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

[R L Measures]

On Jul 20, 2006, at 3:27 AM, Tom W8JI wrote:

>> 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.
>> On the other
>> hand, if most damage to the grid occurs very quickly, then
>> one would
>> probably want fast-acting, electronic protection.
>>
>
> It depends on the tube Paul.
>
> It's actually accumulated damage over time in gold plated
> grids. Any period of excessive grid current will cause some
> gold migration since the kinetic energy of the electrons can
> displace gold without heating the entire grid.

I have never read this in any scientific literature, Tom.  Reference?
   Every grid that I have seen with gold-plating damage has had  
smooth bare patches, as well as popped bubble marks -- which I  
interpret as evidence of boiling.

> We are
> limited how much accumulated time we can exceed grid current
> ratings.

How about the 3-500 grid you ran red-hot?
>
> Most unplated grids are very rigid metals,

(Molybdenuum, MP 2617ºC)  )

> and can be heated
> nearly to the point of incandescence without damage. All
> they do is start to emit electrons.
>
> If a grid doesn't need protection, a fuse would work well.

Say what?
end

> ...

R L MEASURES, AG6K. 805-386-3734
r at somis.org