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Re: [Amps]  Runaway Plate Current

To: <>
Subject: Re: [Amps]  Runaway Plate Current
From: "Jim Garland" <>
Date: Mon, 30 Dec 2013 09:29:49 -0700
List-post: <">>
I've not given it a lot of thought, but I'd think a cathode resistor in a
tetrode amplifier may be problemmatic. A residual cathode current (near
cutoff) through a large cathode resistor lifts the cathode to a positive
voltage with respect to ground. That effectively reduces the screen voltage
by a similar amount and also makes the grid more negative with respect to
the cathode. Both effects tend to push the tube toward cutoff. What I'm
unsure about is whether the amp might be prone to instability because of the
large cathode resistance, even near cutoff There's no problem with triodes,
but the higher voltage gain of tetrodes could possibly pose a problem. The
cathode resistor doesn't really cut the tube off entirely, but makes it
hover near cutoff. 
Jim W8ZR

> -----Original Message-----
> From: Amps [] On Behalf Of Paul Baldock
> Sent: Monday, December 30, 2013 8:35 AM
> To:
> Subject: Re: [Amps]  Runaway Plate Current
> Can you bias a Tetrode off like a Triode with a large resistor in the
> - Paul
> At 02:03 AM 12/30/2013, you wrote:
> >I agree that biasing the control grid off is better.
> >
> >Because of negative screen current, an active (rather than VR tubes
> >or zeners) shunt regulator is best, and not difficult to do,
> >although the heavy bleed current approach worked.
> >
> >Question: Why do these modern ceramic tubes show negative screen
> >grid current under some conditions? What is the mechanism that
> >causes it? Straight thermionic emission from the screen seems
> >unlikely, especially as a gold sputter would effectively prevent it.
> >
> >
> >73 es HNY
> >
> >Peter G3RZP
> >
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> >Amps mailing list
> >
> >
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