DIY tube synth from Eric Barbours site

[Manfredsmirror]

Hello all,
I am constructing Eric Barbour's DIY tube synth modules from http://www.cgs.synth.net/tube/index.html. I have constructed his power supply that is described, but I noticed that two of the modules the VCA, and the VCO have connections for -150 VDC along with the +150 VDC. Please excuse my newbieness but where does the -150vdc come from? Do I have to build a different supply? I am probably just missing something but any light that someone could shed would be greatly appreciated.

Thanks everybody,
David