[Milsurplus] [ARC5] Why "Noodling" About Sweep Tubes

mac w7qho at aol.com
Sat Oct 26 19:56:45 EDT 2013


The electron "beams" in a beam tetrode are formed by a particular  
alignment of the control and screen grid wires.  In tubes like the  
6L6, 807, etc., this alignment can be maintained only in the central  
areas of the grid structures and beam formation is disrupted in the  
areas on the sides where the grid supporting wires get in the way.   
The "beam forming" plates found in these tubes serve to mask off from  
the plate those areas on the sides of the grid structures where this  
disruption of the beams takes place.  The plates (which normally run  
at cathode potential) are thus much more accurately described as beam  
"confining" as opposed to beam "forming" structures.  In other tubes  
such as the 4-250, 4-400 and the like the grids are structured and  
supported so as to allow full 360 degree beam formation without  
obstruction, hence no confining plates or other such structures are  
required.

The 803 is a power pentode and I find no description of it as a "beam"  
power tube in the literature.  If it were such there would be no need  
for the suppressor grid (g3).  No beam confining/conforming plates in  
an 803 either as you point out.

Dennis D.  W7QHO
Glendale, CA

**************
On Oct 25, 2013, at 8:04 PM, Kenneth G. Gordon wrote:

> Also, I have never really been able to understand why 803 is termed a
> "beam-power pentode". AFAIK, it has no "beam forming" plates.



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