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Re: [Amps]  negative screen current

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Subject: Re: [Amps]  negative screen current
From: "Gary Schafer" <>
Date: Mon, 30 Dec 2013 17:48:14 -0600
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>From what I have read in Eimac's "Care and feeding book" the secondary
emission is a result of electrons being collected by the screen (from the
cathode) and lower energy electrons being released from the screen. The
secondary emissions from the screen has nothing to do with secondary
emissions from the plate. 

The grids in these type of tubes are operated at relatively low voltage and
are different than other tubes. Not all the electrons pass thru the screen
and some of the electrons are collected by the screen.

If the secondary electrons feel a stronger attraction to the screen they
fall back to the screen.
If the secondary electrons happen to be nearer to the plate and with the
much higher plate voltage they are attracted to the plate and you have
electron flow from screen to plate.

So yes, in order to have negative screen current the screen must be
releasing more electrons than collected.

This is the reason that a low impedance screen circuit is important with
these type of tubes. With negative screen current the screen voltage will
tend to rise up towards the plate potential which raises the gain of the
tube and increases the plate current causing plate current to run away. 

So if cutting off screen voltage to put the tube into standby is done by
removing screen voltage then either the proper screen resistor to ground is
required or the screen needs to be grounded.

Gary  K4FMX

> -----Original Message-----
> From: Amps [] On Behalf Of peter
> chadwick
> Sent: Monday, December 30, 2013 2:18 PM
> To:;
> Subject: Re: [Amps]  negative screen current
> Were that to be so Don, then all screens would emit electrons, which we
> know is not the case. There is a problem between 'electron current flow'
> and 'conventional current flow' here. Screens collect electrons
> generally. The more electrons they gather, the greater the screen
> current measured with a meter which has the + terminal connected to the
> supply. To get that meter to read effectively backwards, electrons must
> be leaving the screen grid in greater quantities than they are reaching
> it.
> The question is "Why?" Thermionic emission could be countered by using a
> sputtered layer of low work function material - as was done on the 6J4
> grounded grid triode with its close grid - cathode spacing, back in
> about 1944.
> 73 es HNY
> Peter G3RZP
> ========================================
>  Message Received: Dec 30 2013, 07:59 PM
>  From:
>  To:
>  Cc:
>  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...
>  No, they are going in the opposite direction .
>  Don W4DNR
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