> >> Fuses are bad enough because of time delay and internal arcing, and
> >> resistors make very poor fuses.
> There are HV fuses that address these issues. They are readily
> available surplus, though a bit pricey. The arc quench technology is
> the essence of KISS as they fill the fuse with sand.
Filling the fuse with sand (HV fuse) does not address the long
delay time as the fuse opens, nor the unreliability in fusing current.
It does allow the fuse to eventually open and quench the arc, but
the time that requires is VERY long compared to the time it takes
to damage other components.
The sand actually slows the response time of the fuse. Not only
that, you have to wait for the sand to physically move for the sand
to quench the arc.
Get a fuse catalog out, and look at delay time vs current. You'd be
amazed at how long it takes to open a fuse with 150% current
overload, and such a close-rated fuse would only limit grid
dissipation to much more than two times operating dissipation
Remember, accelerating voltage between the grid and cathode
increases along with current, so dissipation does not follow I^2, it
can be more like I^3 or I^4.
If you rate a fuse within 150% of steady operating current, it will
eventually go through internal changes and fail randomly.
Anyone who suggests fuses make good grid protection devices
either has designed a PA with very large amounts of headroom on
the grid in normal operation, or has never looked at the problem.
Fuses are also poor or useless for HV faults. They might save the
transformer, or rectifiers (only because the filter capacitor buffers
the rectifiers and supplies the energy to open the fuse), but they
certainly never protect components from the HV dump. A 1 ampere
fuse will let thousands of amperes of transient current through.
Worse yet, some people think resistors make good fuses! They
make good current limiters, not good fuses, and only them if they
have enough resistance. Ohm's law still works.
The ONLY reliable device for protecting grids from excessive
current is a properly designed electronic breaker of some sort.
The HV needs a glitch resistance of suitable value (part of it can be
in the plate choke and other components), a fuse or low value
resistor simply is not enough.
73, Tom W8JI
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