[Amps] How do I determine Class

[Bill Fuqua]

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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