>I am totally lost on this excessive grid current
>discussion. If there is a
> resistor or fuse tied between the grid and chassis and it
> blows, the grid
> obviously has a potential above or below ground. Which is
> it? Can it be
> either depending on the fault?
>
Keith,.
There are two major types of faults. One is RF generated
where the grid has excessive drive from RF (alternating
current) drive, the other is a hard fault caused by a
flashover. There are multiple things inside a tube that can
cause a hard fault, and they are very common.
When tubes are designed the designer sometimes tries to work
a gap into the tube in a harmless area, knowing there has to
be a safe point for hard faults caused by gas. The point
where a 3-500Z arcs is either from the bottom of the anode
to the grid support cone, although I have seen a few fire
off the top of the grid structure to the anode.
Arcs generally have very low resistance, and when an arc is
struck it can continue at much lower voltage than the
initial breakdown voltage. During an arc molten metal or
ions from gas can be involved. Eimac and other companies
actually use intentional arcs to clear faults, rounding
metal points inside the tube to restore voltage holdoff to
acceptable levels. Arcs can also help getter or clear gas by
the tube by changing it into harmless compounds that
harmlessly bond with elements.
If the tube was absolutely gas free, which large power tubes
never are, and if it was a soft fault caused by drive a
floated grid in a SOFT fault would go negative. However, a
fuse (or worse yet a resistor) is a very poor way to protect
for soft faults. Soft faults require electronic protection.
If the tube has a hard fault, the grid has a direct
conductive path to the anode. During the arc, even if you
open the grid, it will simply pull up towards anode voltage
and turn the tube on. Every tube book I have looked at warns
about this condition!
If you open the grid during a hard fault two things happen.
Whatever you are opening has to hold off nearly full supply
voltage. Fuses and resistors normally can't take this
voltage, and neither can the grid-cathode path.
If by some strange stroke of luck the arc quenches just
after the grid path opens, you still have a grid hanging
free. That's something every good engineering book I have
warns about doing in high voltage high power tubes, and I
put the text up on my website from three of those books. One
of those books, by Giocoletto, is one of the most
comprehensive tube design books around. When you float or
have a very high grid resistance in a power tube, stray ions
that are always present will collect on the grid and
collect. They actually pull the grid positive, and the
additional electrons striking the plate from increased
cathode current free more positive ions to return to the
grid. A runaway situation develops, and the tube goes into
another fault!
This isn't something I'm just making up. It is a common
thing. That's why several other people piped up an all said
the same thing as I have been saying, and why I can point to
engineering text that has the same warnings. I don't have to
say "the author said that but really meant this". It isn't
an issue if I am right or wrong, but rather one of asking
actual tube design engineers or reading scholarly
peer-reviewed texts on the subject.
It's a lot like the moly anode with zirconium coating for
gettering Eimac used in the 3-500Z. People don't have to
assume, guess, and say what their "opinion" is. They can
pick up a phone or send an e-mail and ask a manufacturer
before reaching false conclusions. It's like the 50-amp
fault protection transistors that never really were even in
the grid circuit, but the person espousing that stuff for 20
years never even bothered to look at a schematic!!
73 Tom
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