[Amps] Heater chokes on 1/2" ferrite

[Thomas Hoeppe]

Hello OM's,

in most of the american handbooks they recommend a bifilar choke on a 1/2" ferrite rod, when the tube's cathode is in direct contact with one side of the filament or even directly heated. (3-500Z...) The 8877, of course, has no need for such a choke.
Did anyone check the inductance? What is the permeablility of that ferrite material?

I want to use a square rod with permeability of about 1000. It is a U/I shaped material. With 17 bifilar windings on the bottom of the U, I got 24 uH, and when I close it by using the "I" I measure 230 uH. This is much more then needed on that point. Should I expect problems? The input of the tube is around 50 Ohms (GS35b).

I have already built a linear, which shows less gain on 160m then on 80m. I am afraid some input power on 160 is lost in that choke. Unfortunately I did not check the choke when I made that amp years ago.
Now, I want to avoid that problem in a new desktop project.

Thanks for your comments....Tom, DJ5RE