[Amps] grid fuses

[Tom W8JI]

> 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