It is not a center tap on the transformer but its taped towards one end creating a autoformer, yes you do need this tap. The rheostat you will need is a 300 ohm 25 watt.
Primary resistance reads 8 ohms, 4 ohms to tap from either leg. This pot has a 4th terminal at 150 ohms. I can find replacement pots but they don't have the extra terminal. Is there a way to work around this?
Could I tie the transformer tap to the 2 outer terminals of the pot with 2-150 ohm resistors?