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Re: [Amps] 8877 cathode-filament question

To: amps <amps@contesting.com>
Subject: Re: [Amps] 8877 cathode-filament question
From: Ron Youvan <ka4inm@gmail.com>
Reply-to: ka4inm@gmail.com
Date: Fri, 03 Aug 2012 20:48:36 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
   Jim Barber wrote:

> 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?

   The RF Voltage from 100 Watts into 50 Ohms is like 50 Volts, so that may be 
a problem.
You didn't say both are operated as grounded grid.  Are they? What is the F-K 
V. rating?
-- 
    73 Ron KA4INM - All E-mail sent to this address shall linger in the Google 
cloud forever!
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