PT centertap for bias supply?

[dms]

Can anyone please take a shot at explaining this?

If you look at the common Trainwreck Express schematics, you will usually see the bias circuit feed coming from one leg of the PT's high voltage secondary winding; other schematics, like "A0" and "A2" show the feed coming from a centertap on the PT, but in those cases, that "centertap" appears to not really be in the center at all, furthermore, there is a second centertap shown and it IS grounded...

Here finally is my question: if I want to run the bias feed in this way, do I need a special kind of PT that has an extra HV winding tap that is slightly off-center, or can I take the bias feed off the UNGROUNDED centertap?

Thanks, Davids

[mhuss]

The extra tap is called a bias tap, and is typically 40-60vac with respect to the center tap. The other way of doing bias (just dividing down the full secondary voltage) works just as well, and doesn't require a special transformer.

--mark