[Amps] Grid fuses (was: Life and gain of 3-500Z)

[R L Measures]

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 at somis.org