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Re: [Amps]  negative screen current

To: <>, <>
Subject: Re: [Amps]  negative screen current
From: "Jim Garland" <>
Date: Mon, 30 Dec 2013 12:00:14 -0700
List-post: <">>
I?ve heard this explanation before but don?t understand it. If electrons are
captured by the screen grid, then that corresponds to current flowing into
the screen grid from the external circuitry (i.e., positive screen current),
no matter whether the electrons are coming from the cathode or the anode.
After all, the electrons don?t carry little name tags that say ?Hi, I?m an
electron and I?m coming from the anode!? 

Seems to me the only way to have a negative screen current is if electrons
are being emitted from the screen grid. I?d think the most likely
explanation would be caused by collisions with the screen by high energy
electrons on their way to the anode from the cathode. If the kinetic energy
of the collisions was great enough, then more electrons could be knocked off
the screen than were captured by it. I?m just speculating,, of course, so
I?m open to other explanations.
Jim W8ZR

> -----Original Message-----
> From: Amps [] On Behalf Of
> Sent: Monday, December 30, 2013 11:00 AM
> To:
> Subject: Re: [Amps]  negative screen current
> Peter asked...
> > 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.
> It is caused by 'secondary emission' of electrons from the anode.  Under
> conditions, more electrons are being gathered (by the screen grid) from
the anode
> than from the cathode.  For a bit more detail without going into the
physics and
> mathematics of the phenomenon, see
> for example.
> I had remembered that this was covered in Eimac's 'Care and Feeding...',
but I can't
> find it there now.  That shows the state of my memory, I reckon.
> 73,
> George T Daughters, K6GT
> CU in the California QSO Party (CQP)
> October 4-5, 2014
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