Re: [Amps] toroid filament choke?

[Tony King <amps@w4zt.com>]

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



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