Peter,
Henry made filament chokes using copper tubing and what looked to be the
center insulation and conductor from RG8U slid into the center of the
tubing. This tubing was then formed into a coil of maybe 8 turns of so. The
copper tubing and the center conductor formed the choke. This was on their
old 13.56 MHz 3cx3000a7 RF generators. Just thought I'd throw that one in.
Larry
_____
Peter Chadwick wrote:
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.....
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