[Amps] Grid fuses

[Tony King - W4ZT]

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