Almost like with meds, any improvements I make come with less then desirable side effects. Here are my latest developments:
First, I completely re-arranged AC power input. Twisted pairs inside of grounded shielding sleeve. This made no improvements in hum but it looks better.
Then I temporarily installed sheet aluminum grounded shielding divider. I expected this divider to shield leading choke inductance ripple from OPT outputs and NFB and ground wires.
No improvement noted on the scope.
Next, I doubled capacitance out of rectifier and powered up with variac. Output is a lot quieter through speakers. Hum is almost not there even with ear next to woofer, and looks twice cleaner on the scope. Before and after scope screenshots attached below. 5mv/div. leading choke side still shows higher ripple but not audible, so no concern, I guess.
Obviously, additional capacitance bumped DC voltages. I'm achieving proper DC voltages at approximately 115vac input. My wall voltage varies between 120 and 122.5v.
I already have two CL-80 limiters installed to control filament voltage. One limiter on Neutral and another one on Line.
I tried increasing Drain dropping resistor in EFB but results are not significant within reasonable resistance increase. I tried up-to 2.5k from Dave's spec 1.8k
Here are options I know to consider:
1. Build bucking transformer. I don't know how to properly calculate what transformer I will need.
2. Replace CL-80 (3A/47ohm) limiters with MS22-12104 (4A/120ohm)
https://www.mouser.com/ProductDetai...=sGAEpiMZZMuqZeNK75brDxRgT1KUh2F4xWvLBVTVbw4=
This will bring voltage down but will generate more heat inside chassis.
3. Resistor out of rectifier. Probably the least desirable option due to heat generation and sag under load that was the reason to adapt EFB to avoid that.
Are there any other ways to keep B+ no higher then 455v?
Thanks