On Sep 2, 2004, at 6:43 AM, Bill Fuqua wrote:
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.
Bill -- This makes some sense, however, reverse screen current
reportedly happens in tubes that show no reverse screen current region
in the published characteristic curves. Thus, I am using a 50k-ohm,
100w screen bleeder in the current project.
...Many of the old designs were for maximum gain so that very low
power exciters could drive them. Here you can vary the drive quite a
bit by changing the link coupling.
For a Bruene bridge neutralized tetrode or pentode, there is no link
coupling. The grid is driven either directly (50-ohm termination) or
through a Z-step up broad band xfmr. (200 or 450 ohm termination) ,
and the grid XC is tuned out by a parallel roller L.
The MB-40L has a link coupling. And we are talking about a 1956
tetrode amplifier from the handbook.
Was the amplifier neutralized?
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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