Creating a bias supply

[jon]

I am working with a pt that is designed for voltage doubling and has no ct. I need to add a Bias supply for my next project. Any suggestions.

[Phil_S]

Here is an example, the bias circuit from a JTM45, which is taken from one leg of the HT secondary. Ignore the center tap on the diagram, as it isn't relevant. Depending on the output voltage you require, you may want to adjust the first dropping resistor, which is often something between 150K and 220K. Then you can adjust the range by playing with the combination of the 25K pot, 47K, and 15K, which are just a simple variable voltage divider. The caps can be smaller, 8uf, 10uf, or 16uf, 100v or 150v rated to be safe and keep the physical size of the caps to a smaller form factor.

[jelle]

Derive from the junction of cr1 and cr2?

[Phil_S]

Yes, you need to take the supply from one leg of the HT secondary as VAC, that is, before the pair of solid state rectifier diodes. The bias supply circuit has it's own diode configured as a half wave rectifier.