Date: Wed, 3 Jan 2018 14:51:23 -0500
From: Peter Bertini <radioconnection@gmail.com>
To: Amps@contesting.com
Subject: [Amps] Toroidal filament choke and DC current
<I'm building a four tube 813 amplifier and I am planning to use a 240-43
toroid core for the filament choke.
I've seen some concerns about core saturation due to the DC cathode current
involved in this application.
I was planning to add a third winding (trifiliar) to allow bringing the
filament transformer CT back to the top of the windings, thus cancelling
the DC field. I'd have to add a second ferrite rod choke to fully decouple
the third winding, but it would be a trivial task.
I was also going use the core to provide a bifiliar winding for
neutralization, if needed. All windings are 7 turns, which yield about 50
uH and over 500 ohms Z at 160 meters. Core is wound and ready to go.
Any advice, encouragement, or warnings?
Pete k1zjh
## A looong time ago I had a hb 4 x 813 GG amp. But it used a .5 inch
x 7.5 long ferrite rod for the bifilar. Wound with 12 ga magnet wire to handle
the 20 A CCS of fil current. Then a 10 vac at 20A fil xfmr, with CT.
Zero issues
with rod saturation with the dc component. Of course there will be zero issues
with the
20A component, since current is always flowing in opposite directions due to
the bifilar
design. The magnetic fields cancel.
## What bands are u using it on ? 50 uh is more than ample even for 160m.
A buddy has a 3x6 metal tube in GG..and 24 uh used for the fil choke...zero
issues
on 160m. The 3 Amp dc current component is also a non issue. But in this
case, 2 x
identical bifilar rods are used. Any one winding of rod A..is in parallel with
any one winding
of rod B.. This is done at the tops and bottoms of each bifilar rod assy.
8 gauge magnet
wire used. Uh does not drop exactly in half either, but a little over one
half..go figure. Done
this way, since 8 gauge magnet wire will handle 40A CCS..and 2 of em in
parallel will handle
the 80A CCS required by that tube.
## I use the identical setup on my hb 3CX-3000A7 fil assy, except 10 ga
magnet wire used.
10 gauge will handle 30A CCS, and 2 in parallel will handle 60A. Tube fil
sucks 50A CCS.
Plate current is between 1.5 to 2.5 A. I have also seen similar setups used
on even bigger tubes,
with 5+ amps of plate current
## IMO, I cant see even a single 240-43 core saturating from the plate
current of 4 x 813s, but dont
quote me on that, as I have no experience using a 240-43 core as a bifilar. A
buddy used type 43
torroid cores on his hb 3CX-3000A7 fil setup, using 6 gauge magnet wire.
Dunno how big in diam the
core was, or how many used. Dunno if it was used on 160m or not.
## You could always use 2 x 240-43 cores in a stack, and even less than your
7 turns, to get the same uh.
provided you have the room for the 2 x stacked cores.
Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|