Tom,
The problem is, you stated all this can occur after the fuse or resistor opens.
It can not, only before it does.
"Now when the tube faults and if the resitor opens, the grid can rise to full
anode voltage"
If the grid doesn't make a connection to anything, it can't conduct and hurt
one thing, it's just there in space.
The only possible scenario after the resistor or fuse were to open is the
triode act as a diode. I agree it can
do the other things while connected to ground, but not un-connected. The tubes
anode can't arc to the grid,
nor can there be any form of grid cuurent without a connection to ground to
make a circuit.
So, if a resistor or fuse were placed from the grid to ground, that did open
very quickly in the event of an arc,
or high grid current, and the amp was powered off when the tube(s) started
acting as a diode (increased anode
current I would think where there's no grid to control it), what would be the
harm? If we had a gassy tube that
was arcing continuously to the grid, or something causing high enough grid
current to hurt the tube, and the
fuse/resistor blew, the tubes grid would be saved if it blew as soon as the
problem started. It would look to me like
the tube could be harmed if the problem kept up without some sort of protection.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/20/06 at 7:26 PM Tom W8JI wrote:
>> I'm having a hard time seeing this. I'm not sure if any
>> others do. If the grid is not connected to anything after
>> the resistor or fuse blows, how can any damage be done
>> where the grid can't make a circuit to anything else?
>
>It's really pretty simple Will. I'll try to explain it one
>last time.
>
>There are two main catagories of excessive grid current. One
>is from excessive RF drive. This is caused by any number of
>scenerios, which in a grounded grid amp includes loss of HV
>or excessive drive for the plate load impedance. Remember
>the negative feedback that diverts drive power from the grid
>to the output is tied to the ratio of load impedance to
>input impedance of the tube, so anything that causes that
>feedback to decrease will increase grid drive.
>
>The second is a HV to grid fault. This is almost always a
>severe fault, since once an arc starts the only way to stop
>it is to have things disappear or to remove the HV.
>
>There are no other causes of excessive grid current.
>
>In either case, there isn't a tube manufacturer or a good
>engineer around that would ever recommend allowing the grid
>to suddenly float. The reasons are quite obvious.
>
>In the first condition the grid would likely be at excessive
>temperature. Every grid dissipation specification set by
>Eimac was set by measuring the start of secondary emission
>in the grid. They measured that point by increasing grid
>current until the grid started emitting electrons. If we
>suddenly yank the grid off ground while the grid is too hot,
>it will rise towards anode potential. That isn't good news
>when HV is still there and the cathode is grounded. Also, it
>cannot always be gauranteed that the leakage current in a
>power grid tube is so low that a cold grid would move up to
>negative bias values with respect to the cathode.
>
>In the second condition, which is probably the single most
>common very high current fault, the grid is involved in a
>high vacuum arc or other fault to the anode. The grid is
>effectively tied to the anode, and that connection won't go
>away until the HV is removed or the cause of the fault blows
>away. Something has to quench the arc. In this case, and
>this is the most common "big bang" fault, the grid would
>instantly try to go right up to full anode voltage. That is
>not a good idea at all, because the grid can then flash over
>to the cathode instead of shielding it.
>
>This is why a good design places fault current limiting
>resistance in the high voltage lead, NOT in the grid lead to
>handle the first problem.
>
>It would also remove drive or bias the cathode into cutoff
>if the problem was excessive RF drive current, as opposed to
>waiting for a resistor or fuse to melt. It isn't good
>engineering to place a control grid in a drag race with a
>fuse that might take a few seconds to open, or a resistor
>that might take a day to open if it ever actually does.
>
>> There cant be any grid current from the cathode to the
>> grid since it's dis-connected from ground.
>
>You forgot the path to the anode, which might be either
>leakage or an arc.
>
>>There can't be
>> any circuit between the grid and the anode either unless
>> for an arc
>
>Secondary emission and leakage also count. There isn't a
>power grid tube in the world I'm aware of, even small stuff
>like 6V6's, that allows infinite control grid resistance in
>operation. Small tubes like the 6V6 might alow up to 1/2 meg
>ohm or more under certain conditions but a good designer
>would never want to have much resistance in a tube with a
>hot grid, especially .
>
>This isn't all that esoteric. Eimac themselves warned
>against fusing the grid path to ground. Most people just
>know better than doing something like floating a grid on a
>large high voltage high power tube.
>
>73 Tom
>
>
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