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[AMPS] RF filament choke for indirectly heated cathodes?

To: <amps@contesting.com>
Subject: [AMPS] RF filament choke for indirectly heated cathodes?
From: 2@vc.net (measures)
Date: Wed, 21 Jun 2000 09:39:11 -0700
>
>I've noticed something strange in amp articles
>featuring indirectly heater cathode tubes like the
>3CX800A7 or 8877.  Some have a bi-filar choke in the
>filament line, while others feed the filament directly
>from the filament transformer.  A variation of the
>latter is grounding one side to provide a return
>through the chassis, and bypassing the live side to
>ground with a .01 uF disk (Ten-Tec does this on the
>Titan).`What's the best approach?
>
?  I have seen several kaput 8877s that arced between the cathode and 
heater during some kind of glitch.  //  In my opinion the safest approach 
is to not stress the (unrobust) cathode/heater insulation. .   In other 
words, put a jumper between the cathode and the heater, and use a heater 
choke - the same as one would do for >>30MHz operation.  / Also:   Do not 
ground - or bypass to ground -  either side of the heater.  /  Use glitch 
diodes in the negative hv return .  /  Use a glitch diode in the hv 
positive lead.  On the other hand, if you have a free supply of tubes, 
the safe approach really doesn't matter.  

cheers, Kevin

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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