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[Amps] 8877 cathode-filament question

To: "Amps@contesting.com" <Amps@contesting.com>
Subject: [Amps] 8877 cathode-filament question
From: Jim Barber <audioguy@q.com>
Reply-to: audioguy@q.com
Date: Fri, 03 Aug 2012 17:15:09 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Looking at 8877 HF amp schematics published on the net I came across two 
designs that
raised a question: The two I'm talking about are the Henry 3KD Premiere, 
and a QST article also
for a single 8877. (don't know the date, sorry)

(1) In the Henry schematic, the filament is powered through a RFC, but 
the filament transformer has
no grounded center tap. There is also no apparent circuit between the 
filament and cathode, as the filament
appears to float above the chassis.

(2) In the QST schematic, the filament is powered directly from a 
transformer,
(no RFC, just bypassed) but again with no grounded secondary center tap 
or other
obvious circuit between the filament and cathode.

My problem with these is simply that with the filament floating I can't 
see what keeps the
filament-cathode potential from exceeding a safe value, whatever that 
might be. I remember
an article somewhere stating it shouldn't exceed 150 volts, but I can't 
find it and memory
is a tricky thing. The newer CPI, older Eimac and the Penta Labs data 
sheets don't discuss
it that I've seen

Comments?

Thanks,
Jim N7CXI

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