[Amps] How do I determine Class

[Will Matney]

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. 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?

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.