Tom said:
>We *never* want to do this with high current filaments. The
reason being nothing cancels magnetizing flux caused by the
many ampere turns of the single winding and the core often
saturates. The result is at best a loss of RF choke
impedance at the filament, at worse it will introduce hum as
the filament current modulates the reactance causing large
reactance swings at a 120Hz rate (two impedance nulls and
two peaks per AC cycle).<
I was looking into filament chokes last week for my 4-250A amp rebuild. Here,
I'm running passive grid, and the filament chokes are there to allow the
filaments to be held off ground by a 20 ohm resistor for NFB. (Not necessarily
the best way of NFB, as it raises output impedance, but it works, and is
relatively broadband). I used a bifilar winding of 4 turns on a Siemens N30
grade pot core about 1.25 inches diameter. This gave (from memory - the actual
numbers are in my notebook at home, and I'm at work)something about 200
microhenries with no DC, dropping to about 70 microhenries with 1 amp -
measured at 1.8MHz, using my GR bridge. I figured this was probably acceptable,
since the choke is shunted by 20ohms in series with a 0.01 microfarad. But the
effects of the DC plate current probably needs considering on the bigger tubes
with ferrite rod chokes. I vaguely remember a big GG stage (100kW or so) that I
saw that used parallel copper bars, about 3/16 inch thick and and 3/4
inch wide, and large 'u' shaped pieces of ferrite around them, with quite a
lot of spacing. I'll have to look up the description of the STC 100kW tx in the
1963 IEE HF Convention proceedings and see if there's any info there on what
they did for filament chokes. I know the rotary tank coil was tube with cooling
air blown down it.....
73
Peter G3RZP
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