Grid current limiting

[tubeswell]

I'm trying to come to grips with understanding something I've read about this on the Valve Wizard site. www.freewebs.com/valvewizard/index.html

Essentially, What is the "space charge" that supplies electrons to the grid when grid voltage approaches cathode voltage? I thought electrons flowed from the cathode usually, but I can't get my head around where they come from when the bias voltage on the grid is 0V (and you are still supplying a voltage swing into the grid), i.e. on the +ve half of swing when the quiescent grid voltage is 0V.

I want to understand, in intuitive terms, is howcome the current flows into the grid and down thru the cathode. If the grid is swinging positive (above 0V), surely it would be attracting electrons if anything. How come they are flowing away through the cathode?

[Rob Mercure]

Well, unfortunately, the link your provided didn't point to a specific article so I'm not sure which one you're referring to. But whatever if your comments are accurate the statements must be incorrect as current isn't not going to flow from the grid to the cathode as the grid is too cold to emit electrons. As to "space charge" this is the build up of electrons around an element or a charge put upon an element with a voltage - there are a variety of low voltage tubes made for 12V B supply that have a grid that's sole purpose is to help attract electrons to the plate and even draws more current than the plate - it's in the location usually reserved for the screen grid. But this is another whole use of the term from what you're referring to I suppose.

Rob

[tubeswell]

Hi Rob

Sorry 'bout that - this link to go to the article

http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

Looking at the stuff on Page 6 and 7 .

I also don't quite get what he means by 'source resistance'. Is that where the electrons come from? (Pardon my complete lack of knowledge). And how does it relate to a gain stage?

[Steve Conner]

We define conventional current as flowing from + to -. But that's just a convenience: real electrons flow from negative things to positive, cathode to anode.

So the reason why you can't understand how the current "flows into the grid and down through the cathode" is that it doesn't. The electrons flow out of the cathode onto the grid: the grid is just acting like the anode of a rectifier when you try to pull it positive. So if you understand how a rectifier tube works, you understand where grid current comes from too.

[tubeswell]

We define conventional current as flowing from + to -. But that's just a convenience: real electrons flow from negative things to positive, cathode to anode.

So the reason why you can't understand how the current "flows into the grid and down through the cathode" is that it doesn't. The electrons flow out of the cathode onto the grid: the grid is just acting like the anode of a rectifier when you try to pull it positive. So if you understand how a rectifier tube works, you understand where grid current comes from too.

Well sure. I know about electrons flowing to a postive charge, and I have a pretty good intuitive graps how a rectifier tube works and I understand understand that the grid acts like a plate as grid voltage goes into positive w,r,t, the cathode.

What I was having difficulty with grasping intuitively is the way jargon in the following paragraphs:

"Grid-current limiting:
... What happens here is that as the grid voltage approaches the cathode voltage, electrons being drawn from the space charge get attracted to the grid rather than to the anode (since the grid is very much closer), rather as if we have place a valve diode between grid and cathode. Conventional current flows in the opposite direction - into the grid and down through the cathode - and this is known forward grid current. This causes a voltage drop across the source resistance, the actual voltage at the grid falls. To put it crudely, less of our input voltage actually makes it to the grid. And the more we attempt to make the grid positive, the more current flows in to it to prevent us doing so, as if some invisible volume control is being suddenly turned down. In technical terms, the input impedance of the valve quickly falls, and this effect is known as grid-current limiting, and it causes the output signal to appear ?chopped off? or clipped on the negative side."

Rob has since reminded me what 'space charge' is. (Thanks Rob - I had come across this term before - but I had forgotten what is was)

But what I am still trying to understand what they mean by how 'foward grid current" works and how "it causes a voltage drop across the source resistance". Having read the whole chapter again I think by the term 'source resistance' they mean plate load (or is it something else?). I can intuitively understand how raisng the grid voltage to 0V would increase current flowing into the grid, but I can't understand how conventional current 'flows into the grid and down through the cathode". If somebody could enlighten me, it would be most kind.

[Rob Mercure]

TW,

Just going from your quotes - haven't returned to your source - and addressing this in a jumbled order <grin>: but I can't understand how conventional current 'flows into the grid and down through the cathode". If somebody could enlighten me, it would be most kind. The key to this phrase is "conventional" - when electicity was new it was thought that electrons flowed from the positive terminal of a battery to the negative - this is how the terminal lacking electrons got called "positive" and vice-versa. So this view of electrons flowing from the positive was referred to as the "conventional" one and used solely, as best I can tell, as an early instructional tool when teaching theory. When I took my first electronics instruction in the late 1960s it was still being referred to but soon afterward it was dropped, as best I can tell, as this is the first time anyone has referred to it in a quite long time. Why the author of the material you're looking at would bring it up I have no idea. Conventional current flows in the opposite direction - into the grid and down through the cathode - and this is known forward grid current. Again more of the same view. One thing to note is that the feller is referring to a "2" (such as AB2) where the grid is drawing current. Sometimes in a 2 situation the grid gets slammed with sufficient electrons that it heats up which makes the electron cloud (space charge) mobile enough to emit electrons and these can provide current that flows to the plate - but, not, as best I know, back to the cathode. There are some industrial very high power tubes that have grids that glow like a filament due to this in which amps of grid current flow but again I'm referring to industrial situations and nothing that we'd worry about in the consumer electronic world we swim in. For all practical purposes the highest power tubes we deal with - 6550s and KT 88s - might pass a few tens of milliamps at most before the grid assembly heats up, softens, sags, and shorts to the cathode. I've never seen it but I've seen screen grids fail in a similar manner. I guess I'll have to read the article but I suspect that his guy is shooting the shit and introducing factors that apply to industrial and military situations to areas where they don't apply - sorta like the HiFi nuts applying the skin effect to copper wire and demanding O2 free wire for their O2 free brains and hyping balance in dual triodes when this only makes a difference in laboratory equipment and ancient tube analog computers.

Rob

[Rob Mercure]

TW,

Just going from your quotes - haven't returned to your source - and addressing this in a jumbled order <grin>: but I can't understand how conventional current 'flows into the grid and down through the cathode". If somebody could enlighten me, it would be most kind. The key to this phrase is "conventional" - when electicity was new it was thought that electrons flowed from the positive terminal of a battery to the negative - this is how the terminal lacking electrons got called "positive" and vice-versa. So this view of electrons flowing from the positive was referred to as the "conventional" one and used solely, as best I can tell, as an early instructional tool when teaching theory. When I took my first electronics instruction in the late 1960s it was still being referred to but soon afterward it was dropped, as best I can tell, as this is the first time anyone has referred to it in a quite long time. Why the author of the material you're looking at would bring it up I have no idea. Conventional current flows in the opposite direction - into the grid and down through the cathode - and this is known forward grid current. Again more of the same view. One thing to note is that the feller is referring to a "2" (such as AB2) where the grid is drawing current. Sometimes in a 2 situation the grid gets slammed with sufficient electrons that it heats up which makes the electron cloud (space charge) mobile enough to emit electrons and these can provide current that flows to the plate - but, not, as best I know, back to the cathode. There are some industrial very high power tubes that have grids that glow like a filament due to this in which amps of grid current flow but again I'm referring to industrial situations and nothing that we'd worry about in the consumer electronic world we swim in. For all practical purposes the highest power tubes we deal with - 6550s and KT 88s - might pass a few tens of milliamps at most before the grid assembly heats up, softens, sags, and shorts to the cathode. I've never seen it but I've seen screen grids fail in a similar manner. I guess I'll have to read the article but I suspect that his guy is shooting the shit and introducing factors that apply to industrial and military situations to areas where they don't apply - sorta like the HiFi nuts applying the skin effect to copper wire and demanding O2 free wire for their O2 free brains and hyping balance in dual triodes when this only makes a difference in laboratory equipment and ancient tube analog computers.

Rob

[tubeswell]

Thanks Rob

So then, is there such a phenomenon as 'grid current limiting' when the grid bias is getting close to 0V w.r.t. the cathode. And how does it work (if the current doesn't flow through the grid and down the cathode?)?

I understand how the plate goes into cut off when the grid is biased too cold. What is the equivalent behaviour when the grid is biased too hot?

[BackwardsBoB]

At least in a tube amp, the drive to the grid is either AC coupled through a capacitor, or it isn't. Usually it is, so lets deal with that first. When the AC driving signal forces the grid positive, to 0V with respect to the cathode, electrons flow to and out of the grid, which is what normal people call current into the grid. This current passes through the coupling capacitor, and the grid side of the capacitor develops a charge, which reduces the grid voltage, limiting the current to what the grid leak resistor can provide. The charge on the capacitor sticks around until the signal stops trying to drive the grid to a voltage higher than the cathode. Then it begins to discharge through the grid leak resistor. Until it discharges, output of the tube is dramatically reduced, and the tube is easily driven to cutoff. This is called blocking distortion, and the amount of time it lasts depends on the value of the capacitor and grid leak resistor.

Sometimes a cathode follower's grid is driven directly off the junction of the plate and plate resistor of the preceding stage. In this case, the grid saturation current pulls the combined resistance of the plate resistance and plate resistor negative, maintaining the grid near zero volts.

If you could drive the grid positive directly with a low resistance source, the cathode-grid structure would act as a tube rectifier, and lots of current would flow, causing the tube innards to misbehave, and causing the tube to die a horrible death.

The current does "flow through the grid and down the cathode". Only the electrons flow from the electron cloud around the heated cathode to the grid.

[tubeswell]

Thanks Backwards Bob. I appreciate that explanation thanks. Why these kind of introductory texts have to be written in as-codified-a-way-as-possible is irritating. I could do with a 'zen' type book on tube amps that explains everything in a natural easy-to-understand way. Your explanation is much easier to understand.

I just couldn't make any sense of that statement about the current flowing 'down the cathode". It seemed to defy logic. Relief to know I'm not going mad :-)

[Enzo]

... What happens here is that as the grid voltage approaches the cathode voltage, electrons being drawn from the space charge get attracted to the grid rather than to the anode (since the grid is very much closer),

I thought that was pretty clear.

The cathode when hot just sits there emitting electrons. They would gather in a cloud around it if there were no other influences. There is a positive voltage on the plate, and the negative electrons are attracted to it. And there they'd fly without a grid. Add a grid and things change.

As the item states, the grid is far closer to the cathode and its electron cloud - the space charge - than is the plate. Without a grid, the tube would just conduct as hard as it could - like a recitifer tube. The grid is there to control the flow by interfering with the electrons path to the plate.

SInce like charges repel, a negative charge on the grid will repel electrons coming its way. But if the grid is made positive, then all of a sudden, electrons are attracted to it. When electrons flow from point A to point B, that is current.

When signal sits on a grid, the positive portion of the wave makes the grid more positive, but that doen't mean the grid is positive with respect to the cathode. If the cathode sits at +1.5v, then the grid signal has to top +1.5v for the grid to see any current.

When the grid goes positive, that current starts to flow, and the current path is down through the grid resistor to ground. Current through a resistor will cause a voltage to appear across that resistor.

I think in conventional current, all the while totally aware that electron flow is what is really happening. One is the opposite direction of the other. Look at a rectifier tube. Electrons flow from the cathode to the plate. In a typical amp, that means the B+ filter cap is supplying electrons to the plates of the recto tube. They flow through the transformer adn into the other end of the filter. But it is mostly more convenient to think of a B+ trying to find its way to ground than it is to think of B+ as a hole for electrons trying to find their way up from ground to fill.

When grid current flows, the electrons are coming from the cathode to the grid. Therefore, "conventional current" would have a positive current flowing the other way. This is not really different from B+ electrons flowing from ground up to the B+ rails throughout the circuit. But in the grid we usually don't think of it in those terms.

[tubeswell]

I thought that was pretty clear.
I think in conventional current, all the while totally aware that electron flow is what is really happening. One is the opposite direction of the other. Look at a rectifier tube. Electrons flow from the cathode to the plate. In a typical amp, that means the B+ filter cap is supplying electrons to the plates of the recto tube. They flow through the transformer adn into the other end of the filter. But it is mostly more convenient to think of a B+ trying to find its way to ground than it is to think of B+ as a hole for electrons trying to find their way up from ground to fill.

When grid current flows, the electrons are coming from the cathode to the grid. Therefore, "conventional current" would have a positive current flowing the other way. This is not really different from B+ electrons flowing from ground up to the B+ rails throughout the circuit. But in the grid we usually don't think of it in those terms.

Thanks Enzo - your explanation of conventional current makes my quest in this regard complete. It was what was throwing me - what with my understanding that electrons flow towards a positve charge'n'all - until you explained it I couldn't conceive how current could flow from the +vely charged grid down through the cathode. :-)

[Robert M. Martinelli]

But what I am still trying to understand what they mean by how 'foward grid current" works and how "it causes a voltage drop across the source resistance".

After all that's been said I think I have very little contribution to bring indeed, anyway, here they are my "1 cent worth" words.

The reason it causes a voltage drop across the source resistance ( impedance ) - is exactly the same : Grid has normally a very high input impedance ( in the no-grid-current-flowing-normal-operating-conditions ) but, when grid current starts to flow, input impedance falls considerably ( because it' s like having a rather small resistor in parallel between the signal on the grid and the cathode, whose value is determined by the potential difference between the grid and the cathode divided by the grid current flowing ) and that causes the input signal to drop dramatically, because it is applied to a greater load ( much smaller resistance/impedance ).

Hope this helps

Best regards

Bob

[booj]

So when the grid shorts electically to the cathode, the source voltage sees the cathode resistor in series with a short. That knocks down the source voltage. I guess that's about $0.01'th worth