Chris Howard wrote:
> On Fri, 2006-07-21 at 12:40, Tony King - W4ZT wrote:
>> If the
>> amp contains series elements between the grid and ground, once they
>> fail, it will still be very unlikely that the arc will extinguish before
>> the energy from the filter caps is depleted. But, should that happen,
>> then significant cathode current could occur and bias would have no
>> effect since the grid would no longer be connected to ground.
>
> I'll cogitate on that...
>
> I guess that is what I was getting at. If grid is hard tied to
> ground, then arc between cathode--grid could continue
> even after the plate--grid--cathode arc is done (?). (Depending on the
> configuration of the power supplies as you said.) So if that is true
> than having some series elements between grid and ground would make
> sense. It looks like if arcing happens you have to stop two routes
> to stop it.
>
Only one... kill the HV with either a proper HV fuse and/or trip the HV
supply off line. For the same reason you can't depend on a blown
resistor to stop the arc, you can't depend on a low voltage fuse to stop
it either. That's a big mistake that some make, using a conventional
3AG fuse in the HV lead. When a 250 fuse is used in the HV lead, when it
blows, the vaporized metal plasma will allow the arc to continue until
the fuse literally explodes.
73, Tony W4ZT
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