See below;
*********** REPLY SEPARATOR ***********
On 7/25/06 at 3:48 PM Tom W8JI wrote:
>> Uhhhh Tom, your mis-quoting me again. I said the grid
>> would not go over 0 Vdc if connected to ground. I also
>> said it would stay negative or at 0 Vdc if those ions
>> flying around in a gassy tube were happening. A tube can
>> have the ions without an arc. Also, which direction does
>> an arc occur, from the anode to the grid, or from the grid
>> to the anode, or which way is the electrons in the arc
>> flowing?
>
>Will,
>
>Early on you said contact bias and grid leak bias were the
>same, they are not. You also said when the grid was floated
>during a fuse or resistor blow, it would go negative. It
>does not in the case of a power grid tube. I'm sure most
>people remember that, and the archive clearly show it.
>
>Now it appears, running out of escape avenues, you want the
>grid grounded at all times.
See the post to this same text I made earlier about what I said in the past.
All the quotes are there.
>
>Since that is what I said all along, that the grod belongs
>grounded, it appears you finally agree and see the point.
>
>>> We
>>>need somehow to bring the grid up positive to anode
>>>potential to reduce current and quench the arc.
Are you saying the grid in a 3-500Z could have a potential of +3000 Vdc and it
connected to ground?
>>
>> Or is it bringing down the anode potential to match the
>> grid? Do you have a voltage drop with current rise?
>
>We now agree again. You now appear to see the merit of a HV
>fuse and current limiting system. Everyone agreed on that
>all along, and now you are on the team.
I said in fact that once a gird was floating, the amps should be shut down or
the HV removed. See my quotes in the other post about this. However, I do not
agree that the grid can ever be made positive with a potential above 0 Vdc.
Also, who's everyone? I've only seen two people saying a grid could be positive.
>
>> In order for
>>>the grid to go to anode potential, the grid has to move
>>>significantly positive with respect to the chassis (and
>>>cathode).
>>
>> Remember, the grid is still connected to ground at this
>> point.
>
>Yes, because NOW you have removed the silly poorly
>engineered grid fuses. We again agree.
How can it be at gorund (0 Vdc) or tied to the negative rail if the fuse is
removed? That would make the grid float thus being un-connected. If it's a
fuse, it should be close to 0 ohms, the same as a short circuit across it. This
sould not allow a grid to ever go over 0 Vdc.
>
>> Once the grid is even remotely far enough positive
>>>to reduce the arc, it is so far positive it almost
>>>certainly
>>>has exceeded the breakdown voltage of the grid to the
>>>cathode. Now we have a grid, arcing to an anode, that is
>>>also arcing to the cathode
>>
>>
>> In what direction does the cathode to anode arc occur or
>> which way is the electrons in the arc flowing?
>
>Electrons normally move from cathode to anode Will.
Correct, so how can an arc make a grid positive when the grid is tied to
ground? It can be no higher than 0 Vdc. If the arc is going from the 0 Vdc grid
to the positive anode, the potential at the grid would still be 0 Vdc. In fact
there would be a voltage drop across the resistance of the arc or the ionized
gas.
>
>>>The entire idea of pulling the grid off ground to reduce
>>>grid-to-anode fault current is, quite frankly, very poor
>>>design.
>>
>>
>> I couldn't say, I've never tried it.
>
>Then why did you waste 50 posts arguing with me and others
>about it, only to finally agree it is a bad idea?
I have never once said this is a bad idea. I said it wouldn't protect the tube,
which would be bad anyhow. What it would do is possibly help save the B+
supplies rectifiers, etc. If I recall, Rich never said this either. You are the
one saying it's a bad idea like now, and saying something catastrophic may
happen when a grid opens. In fact, it would be bad if that happened as far as
the tube and system, but the grid fuse could help protect the B+ supply by not
allowing another arc from it to the anode. That's why I have had to post
umpteen posts for this, and to show a control grid can't become positive or be
over 0 Vdc.
>
>
>> Ehhh, there you go again Tom. Nowhere did I ever say
>> arcing was less likely if the grid was fused. Matter of
>> fact, I said it wouldn't help the tube at all because the
>> tube was bad, it was gassy. The only thing it might help
>> is saving the B+ supply from ruin. Evidently, you don't
>> read all the threads, or have one helluva time
>> understanding them.
>
>1.) I understand what most people say very well Will. The
>only time I have a hard time understanding someone's
>position is if the change it constantly.
I've not changed my tune one time. See the first post pertaining to this one
with my quotes.
>
>2.) I can't say the same for other people. As an example
>look just above. I very clearly said, "The entire idea of
>pulling the grid off ground to reduce
>grid-to-anode fault current is, quite frankly, very poor
>design."
>
>3.) You will not find the words "arcing is less likely" in
>my text above Will, so I certainly don't know how you came
>up with that.
No, you were agreeing to that which Chris said, which to me is the same as
saying it.
>
>4.) Your statement above, "Matter of fact, I said it
>wouldn't help the tube at all because the tube was bad, it
>was gassy ", is totally incorrect. Anyone inside the
>industry or using large power grid tubes knows that
>occasional hard faults are not unusual. Dozens of people
>have repeated that here on this thread. When a tube arcs, it
>does NOT mean the tube is bad. This is why it is important
>to not cause more damage than we need to during a hard
>fault.
Are you calling a hard fault with a totally gassy tube the same type arc that
might happen if just enough gas were there to cause a slight one like a flash
over that happens very quickly? There's a huge difference between those two
types of arcs in how destructible they are. Anyone that's seen a totally gassy
tube arc can verify that.
>
>Are we done Will, now that we both agree fusing the grid
>does nothing good?
It may, I can't say. By what some others have said, it may well have did some
good. If I recall, their HV supply was still intact after the occurance.
>
>73 Tom
>
>
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73,
Will
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