Peter,
I may have misunderstood yout comment, and if so I apologize, but if not
then I believe you are harboring a misconception about charges and current
flow.
Electrons always have a negative charge, and there's nothing we can do to
them that will change that fact (except annihilate them with a positron,
which is a postively charged electron, and which turns them into a gamma
ray. Or we can combine them with a proton, which turns them into a
chargeless neutron. Neither of these processes are believed to occur in
vacuum tubes.) This means that if electrons are absorbed by the screen grid,
no matter where they come from, then that corresponds to a positive grid
current. The only exception is if the screen grid is floating (not connected
to any external circuitry), in which case no screen current flows and the
screen voltage becomes more negative, thus creating an electric field that
repels the approaching electrons until no more of them accumulate. The
bottom line is that electrons coming from the anode are indistinguishable
from electrons coming from the cathode, so far as their charge, mass, and
spin are concerned. They will differ only in their kinetic energy, which is
basically how fast they are moving.
If the screen grid is grounded, then there is no charge accumulation on the
screen. Electrons striking the screen from the cathode or anode flow
immediately to ground, with no charge buildup. If the screen is connected
to a positive power supply, then in equilibrium there will be a charge
deficit on the screen (resulting in an outward electric field), but this
charge will neither decrease nor increase as cuurent flows into the screen.
73,
Jim W8ZR
---Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Peter Voelpel
> Sent: Monday, December 30, 2013 7:12 PM
> To: amps@contesting.com
> Subject: Re: [Amps] negative screen current
>
> 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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