Pete, the TL-922 has the 3-500Z filaments in series and the flat ferrite
flat bar filament choke is quite small and 22uH. but I have no idea of its
ferrite specs. That amp makes full power on 160 with ease. If you have
access to a 20 or 40V transformer or a small Variac the wire size drops
dramatically.
You could then use a pair of 4" x 1/2" 33 mix rods taped together. The mu
is 800 and I have plenty on hand at 1990 prices.
Or at 10V use the classic 7.5" x 1/2" rod and #14 wire.
Many amps use the filament choke as part of the L for the 160 or 80 input
network. The RF only "invades" a part of the choke which is also bypassed at
the cold end so noRF gets past that point in any case.
Just a couple of ideas.
I have also used ferrite rods with a neutralizing winding added to stabilize
twitchy Heath and other amps on 10M.
With a good layout and the 813 shells grounded they should be stable on
10M....at least mine was (-;
Carl
----- Original Message -----
From: "Peter Bertini" <radioconnection@gmail.com>
To: <Amps@contesting.com>
Sent: Wednesday, January 03, 2018 2:51 PM
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
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