voltage doubler
 

To elaborate just a bit on Phil's correct answer:

Hot to hot = doubled voltage.
Hot-to-ground = half-voltage.

https://img.bhs4.com/e7/1/e71df333b1...e9a5_large.jpg

One side will be - to ground +.
One side will be + to ground -.

To see this in action - exactly this - look at the filament voltage schematic for a Dynaco PAS. As these are all dumb components, loading does not change anything - to the capacity of the source in any case.

Peter Wieck
Melrose Park, PA