[Amps] toroid filament choke?
Tony King
amps at w4zt.com
Thu Oct 28 05:33:25 EDT 2004
First I want to thank each of you that responded with your valuable
opinions on the use of the toroid to make a filament choke for the GS-35B.
The overwhelming opinion is to use the toroid if you want to.
The majority of you have taken the issue down to the question of wire size
limited by heating due to the filament current. The GS-35B filament
requirement being 12.6 V at 3 Amps will allow me to use wire at least as
small as #16 and possibly smaller. In the case of this tube, the choke
will be in the cooling air stream so technically it could be much
smaller. I liked Will's comment about building things like a tank because
I often do the same thing. That said, #18 or #16 wire is small enough to
keep the choke in a smaller package and safe from resistance heating.
Opinion places the choke inductance between 9 and 15 uh to safely cover 1.8
MHz on the low end. Since the toroid keeps the magnetic field contained,
the smaller cathode compartment is easily accomplished. This is a plus for
new construction but even more so for the re-builder/upgrade project.
Great discussion of the way the filament types respond to inrush current.
Whether it is a directly or indirectly heated cathode, the filament will
experience stress due to the surge of inrush current. Physical changes
internally due to heating and magnetics are limited by raising the filament
voltage to a nominal level, say half, then waiting a few seconds to raise
it to full filament voltage. This is good practice whether the cathode is
indirectly heated like the GS-35B at 3 amps or directly heated at 21 amps
like the 4-1000. Using a timer to allow the filament/cathode temperature to
properly stabilize before applying anode voltage is a good practice. I
hadn't thought much about the full process of heating the cathode until
now. Even with the current stable, the cathode takes more time to come up
to temp and longer for the indirectly heated one. Thanks!
I appreciate the tips about using silicone-varnished wire over other types
that may be available or on hand. Hopefully the issue of heating the wire
in the choke wont be an issue in this case but it is certainly something
that we should all be aware of.
Thanks again. I'll update you on the filament choke and other parts of this
thing as it is finished.
73, Tony W4ZT
More information about the Amps
mailing list