[Amps] toroid filament choke?

[Will Matney]

Gerald,

Some heaters are quicker than the others that I've seen. One must 
remember in indirectly heated cathodes, the heater acts more like a 
resistance wire space heater used in ones home than a filament in a bulb 
or a directly heated cathode. Your correct, in directly heated, they 
heat up right now. In the indirectly heated ones, I've seen them take 
several seconds to build up to the bright orange color. Maybe not a full 
minute but it took a while. So, the colder heater would draw more 
current than the hotter one would. What I'm figuring, you could use a 
small enough wire that either the surge would open the wire like a fuse, 
or get it hot enough to ruin the insulation. 20 gage wire is pretty 
small stuff and I've seen it melt insulation off pretty darn quick in 
instances where there was a low ohm short. A lot of the magnet wire 
being used now is like Sodereze (low temp insulation). It was designed 
so that the heat from a soldering iron will strip the insulation off. 
Mostly, all the small rolls of magnet wire you see at electronics 
dealers is like this. What I was saying may be over-kill, I'm not sure. 
Just from experience with the wire, I just wouldn't run really small 
wire for that use.

Will Matney

TexasRF at aol.com wrote:

> The filament transformer will limit the current to a large extent. 
> Also, the resistance of the filament or heater will rise very quickly 
> when the voltage is applied. For example, the TH347 filament reaches 
> rated current in just a few milliseconds. I haven't measured the 
> current of an indirectly heated cathode tube but you can see the color 
> of the heater turn red almost immediately when voltage is applied, 
> suggesting that the resistance has increased substantially.
>  
> 73/k5gw
>
>
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