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[AMPS] temperature variable, varistor or globar resistor source wanted

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Subject: [AMPS] temperature variable, varistor or globar resistor source wanted
From: W8JI@contesting.com (Tom Rauch)
Date: Tue, 12 Sep 2000 10:52:27 -0400
> Dont know who manufactures these beasts but RFparts sell them....
> I used two as filament inrush limiters in a single 3-500zg 50 mhz amp ....
> One point to consider in using them is that if the supply is switched off
> and them back on before they go cold they no longer limit the current!!!!
> 
> 73
> Stephen G7EUW

Unless you use a greatly oversized transformer and oversized 
leads and chokes, most inrush worries are a waste of time.

Inrush was a craze in the 70's, like parasitics are today. All the 
problems you see mentioned today with tubes and attributed to 
"parasitics" were attributed to inrush years ago, when that was the 
popular hysteria.

Inrush is more of a problem with large MOX cathode indirectly 
heated tubes, like the 3CPX5000. That tube can draw excessive 
current for many many seconds at turn on. Inrush is also more of a 
problem with series filaments. 

The best thing to do is measure the inrush. In most cases you'll 
find you are worrying about nothing. The exceptions are generally 
when the filament winding is on the plate transformer, or you have 
unwisely oversized the filament transformer and filament wiring and 
wire sizes in the filament choke.

NEVER use a globar unless you have a way of monitoring the 
filament voltage , and if you slow-start a MOX cathode tube be sure 
you extend the warm-up time enough to makeup for the delay in 
heat at the beginning.


73, Tom W8JI
w8ji@contesting.com

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