[Amps] Toroidal filament choke and DC current

[Peter Bertini]

I'm building a four tube 813 amplifier and I am planning to use a 240-43
toroid core for the filament choke.

I've seen some concerns about core saturation due to the DC cathode current
involved in this application.

I was planning to add a third winding (trifiliar) to allow bringing the
filament transformer CT back to the top of the windings, thus cancelling
the DC field.  I'd have to add a second ferrite rod choke to fully decouple
the third winding, but it would be a trivial task.

I was  also going use the core to provide a bifiliar winding for
neutralization, if needed.  All windings are 7 turns, which yield about 50
uH and over 500 ohms Z at 160 meters.  Core is wound and ready to go.

Any advice, encouragement, or warnings?

Pete k1zjh