In this illustration I have used 2x transformers with both primary and secondary windings in series to exhibit a Center-tap. As with any isolated circuit, you must reference ground to the secondary circuit before the software can make it work. Therefore I have connected the ground to the center tap for the purpose of giving the output polar opposites from ground.
Alternately, you can change the number of windings to give your transformer a skewed tap if you so desire. For instance a 12v output for tube heaters and an 85v output for drive B+.
