Bias supply question

[trem]

Page 3 and 6 show the schems I was looking at. How does Mesa get positive and negative voltage at the same node.? Obviously, I have to be missing something. It looks like they supply -12VDC to the power tubes, and +2.3VDC to the PI cathode, all from the same source. What is happening there.?
Thank You

http://www.webphix.com/schematic%20h...aliber_dc3.pdf

[g1]

From what I can see, looks like lots of typos.
Supply point F, -8 at supply, -13 at top of page 5, -8 at op amp page 6 (why would you use +12, -8 for split supply?).
Supply point C, 200V everywhere except 100V page 4 to V1B.