[morzh]

Peter

Yes, except instead of the ballast resistor I usually use a capacitor. This will turn the radio in the AC only set, but then I do not intend to use it with DC outlet. I converted a few radios this way.
This worked especially well on those radios that had the infamous "curtain-burner" power cord, with asbestos-insulated 3rd wire that was a ballast resistor and droped about 30W of power to feed filaments. When replacing it, it usually would take 6 to 8uF cap to drop the needed voltage, and without any power dissipation.

In this case this will work like this:

I need 100mA, so I need 1.2K total impedance.
Then the 1,200-squared is 1,440,000. That less 810,000 is 630,000 and the root of that is 794 Ohm.
This impedance gives us a cap of 3.35 uF.

I could make something like that or buy a single 3.3uF cap. Like this one.

https://www.tubesandmore.com/products/ca...ypropylene

I have 3.3uF 100V cap. It needs to drop 80V (those who cannot make sense how 80V and 90V make for 120V, well, this is reactiove and active loads, so the actuall formula is the square root of the sum of their respective squares). And if it gets shorted, the voltage across the 90V-rated load will be 120V. Not catastrophic. Should do.