On 7/25/06 at 2:44 PM Tom W8JI wrote:
>> Well... the other issue of course is _COST_. If a
>> reliable
>> system is obtainable without investing in a sophisticated
>> circuit
>> that's a good reason to stick with the simpler method.
>
>As long as it works the same.
>
>> As for "hard faults should be handled in the anode",
>> I parse that statement as being a design philosophy.
>
>It is a recommended design standard that appears many
>places. Not a philosophy.
>
>> (by Hard Fault do you mean arc? I think so.)
>> Well here we get to a minor point of contention.
>>
>> The textbooks we've been examining, with your generous
>> help,
>> don't cover what happens when a grid under arc is floated.
>> So... we don't have good data on what happens, except for
>> the experience of folks who say things work out ok.
>
>Anyone who has taken basic physics or worked with
>electronics in any technical depth should know what an arc
>does. Pipeline welders even understand arcs.
>
>Once an arc starts, the sustaining voltage becomes very low.
>This means to stop a grid from arcing to the anode the grid
>has to rise to nearly the anode potential.
>
>Now, we have Will claiming the grid goes negative. If the
>grid went negative, the arc would get worse...not better.
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?
We
>need somehow to bring the grid up positive to anode
>potential to reduce current and quench the arc.
Or is it bringing down the anode potential to match the grid? Do you have a
voltage drop with current rise?
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.
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?
or at least sucking all the space
>charge away from the cathode. Once the cathode is exposed to
>positive ions, it instantly suffers some amount of damage.
Correct.
>
>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. I said this much earlier. However there's
been others who posted on here it didn't hurt one thing. Also, there were
several who told me privately the same thing. Are all these people full of bull
hockey?
>
>> If it were dangerous to the tube.. I would think the
>> experience
>> stories would be different than what has been reported
>> here.
>
>Why? Why do you think a Ham staring at an amplifier panel
>would understand the statistics of making a mod over a large
>scale field sample?
>
>We did statistical analysis of exciter failures caused by PA
>tube faults, and the number of incidents of reported
>destructive damage to exciters was higher in triode amps
>with grids that floated. We analyzed a few hundred failures
>known to be arcs, and there were zero exciter failures with
>hard grounded grids. There were about a half dozen with
>grids tied to ground through resistors.
>
>We could actually simulate that failure in the lab.
Where at, Ameritron?
>
>> Or, maybe the position of the pro-fused-grid bunch would
>> be
>> that arcing is less likely with a fused grid altogether...
>> I'm a little shakey on this point.
>
>So are they, and they are shaky on several other points as
>well.
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.
>
>
>
>
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