[Amps] Ferrite Rod

[Peter Chadwick]

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