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[AMPS] Big Tetrodes at UHF

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Subject: [AMPS] Big Tetrodes at UHF
From: G3SEK@ifwtech.demon.co.uk (Ian White, G3SEK)
Date: Mon, 22 Mar 1999 00:29:19 +0000
Peter Sundberg wrote:
>
> If a tube is "gassy" and has internal arc-overs, will these also 
>occur without screen voltage, i. e. all other voltages, HV, G1and 
>filament are on ? 
>
As I understand it, gas-induced arcs wouldn't the tube to be biased on,
because the arc doesn't need electrons in order to get going - it only
needs HV and a source of material to ionize. The arc then makes its own
conducting path like a miniature lightning flash.

Possibly I didn't make myself clear (also had an e-mail from Rich) that
I wasn't talking about tubes that are "gassy" in the normal sense of
being leaky. An arc would only need a very small, very brief release of
gas... if indeed that is the mechanism involved... nobody really knows.

Regarding the YL1050, LF parasitics seem much more likely as you say.
W7QX noticed something similar with the GS23b tetrodes at UHF, with
continuous oscillation that could be identified as HF or LF resonances
involving the external power leads as inductors.

The real mystery is when any form of oscillation only happens
occasionally.  "Continuously" and "never" are both easy to understand,
but "hardly ever" is tricky to explain!

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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