circuit like this, the lower part of the primary could have a short (total or parcial), or somebody could have reversed the wires to the plate and the 1200 ohm resistor (both windings are usually different, with the tap off-center).
The 200uF cap may shorten the life of the rectifier tube.
By the way, without the 1200 ohm resistor, the second half of the primary would short to ground the audio frequencies coming from the plate. Looking from the 50C5´s plate, both halves of the primary work as an autotransformer (the plate winding would be the primary and the lower half the secondary) but both ends of the secondary would be grounded by filters, and the OPT tube would have a bad time trying to supply audio that was being shorted at the other end. The resistor separates one end of the secondary from the 30uF cap (and lowers the plate voltage to the earlier stages).