[Amps] Amps Digest, Vol 166, Issue 26, reference direction of current flow in screen grids

[Gene May]

All,


Concur with Bill W6WRT re "positive vs negative" convention in describing current.  The convention we use now is left over from at least a hundred years ago when a scientist guessed wrong about the polarity of the electron.  To me, thinking in terms of the direction of electron movement is far more clear.


Suggestions by Bill and Gary K4FMX and others to put a resistor from the screen to the cathode, such that the resistor will pass at least the maximum "negative current" (meaning the screen is emitting electrons) that the screen will pass will work fine.  This is absolutely essential if the screen grid supply is series regulated.  Zeners connected in this way (a way to parallel regulate the supply) should pass at least the same.  A little extra current flow is a good idea.


Suggest finding a copy of the Eimac publication Care and Feeding of Power Grid Tubes, out of print but easy to find used online from used book dealers or at hamfests.


I am using a power supply for tetrodes designed and sold by  Ian GM3SEK, which regulates screen current very well.  Ian can be found on this.


Gene May  WB8WKU

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From: Amps <amps-bounces at contesting.com> on behalf of amps-request at contesting.com <amps-request at contesting.com>
Sent: Monday, October 24, 2016 10:28 PM
To: amps at contesting.com
Subject: Amps Digest, Vol 166, Issue 26

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Today's Topics:

   1. Re: Negative current with a Zener screen grid regulator  (Bill Turner)

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Message: 1
Date: Sat, 22 Oct 2016 18:55:25 +0000
From: Bill Turner <dezrat at outlook.com>
To: Amps group <amps at contesting.com>
Subject: Re: [Amps] Negative current with a Zener screen grid regulator
Message-ID:
        <CY1PR0301MB11967B12023C877DBAC2BDBDC0D70 at CY1PR0301MB1196.namprd03.prod.outlook.com>

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On Tue, 18 Oct 2016 05:44:13 -0600, W8ZR wrote:  >Positive screen current means that current flows into the grid, and negative screen current means that current flows out of the grid.

REPLY:

Be careful with this "into" and "out of" business. He is talking about the so-called conventional current, which exists only in the imagination.

In the real world, positive screen current (electrons) flows from the cathode to the screen and out to the  positive power supply.  Likewise, negative screen current means the screen is actually
emitting electrons as if it were a cathode itself, a phenomenon caused by secondary emission where arriving electrons actually "knock off" electrons from the screen and it appears to be emitting electrons of its own. See Wikipedia discussion:
https://en.wikipedia.org/wiki/Secondary_emission

Because of this phenomenon, the screen supply must be capable of supplying electrons as well as receiving them. This is most easily accomplished with a resistor from screen to ground, whose value is such that current through at idle equals or exceeds the value of expected negative screen current.

I wish the concept of "conventional current" would disappear from the textbooks since it purely imaginary. Electron flow is real.

73, Bill W6WRT
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