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Re: [Amps] B&W FC-30 Filament Choke: EPILOGUE

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Subject: Re: [Amps] B&W FC-30 Filament Choke: EPILOGUE
From: "Tom W8JI" <>
Date: Sun, 4 Mar 2007 11:20:38 -0500
List-post: <>
> Hi All, is there no possibility of having the input 
> matching network
> resonate with the FC30 filament choke to allow 160m 
> operation?

There is a possibility but a 4-1000A would be one of the 
worse possible cases for using that choke.

> With the low impedances present it seems that the loaded Q 
> would still be
> fairly low. Seems that the rf related currents would be 
> much lower than the
> filament currents in the choke.

Filament currents, assuming the choke is wired and wound 
correctly, do NOT magnetize the core. The net flux density 
from filament current should be zero or very close to zero.

Common mode currents which are those caused by RF cathode 
voltage do magnetize the core. The flux density  at any 
given frequency relates only to cathode RF voltage swing and 
choke impedance.

If you have 100 volts across the winding and 500 ohms 
inductive reactance RF current is 0.2 amperes.

The resistive portion of the impedance dissipates power in 
the core and creates core heating. Saturation is where the 
core can't handle any more magnetic flux and so any 
additional flux cause by increased current has to travel in 
a path outside the core, and so there is a change in 
permeability so far as additional current beyond a certain 
value. This is why a core **usually** overheats long before 
flux saturation and severe waveform distortion occurs. 
Amateurs almost always call heat "saturation" when it isn't. 
We can have a core that distorts the waveform and does not 
get hot, or we can (and usually do) have a core that gets 
hot without waveform distortion (saturation).

> What parameter actually establishes the lower frequency 
> rating of choke  like
> this?

Heating, flux density, impedance (can we still match the 
input efficiently?), and distortion of waveform from 

The fact it "tunes" or "matches" and doesn't heat or 
outright explode doesn't mean it is a good core or a good 
choice. The fact it carries 100 amperes of filament current 
doesn't mean it can carry even 100mA of RF current.

Eddy's case is particularly bad because:

1.) The B&W choke, while handling filament AC ok, was 
already a marginal RF package

2.) He would have to parallel two chokes in one case. That 
makes things worse

3.) The 4-1000A in cathode driven service has pretty high 
filament RF voltage.  The cathode impedance is fairly high 
and the drive power requirements are high.

With the very easy access to good rod materials, good toroid 
cores, or complete chokes (like an Ameritron filament choke) 
it really doesn't make much sense to use a B&W stack of 
1960's Japanese transistor radio loopstick antenna cores or 
a TV deflection yoke core designed for 15.75 kHz use.

The 4-1000A is already power gain marginal, has fairly high 
IMD levels, and is notoriously unstable. Why risk making 
something so marginal worse?

73 Tom

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