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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