Bias Supply

This is how crazy easy this is. I bought a Triad VPL36-140, which is a tiny transformer that you can wire for either 18VAC or 36VAC.

Wired for 18VAC, I get ~24VDC. I'm not accurately simulating the load current, but 18*Sqrt(2) minus a couple volts for drop across the diode = yep.

Wired for 36VAC, I get ~49VDC.

Since I need only around -21V to bias the 7591s in Bodie, I'm thinking the 18V route is the way to go. Sure, even though I calculated the source impedances for both configurations (secondaries in series or parallel) I'm not sure I trust PSU Designer to accurately predict the load regulation on a tiny little 5VA transformer, especially considering the load current is going to be practically nil. Even assuming ~20k for a bias pot and a resistor to make sure the pot can't dump all the V- to ground, I'm still getting ~23VDC.

Merlin has R1 as 100k in his power supply book, which is probably more realistic for ripple reduction. I'll have to balance that against how long it'll take the bias supply to reach a stable voltage. Considering this is a full-wave bridge rectified supply, the ripple will be at 120 Hz which is much easier to filter than the standard half-wave rectifier in most guitar amps which pumps out 60 Hz ripple.

And hey, finally a little circuit where I can buy almost all the components at Rat Shack!