> 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. We are
limited how much accumulated time we can exceed grid current
ratings.
Most unplated grids are very rigid metals, 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.
The problem is we don't want the grid to float, even in a
tube where we don't care of the grid is protected.
If a grid needs protection, a fuse (or worse yet a resistor)
absolutely stinks as protection. Even the fastest operating
fuses have reasonably long and unreliable opening times.
Fuses are there to prevent fires and damage to things that
have longer thermal lag times than the fuse. Worse yet
placing the protection device from grid to chassis is a
serious mistake. We don't EVER want the grid to float. Fuses
are terrible in a grid circuit, but they are OK in a
transformer primary or to protect wiring where fusing
currents are hundreds or thousands of times more than
working current.
Even in wire circuits, electronic devices are faster and
more reliable. That's why the move has been away from fuses.
If resistors made good fuses, we would have a resistor panel
in our houses and not a breaker box. We would use GFI
resistors, and not GFI breakers, to protect people from
equipment faults.
73 Tom
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