On Jul 19, 2006, at 11:22 AM, Paul Christensen wrote:
>> Why would a person put a fuse between a control grid and
>> ground?
>>
>> First, if we want to protect for grid current from drive
>> power it should be an electronic protection. Any fuse is
>> well-known to be too unreliable in a system like a control
>> grid.
>
> This brings up a point I've often considered...
>
> Is grid damage resulting from over-drive primarily the result of
> instantaneous high Ig, or is it more from a combination of both
> high Ig and
> duration? It would seem that if a grid is relatively immune to
> short bursts
> of high Ig, then a fast-acting fuse would be appropriate.
Correct. Paul --- Both Tom and I have tested 3-500Zs with the grid
running red-hot, and neither of us subsequently found any grid
damage. The 3-500Z grid failures that I have seen weren't really a
grid fault, they were shorts from the filament to the grid as the
result of the filament being bent. As I see it, the lateral EMF from
grid-I during a parasite bends the filament sideways, in some cases
causing it to touch the grid. I have no idea what the magnitude of
grid-I in a 3-500Z is during an intermittent parasite, but it must be
close to the fusing rating of the #28 Cu wire used in the SB-220s
grid RFC.
> On the other
> hand, if most damage to the grid occurs very quickly, then one would
> probably want fast-acting, electronic protection.
... provided that the transistor used to interrupt grid-current was
up to the task.
>
> Paul, W9AC
>
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>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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