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Re: [Amps] How do I determine Class

To: Will Matney <craxd1@ezwv.com>,"amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] How do I determine Class
From: Bill Fuqua <wlfuqu00@uky.edu>
Date: Thu, 02 Sep 2004 13:37:21 -0400
List-post: <mailto:amps@contesting.com>
At 12:07 PM 9/2/2004 -0400, Will Matney wrote:
Bill,
I was reading about this a while back. If I recall, the spacing between the screen grid and the anode in power tetrodes help control secondary emission reaching the screen from the anode.


This has to do with secondary emissions going the other way which causes the Dynatron effect.

If you recall the innards of a power tetrodes, the screen and control grid are in precise alignment and are close together. But the anode is a good distance beyond the screen. In pentodes or beam tetrodes (most which still are called pentodes) of course, the suppressor grid or the beam plates keep the secondary emission from reaching the screen being at ground potential. I'm pretty sure I was reading this in either the Care and Feeding texts by Eimac, or in the RCA Transmitting Tube manual. One of the two spoke about this same thing.

This makes me wonder, why don't more if any amateur amps use power pentodes? I've not really compared the specs between the two but was something that just hit me. It would seem to me that this would help cure the problem of screen current caused by secondary emission in amateur amplifiers. Can anyone comment on this?

Triode GG amplifers are simpler, much more forgiving in tuning up, and less expensive. Also, I think the FCC rules requiring a minimum drive power for a HF amplifier had a lot to do with it. GG amplifiers also have some inherent negative feedback that improves their linearity.



Will Matney


I think it has to do with the close proximity and precise alignment of the screen grids with the control grids so that screen grids are in the electron shadow of the control grids. I have not given it much thought. In any case, any electrons hitting the screen grid will produce secondary electrons provided they have enough kinetic energy. At the high screen potentials that transmitting tubes use each electron that hits the screen grid will produce 2 or maybe more secondary electrons. The question is where will the secondary electrons go? Will they got back to the screen grid or will they go to the plate? With suppressor grids the answer is clear because the electric field will be favor the secondaries going back to the screen grid. But there must be something in the average tetrode that forces the secondaries back to the screen grid rather than going to the plate.


Here is one possibility. In a non-shadow type tube most of the electron impact will be on the side of the screen grid closest to the control grid. In this case the secondary electrons will be emitted with a low kinetic energy ( low speed) back toward the control grid. In this region the electric field in going to push the secondaries back to the screen grid before they get very far from it. They will only have a few electron volts of kinetic energy and easily drawn back to the screen grid.
But in the case of the control grid shadow around the screen grid. I can only think that there are situations that allow the electrons to miss the front surface of the screen grid (nearest the control grid) and then strike it on a surface that is nearer the plate than the control grid thus the secondary electrons would be attracted to the plate causing a negative net current flow to the screen grid.
Just some thoughts.



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