[AMRadio] Filament choke

sbjohnston at aol.com sbjohnston at aol.com
Wed Apr 29 22:45:13 EDT 2009

Tonight I ransacked my parts bins and found a 3/8" dia ferrite rod 
about 4 inches long.  I covered it in two layers of quality electrical 
tape, and wound a bunch of pairs of turns from two spools of identical 
enameled magnet wire about #18 or#20.  Covered about 3 inches with 
close turns, with a half inch open on each end.  Secured the wire at 
the ends with superglue, then covered it all in a sleeve of heatshrink 

Each winding measured about 135 uH on my LC meter, with about 0.2 ohms 
of DC resistance.  I then put it across the high-Z output of my URM-25 
signal generator and tuned the spectrum.  I saw no dips anywhere, and 
the coil seemed to load the output down about the same amount as a 2k 

I also tried putting the coil in series with various resistance values, 
and at 1.9 MHz it appeared that the voltage division was even when the 
resistance was around 1.5k ohms.  The calculator says that a 135 uH 
coil would have a reactance of 1611 ohms, so this data all hangs 

I think I've got a bifilar filament choke suitable for my untuned input 
160m amp.

Steve WD8DAS

sbjohnston at aol.com
Radio is your best entertainment value.

-----Original Message-----
From: sbjohnston at aol.com
To: glowbugs at piobaire.mines.uidaho.edu
Cc: amradio at mailman.qth.net

Sent: Wed, 29 Apr 2009 5:48 pm
Subject: GB> Re:  Filament choke

Excellent ideas today, guys - thank you very much.  I 've got 100 watts
+ of drive, so I don't believe I'll use a tuned input in this design,
instead I'm going to swamp the input to the cathode with 15 1.5k 2W
resistors as was successful in the SB230.  So the
cathode/filament/heater power circuit just needs to look open to the RF
but pass the low frequency AC to light it up. 

Steve WD8DAS 

sbjohnston at aol.com 



Radio is your best entertainment value. 



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