Chris Howard wrote:
> On Fri, 2006-07-21 at 12:05, Tom W8JI wrote:
>
>> If you open the grid path, the anode now dumps energy to the
>> cathode via the grid.
>
> If you only cut HV when this is happening, couldn't the bias
> voltage (let's say 100v) continue the arcing between cathode and grid?
>
No. The bias is normally derived from the voltage drop across a bias
circuit (a zener diode(s), a string of silicon diodes or an adjustable
bias circuit) in the cathode current path. When you remove HV and the
cathode current ceases, so does the bias. Also, during the glitch, when
the current path is B- to ground to grid to anode, the cathode is no
longer a serious contender for current so bias isn't a factor. 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. At that
point there's a good possibility of getting damage to the cathode if the
HV hasn't tripped off.
Remember, as was pointed out earlier, once the arc is started, it isn't
going to stop until you run out of energy or the path is opened wide
enough to extinguish the arc. It would be very difficult to open the
path wide enough to cause the arc to cease once the plasma has formed.
With a proper glitch resistor, the arc may extinguish and restart with
fall and rise of applied HV through that resistor. The best you can hope
for in any case is to sense the over current situation (grid and
cathode) and trip off the HV supply and take the amp off line.
73, Tony W4ZT
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