You have to remember that Wikipedia consist of contributions from people that
don't always have all the
facts.
In most tetrodes the incident electrons that strike the screen grid far out
number the secondaries that
get captured by the plate. However, in some tetrodes there is an attempt to put
the screen grid in the
shadow of the control grid so that little or no electrons strike it. However,
under some combinations of
plate voltage and screen voltage some electrons do strike it but not right on,
you can imagine the shadow
closing in on the screen element. When this happens multiple secondaries are
produced that are not
recaptured by the screen and are attracted by the more positive plate.
Now, whenever an electron strikes a metal object grid or plate it does
produce secondaries if
it has enough kinetic energy. At several hundred Volts of accelerating
potential multiple secondaries
are produced. It is just that most of the time these slow moving secondaries
are recaptured by the
element that emitted them.
It is a problem that I have seen with my transmitting tube tester under the
right conditions and
has nothing to do with the dynatron region where the plate voltage is near or
below the screen
voltage.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of Peter Voelpel
[dj7ww@t-online.de]
Sent: Monday, December 30, 2013 7:10 PM
To: amps@contesting.com
Subject: Re: [Amps] negative screen current
Extracted from http://en.wikipedia.org/wiki/Tetrode
"As the screen grid is positively charged, it collects electrons, which
causes current to flow in the screen grid circuit. This uses power and heats
the screen grid; if the screen heats up enough it can melt and destroy the
tube. There are two sources of electrons collected by the screen grid—in
addition to the electrons emitted by the cathode, the screen grid can also
collect secondary electrons ejected from the anode by the impact of the
energetic primary electrons. Secondary emission can increase enough to
decrease the anode current, since a single primary electron can eject more
than one secondary electron. The reduction in anode current is because the
external anode current (through the connection pin) is due to the
cathode-to-anode current minus the secondary emission current. This can give
the tetrode valve a distinctive negative resistance characteristic,
sometimes called "tetrode kink". This is usually undesirable, although it
can be exploited as in the dynatron oscillator. The secondary emission can
be suppressed by adding a suppressor grid, making a pentode, or beam plates
to make a beam tetrode/kinkless tetrode."
That explanation is identical to that found in German textbooks
73
Peter
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Garland
Sent: Dienstag, 31. Dezember 2013 00:36
To: amps@contesting.com
Subject: [Amps] negative screen current
Kerry,
Your explanation about secondary emission seems reasonable, except for the
idea that electrons “caught” by the screen correspond to a negative screen
current. Unless I’m grossly mistaken (always a possibility!) the only way
to have a negative screen current is for the screen to emit electrons, not
capture them.
Just to clarify the sign convention, if a milliammeter is placed in series
with the screen voltage supply and the screen pin of the tube socket, with
the positive lead of the meter attached to the power supply, then a positive
screen current corresponds to electric current that flows from the power
supply into the screen grid. A negative screen current thus corresponds to
current that flows into the power supply and which comes from the screen
grid.
I know at first blush it may seem reasonable to think that electrons
captured by the screen grid flow out the screen into the power supply, and
that this electron flow corresponds to a negative screen current. But, alas,
that logic is flawed, because it fails to take into account the fact that
electrons have a negative charge. Electrons flowing out of the screen grid
into the power supply actually correspond to a positive screen current.
When a screen current meter deflects in the negative direction, it means
that electrons are flowing from the power supply into the positive meter
terminal, through the coil in the meter, and out the negative meter terminal
into the screen grid, where they are subsequently emitted. Captured
electrons cause the meter to deflect in the positive direction.
73,
Jim W8ZR
_____
From: kerry keel [mailto:kkninsc@yahoo.com]
Sent: Monday, December 30, 2013 12:11 PM
To: Jim Garland
Subject: Re: [Amps] negative screen current
The electrons captured by the screen grid are "splash back" for an
explanation, from main current stream striking the anode. The anode is being
bombarded by high energy electrons, drawn there by the anode. Upon impact
with the anode, other electrons are thrown off of the anode. This is a
secondary emission, low enough in energy level to be caught by the screen
grid. Some of the electrons in that main current flow will be caught by the
screen, but it is the lower energy electrons, the product of main current
impact against the anode, that causes the problem with the screen current.
This is the "why" for the introduction of the suppressor grid in Pentodes.
The suppressor is supposed to limit, or better yet, under the best operating
conditions, stop the bounce back of electrons from the anode. The older ARRL
mauals, or a good vacuum tibe manual, such as one from Raytheon, or RCA goes
into good detail aboout these issues.
This Message is from Mr. Kerry Keel. It has been checked by AVG, and is
Virus free.
From: Jim Garland <4cx250b@miamioh.edu>
To: gdaught6@stanford.edu; amps@contesting.com
Sent: Monday, December 30, 2013 1:00 PM
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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