You mention six secondary windings, but I only see 3 connections
to the transformer on your schematic. The schematic looks like one
of these:
http://www.ipes.ethz.ch/ipes/Thyrist.../e_ThyrB3.html
...but the description (six secondaries, interphase transformer)
sounds more like one of these:
http://www.ipes.ethz.ch/ipes/Thyrist...ke/e_saug.html


I am not sure whether this describes your Cyber Tig, , but I have
often seen see welder transformers that consist of a 3-phase wye
secondary (which may or may not bring the center tap out) and a
eperate 3-phase delta secondary, each driving 3 diodes or triggered
SCRs. The the delta and wye are 60 degrees phase shifted from each
other. The advantage of this is that the rectifiers/SCRs send twice
as many pulses per cycle to the capacitor, lowering the ripple
amplitude, increasing the ripple frequency, and improving the power
factor -- which is the main reason for doing it in a welder.

--
Guy Macon
http://www.guymacon.com