True, the total number of electrons increase, but their charges are opposite
polarity.
Secondary emission in a tetrode can occur when the anode voltages drops
below screen voltage.
In that case the electrons bounced from the anode flow in reversed direction
to the screen. Their number can become much higher then those intercepted
by the screen and which were charging the screen positive and let positive
screen current flow.
When that reversed electron flow overrides the number of positive charged
electrons on the screen the electrons flow from the screen through the
screen supply and / or bleeder to the cathode which is connected to -screen
and ground as a negative screen current.
In the constant curves of a tetrode you see the kink in anode current where
it happens.
hny
Peter
-----Original Message-----
From: peter chadwick [mailto:g8on@fsmail.net]
Sent: Montag, 30. Dezember 2013 23:22
To: Peter Voelpel; amps@contesting.com
Subject: Re: [Amps] negative screen current
Peter,
Please explain then how electrons from the electron stream cathode - grid 1
- grid 2 - anode that are intercepted by the screen grid are different to
those that hit the screen grid because they were derived from secondary
emission at the anode?
Both cases increase the number of electrons intercepted the screen grid, not
decrease it.
73 es HNY
Peter G3RZP
========================================
Message Received: Dec 30 2013, 09:41 PM
From: "Peter Voelpel" <dj7ww@t-online.de>
To: amps@contesting.com
Cc:
Subject: Re: [Amps] negative screen current
The negative screen current is caused by back bombardment of electrons from
the anode to the screen grid
73
Peter
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of peter chadwick
Sent: Montag, 30. Dezember 2013 20:32
To: Jim Garland; gdaught6@stanford.edu; amps@contesting.com
Subject: Re: [Amps] negative screen current
I too have a problem with this explanation. Electrons gathered by the
screen
are 'positive' current flow, no matter where they come from: 'negative'
flow
must surely be electrons leaving the screen. Or what am I missing?
73 es HNY
Peter G3RZP
========================================
Message Received: Dec 30 2013, 07:00 PM
From: "Jim Garland" <4cx250b@miamioh.edu>
To: gdaught6@stanford.edu, amps@contesting.com
Cc:
Subject: Re: [Amps] negative screen current
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.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of
> gdaught6@stanford.edu
> Sent: Monday, December 30, 2013 11:00 AM
> To: amps@contesting.com
> 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
some
> 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
>
> http://www.r-type.org/articles/art-024.htm
>
> 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
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
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