Tom,
If I said those, things, go back and copy them verbatim, and post them on here.
However, post the whole text I said, not just one small part of it. Matter of
fact, I'll do it for you.
Never did I say those two bias arrangements were the same, I said they gain
their electrons from the grid in the same manner.
Neither did I say a grid would be only negative (maybe slightly negative for
equilibrium), I said it would be what it's charged to which can't be over 0
Vdc. If you want to call that negative, call it negative, but I can assure you,
it will never be over 0 Vdc like you've said several times. The grid is
connected directly to ground in a G-G circuit. If you red Terman where he talks
about space charge, it says a grid can become slightly negative, or to where
equilibrium takes place. I said this when I first got into this after Bill
posted about it.
Original posts about it;
Bill,
I could see this if the tube was operating and the electrons were flowing
between the cathode and grid then back to the cathode via ground like it
supposed to. Once the fuse/resistor opens, the grid could be less some
electrons. Since the cathode is still emitting electrons that are being drawn
to the anode across the grid, the grid would accept enough electrons to come
into equilibrium. I can't see it though taking more charge after it's
equalized, and just keep charging up to some higher potential which is what I
was talking about earlier in other posts. That's just not possible, or I cant
see it would be. Equilibrium yes, but other no.
Best,
Will
> I have observed this phenomenon in small receiving tubes with the grid
> floating. No doubt it applies to their big brothers as well.
>
> My understanding is the grid accumulates electrons from either the
> space charge surrounding the cathode or the cathode-anode electron
> stream and has no way of getting rid of them. Thus the grid becomes
> negatively charged to a small extent. This negative charge repels
> additional electrons from accumulating and a balance is achieved.
>
> Willing to stand corrected if necessary.
>
You got it right, Bill.
> Bill, W6WRT
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
And;
>ORIGINAL MESSAGE:
>
>On Fri, 21 Jul 2006 02:30:16 -0400, Will wrote:
>
>>How can it collect electrons from the cathode and take a charge when the
>path back to the cathode is broken via a blown fuse between the grid and
>ground making an open circuit?
>
>------------ REPLY SEPARATOR ------------
>
>Stray electrons within the tube can attach themselves to the floating
>grid and develop a small charge, typically no more than a few volts.
>To prove this to yourself, set up a tube with only filament/heater
>voltage applied and measure the c-g voltage with a high impedance
>meter. You will be amazed.
No, I agree like I said in another post earlier that this could happen, and
equilibrium be obtained. What I cant see is the grid taking on any more charge
than this thus charging to some other higher potential.
>In the case of small receiving tubes, this charge can actually be used
>to bias the tube when operated class A, although this is not really
>good design because of variation between tubes. It's just an
>interesting experiment.
Correct, this would not at all be a good idea for bias.
>
>Bill, W6WRT
My first responce to this mess;
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?
There cant be any grid current from the cathode to the grid since it's
dis-connected from ground. There can't be
any circuit between the grid and the anode either unless for an arc. However
again, there's no circuit as the grid
is not connected. Operating current also can not go backwards in a tube from
anode to cathode. It can only flow
from cathode to anode. About the only thing I could see is the triode act as a
diode with the grid open. If the tube
can stand the current operating as a diode momentarily until the amp is shut
down, I don't see a problem here. Am
I not seeing something here?
Another one from me;
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
Another;
Lon,
That's exactly the point I was trying to get at. Some protection is better than
none at all. If a tube arcs internally, it's gassy as an arc can't form in a
vacuum. It needs to be replaced no matter what. However, if we was to have an
arc between the anode and control grid, which kept happening for any amount of
time, it could surely damage the B+ supply. I seen a comment in another post
saying that the grid would not be a series circuit in the B+ supply if an arc
occurred between the control grid and anode, but it is. The current would be
drawn from ground (which the B+ supplies - is tied to), through the grid, and
arc to the anode then back to the B+ supply. Since the B+ supply uses the same
ground, it has a path that makes a series circuit which would allow the B+ to
arc across that gap. If it wasn't in a series circuit, there shouldn't be an
arc, the path has to make a complete circuit and the gas acts as the switch
closing the circuit so to speak by allowing the arc. The
resistor/fuse won't benefit the tube any, but may save the B+ supply. In a
gassy tube, even an arc to the cathode could form, which I've never once argued
it couldn't. It's gassy, take it out and throw it in file 13. I'd much rather
to change a tube, than to do it and have to repair the B+ supply with it.
As far as a control grid charging up to some high positive potential, it can't
connected to ground, it's at ground potential. If it's open, the most it might
do would be to attract a few electrons being negative like was mentioned by
Bill. After this, the triode would act as a diode. The electrons being
negative, the same as the control grid can not make it positive. Electrons have
a negative polarity unless something has changed that I don't know about. The
definition of a charge is a substance receiving electrons and storing them
which makes it more negative, not positive. The only way to make it positive
would be to apply a positive potential to it like applying one to a screen
grid. In either case, I don't see this happening in a tube that is in operating
condition, or in one that the control grid is open from ground, both with a
good vacuum.
There was also a mention of a control grid forming a cathode. How can it be a
cathode unless it's heated red-white hot to where it would emit electrons? I
highly doubt it ever getting much warmer than the internal temperature of the
tube. The only way it could would be to short to another element allowing a
current to pass through it to ground heating it up. Then it would emit
electrons towards the anode. It would also have a source to draw the electrons
from. Every electron that is pulled from the cathode by the anode has to be
replaced with one from the negative rail since current only flows in one
direction.
The original question had to do with a 3-500Z if I recall, and some tried to
throw other tubes in the mix. I've tried to stick with discussing the 3-500Z
triode and safety measures for it.
It's also been stated on here, and by several off the list to me that the amp
was ok after a resistor blew that was placed in the grid circuit. The only
reason I think the whole discussion was started was to try and dis-prove and
belittle another member on this list.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/21/06 at 4:48 PM Lon W. Cottingham wrote:
>Greetings to all,
>
> This discussion has deteriorated to that of electing officers
>at a 7th grade 4H Club Meeting. The original point has long been
>forgotten.
>Who cares what you call it; grid protection or grid fusing. The results
>is
>what we should be after, not all this, mostly unrelated, speculation and
>theory.
>
> One thread that I have taken from this bilious discussion is
>that some of you do not realize that most of the commercially built
>Amateur
>triode linear amplifiers throughout the 70's, 80's, and 90's have floating
>grids, of one sort, or another. It is time to pull the heads out of the
>sand and realize this.
>
> More often than not, when the "big bang" occurs, the tube/s
>are
>lost/shorted. Anyone who has been repairing amps and uses "grid fusing"
>knows that the loss of tubes is reduced by approximately 50% with grid
>fusing when the "big bang" occurs (at least that has been my findings over
>the past 25 to 30 years repairing 3 or 4 linears each and every month. I
>do
>not care why it works or what you call it, it works. In my opinion, grid
>protection, of some sort, should be a standard upgrade, along with anode
>surge resistors, in any amplifier repair or restoration. Inexpensive 1/4
>or
>1/2 watt resistors are the most cost effective way to do this that I have
>found. This idea of using anode surge suppressing resistors alone, as has
>been suggested by some of us, to protect the tube/s is like starting to
>brush your teeth after they all have cavities.
>
> I would like to see a thread start on trying to salvage shorted
>tubes. My success ratio over the years has been about 1 in five. Richard
>has mentioned a technique of spinning the shorted tube at a high speed in
>some sort of a centrifuge. I have heard other techniques such as
>applying
>voltage (both AC and DC) across the short. Some have suggested
>discharging
>large capacitors across the short. One Ham I know simply attaches an AC
>line cord across the elements and plugs it into the mains. Apparently
>this
>works, some times. I have always wanted to try Richard's method but
>simply
>have not done it. I would like to hear success rates from the various
>methods.
>
>73 de K5JV
>
> Lon W. Cottingham
>1110 Golden Bear Ln.
>Kingwood, TX 77339
>
>281-358-4207
>281-358-4234 FAX
>281-795-1335 CELL
Another;
Roy,
You got it! Most of the old sweep tube amps used a form of grid leak bias.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/22/06 at 10:50 AM Roy Koeppe wrote:
>"...If it's open, the most it might do would be to attract a few
>electrons being negative like was mentioned...(etc.)"
>
>In the old days this was called 'contact bias,' and was the principle
>incorporated intentionally in a grid-leak biased stage.
>
>
>73, Roy K6XK
!!!!!!!!! Where above did I say anything about contact bias?
Another;
On 7/22/06 at 4:25 PM Tom W8JI wrote:
>> Roy,
>> You got it! Most of the old sweep tube amps used a form of
>> grid leak bias.
>> Best,
>> Will
>> *********** REPLY SEPARATOR ***********
>> On 7/22/06 at 10:50 AM Roy Koeppe wrote:
>>>"...If it's open, the most it might do would be to attract
>>>a few
>>>electrons being negative like was mentioned...(etc.)"
>>>In the old days this was called 'contact bias,' and was
>>>the principle
>>>incorporated intentionally in a grid-leak biased stage.
>>73, Roy K6XK
>
>Actually both of you are mixing applications and very
>different systems. Contact bias or space charge biasing is
>NOT the same as grid leak biasing.
No, this actually forms the basis for grid leak biasing where the grid collects
electrons. In a grid leak circuit, those electrons are fed to a capacitor to
ground which charges up with those same electrons during the positive half
cycle. Generally, a resistor is in parallel with the capacitor making the grid
leak circuit (grid leak capacitor and grid leak resistor). One can also add
some negative voltage from a power source giving it a minimum amount of bias.
The grid still bleeds off into the capacitor when doing this. Most of the old
amps in mention and other equipment used this method unless all grids were tied
to ground like a few did.
>Grid leak biasing is dependent on grid current to establish
>bias.
Not necessarily. A small negative voltage can be fed to the circuit giving it a
minimum amount the bias can go to. One can do it either way, but adding the
minimum bias voltage is done a lot in some of the older amps and other
equipment I'm speaking of.
A common application is oscillators where we might
>want zero bias at start and automatically adjusted bias that
>increases with oscillator output. This type of bias works by
>grid current from a positive going grid with respect to
>cathode placing the grid/cathode path into conduction!
Correct, but see above.
>Space charge bias, which is more commonly called contact
>potential bias, depends on the control grid having no
>leakage to the anode (no gas and low anode voltage) and no
>grid emission (cold grid) to allow a very small amount of
>charge buildup from the space charge around the cathode to
>develop self-bias. From "Electronic Communication" page
>13.15
Correct, but if using a grid leak circuit, the grid can still obtain some
electrons from the space charge around the cathode. You just said the grid can
charge, so the grid leak capacitor can be charged to a small value by it also.
>" This type of bias is useful with high-u tube operated with
>very small input-signal levels only. The diagram resembles
>grid-leak biasing used in oscillator and RF amplifiers, but
>the theory of operation is not the same." It goes on to
>warn inexperienced designers NOT to use this type of bias
>unless they are experienced since there are few applications
>where it is useful. It is useful with small tubes in very
>low level stages with low bias requirements, primarily where
>AC from a heater might modulate the cathode.
>
>The stuff Will remembers from cheap ratty poorly designed CB
>amplifiers is NOT contact potential bias, but rather bias
>developed when a grid is driven positive with respect to the
>cathode during part of an RF cycle in an amount that exceeds
>any negative bias. Don't hang your hats on that as something
>justify gross misstatements that the grid in a large power
>grid tube will go negative when floated! I especially
>wouldn't point to grid current biased sweep tube amplifiers
>as panicles of good communications system engineering.
Yes I mentioned them for others who may not know of this method. However, don't
degrade this method by calling it "cheap" or "poorly designed" as it was used
in numerous pieces of military and commercial tube equipment. The control grid
in a power tube can be charged negative the same as any other tube. Just
because your wrong in some areas, makes no need to try and belittle others by
your tone you use to try to prove a point that's partially incorrect.
>
>73 Tom
All I can say is, I rest my case on the mis-quotes from Tom.
Best to all,
Will
Post all the above applies to;
*********** 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.
>
>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.
>>
>> 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.
>
>> 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.
>
>> 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.
>
>>>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?
>
>
>> 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.
>
>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.
>
>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 we done Will, now that we both agree fusing the grid
>does nothing good?
>
>73 Tom
>
>
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