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
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|