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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