[Amps] Toroidal filament choke and DC current

[Manfred Mornhinweg]

Pete,

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

I just did the maths. Unless I messed up while typing numbers into my 
trusty old Sharp calculator, the choke you describe would run at a flux 
density of 45mT for every ampere of cathode current. This is just a 
first-order calculation, not including nonlinear effects in the ferrite.

Material 43 has a somewhat nasty behaviour with moderate to high DC 
flux, that varies a lot with temperature, so don't try to push it near 
its saturation flux density. But 2A of cathode current would very likely 
be safe with that choke, and I doubt your 813's will pull more than 
that. Probably even 4A would work as long as the core is kept cool, so 
that the more nasty area of that material's behaviour isn't reached.

> 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. 

As far as I can see, that would negate the usefulness of that choke!

> I'd have to add a second ferrite rod choke to fully decouple
> the third winding, but it would be a trivial task.

In that case you could as well wind the whole choke on the rod, and save 
the toroid.

> I was  also going use the core to provide a bifiliar winding for
> neutralization, if needed. 

I can't claim being an expert in the field of tubes, but I understand 
that you can use an 813 either in its intended configuration, with 
grounded cathode, grid drive, and high gain, or in grounded-grid 
pseudo-triode configuration, at low gain. If you use grounded cathode, 
you don't need a filament choke, and if you use grounded grid, you 
shouldn't need neutralization. So I don't think you will need that 
additional winding...

Manfred

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