> 1. Why not use a real fuse? There are some micro-miniature
> fast blow
> fuses available which would not add much lead inductance,
> if that is a
> consideration.
>
> 2. If the fuse blows, you wouldn't want the grid to be
> floating I'd
> think. Do you parallel the fuse with a high value resistor
> to maintain
> DC continuity?
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.
Fuse failure is always a time issue related to ambient
temperature and overload amount. Resistors are totally
useless in this application.
Second, if the issue is protecting from a HV fault the fuse
should NEVER be in the grid circuit. It should be in the HV
path to the tube. The very last thing in the world we would
ever want to see happen is a fault open the grid to chassis
path and allow the grid to go positive to the anode supply
rail. If the grid goes up to the supply rail, and it will
during an anode-to-grid fault, a shunting resistor won't
help a bit. All that HV will dump into the cathode circuit,
and perhaps right back to the exciter.
Lifting a grid in a GG amp is a very last resort. It was
only done in the 811H and 572B to equalize tube drive power,
and intentional protection was added to minimize damage to
exciters during a flash over. That was a calculated tradeoff
between tube life (from unequal current in four tubes) and
risk of exciter damage, and clamps were added to protect the
exciter.
Common sense should tell us using a resistor as a grid fuse
is one of the worse ideas possible unless there is a
compelling reason to add intentional grid path resistance.
It is the worse kind of fuse, it does nothing for normal
grid overload, and it is a disaster to have there if the
tube arcs.
73 Tom
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