[AMRadio] Filament choke

Bernie Doran qedconsultants at embarqmail.com
Wed Apr 29 17:16:30 EDT 2009

consider copper tube with a copper wire down the center, some have used 
copper with the varnish used for motor winding etc, I think I would go with 
THHN insulation on a 16 gauge wire. Deburr the tube and push the wire 
through the center then wind the coil for the necessary inductance, it 
becomes the tuned circuit for the filament.   I would use a Q of about 5 - 
10, the coil should not be too big, will take a lot of C to resonate.  if 
the cathode Z is high, you can tap the coil to match your driver, or wind a 
separate winding to link couple in and match. Of course the transformer end 
is heavily bypassed to gnd.    Solder tabs on the end of the tube before 
running the wire in it!!!  If you really want a choke, I would think  a 
choke would not be difficult with that low amperage, just wind a bunch of 
turns on a toroid and check the inductance, but you still need a tuned 
circuit, that why I suggested the  wire in the copper tube.     Bernie W8RPW
----- Original Message ----- 
From: <sbjohnston at aol.com>
To: <amradio at mailman.qth.net>
Sent: Wednesday, April 29, 2009 2:32 PM
Subject: [AMRadio] Filament choke

> I'm building a 160m grounded-grid amp around the GI-7B tube
> (it worked so well in my SB-230 retrofit project:
> http://www.wd8das.net/SB230.html    )
> Anyone know of a source for a filament choke appropriate for this tube?
>  The filament draws only 2 amps, so the 30-amp ones I see available
> online are really overkill (and awfully large).  But for 160m I'm
> thinking that a somewhat higher inductance than might normally be used
> for 80-10m amps might be in order.  Say >50 uH?.  Bifilar-wound would
> be best, but two separate chokes would probably work too.
> Should I not find a ready-made source, then I guess I could make one if
> I find some ferrite rod suitable for use at 1.9 MHz.
> Thanks...
> Steve WD8DAS
> sbjohnston at aol.com
> http://www.wd8das.net/
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