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Unbypassed cathode resistor and feedback

173.48.30.29

A common-cathode circuit with an unbypassed cathode resistor is an example of series-series negative feedback. This means the feedback signal is sampled from the output current and summed with the input voltage. The effect of current sampling is to raise output impedance, and the effect of voltage summing is to raise input impedance.

It's easy to show that the series-series feedback model is a good fit to the tube circuit in question. That doesn't mean you are obliged to use the model to predict or explain circuit operation. But to claim that the model doesn't fit (that is, that the unbypassed cathode resistor does not introduce negative feedback) raises a logical inconsistency that's pretty much impossible to reconcile with accepted rules of circuit analysis.

-Henry

Follow Ups:

RE: Unbypassed cathode resistor and feedback - casouza 23:24:32 10/27/09 (0)
Jean Hiraga, the French tube guru, wrote a seminal book on "Amplis a lampes" back in 1977.
He had a whole chapter devoted to bias.

The degenerative effect at the cathode is clearly explained in this page;
http://www.bonavolta.ch/hobby/en/audio/t_bas9.htm
Look for the paragraph that starts with automatic Biasing (self bias).
Then read carefully the paragraph that starts wih: This system has a drawback.
Think in terms of local degeneration, not loop feedback.
I hope this helps
- http://www.bonavolta.ch/hobby/en/audio/t_bas9.htm (Open in New Window)
Conceptualization - Triode_Kingdom 05:53:07 10/27/09 (0)
Henry, this is a conceptualization issue. Feedback of all types - internal and external, and both naturally occuring and that introduced by the designer - is recognized as such everywhere but in discussions like this. Actually, referring to this as "conceptualization" might be too kind. Some would say "semantics." :)
RE: Unbypassed cathode resistor and feedback - danlaudionut 16:25:47 10/26/09 (15)
Henry

Since you like diagrams ...
An unbypassed cathode resistor grounded cathode
circuit is like a split load phase splitter
without using the cathode output of course.
The tube produces a signal on the plate
and on the cathode and since the cathode
signal isn't used it is "wasted".
It is degenerative with respect to the grid
but at no time is the plate signal
fed back to the input which is the grid.
Since there is nothing being fed back
then it should not be called feedback.

> > This means the feedback signal is
> > sampled from the output current
> > and summed with the input voltage.

The grid signal is amplified by the tube
and the tube produces a signal on the
plate and the cathode so where is this
so called "sampling" taking place?
The cathode signal is subtracted
from the input signal not summed.
You make it sound like there is some
time delay when it's simultaneous.
The tube is producing both signals
at the same time so there's no delay.
I agree with the effects though.

DanL
- RE: Unbypassed cathode resistor and feedback - Henry Pasternack 17:32:24 10/26/09 (2)
  The forward element of the circuit is a two-port network containing the triode. This network has two input terminals and two output terminals. The cathode is connected to both the input and output ports as shown.
  
  The triode is a transconductance amplifier. The current flowing through the output terminals is equal to gm times the voltage across the input terminals.
  
  The feedback element is a two port network containing the cathode resistor as shown. The feedback network samples the output current and converts it to a voltage that is summed with the input voltage, Vs.
  
  Compare with the first diagram I posted. The rest follows in a straightforward way from the model. With a bit of Googling you can quickly find a full analysis on the web.
  
  -Henry
  - RE: Unbypassed cathode resistor and feedback - danlaudionut 17:56:45 10/26/09 (1)
    Henry
    I will restate my position again.
    The output of the circuit is the plate.
    The input is the grid.
    Both are referenced to ground.
    Feedback by definition says that the
    output is fed back to the input.
    The grid to ground signal is NOT affected
    by signal on the cathode resistor therefore
    nothing is being fed back so there is no feedback.
    The BACK part means a previous point in the circuit.
    The plate being the output is NOT fed back
    to the input which is the grid and of course
    both signals are referenced to ground.
    It is at best a misnomer and at worst stupidity.
    And people say it means more than that
    but the word FEEDBACK speaks for itself.
    DanL
    PS Both Vs and Rl are refenced to ground and
    have nothing to do with the cathode resistor
    in your drawing so I still don't see feedback.
    
    Edits: 10/26/09
    - RE: Unbypassed cathode resistor and feedback - Henry Pasternack 18:26:14 10/26/09 (0)
      Hi, Dan. I agree that by your definition cathode degeneration is not feedback. But I also think your definition is overly restrictive and ultimately your way of thinking about feedback isn't as powerful or practical as the way engineers think about it.
      
      I'd be gratified if after all this you were motivated to study network analysis more deeply and eventually came to see things the way I and other engineers do. But I'm not here to fight, so I can only share what I know and encourage you to work it out on your own.
      
      Have fun.
      
      -Henry
- RE: Unbypassed cathode resistor and feedback - kurt s 17:23:00 10/26/09 (11)
  A cathode follower is 100 percent NFB on a tube stage. That's why vout ~ vin. Your schematic is missing something: Rk=Ra.
  
  Since the follower portion is directly 100% NFB such that vo(+) = vin and vk = va so that |vo(+)| = |vo(-)|, then the anode is in complete control of the voltage of the cathode. So it follows that the anode is under direct influence of 100% NFB and 100% tracking from that. The anode in other words sounds like the cathode, just inverted, and from solid 100% NFB.
  
  It's "instantaneous" NFB by virtue that there is so little lag up to a very high frequency. There is no perfect "instantaneous" response as electrons have delay. It just gets really short in delay. In this manner, no compensation need be there since the phase margin is about the full 180 degrees to a frequency the gain goes to an unimportant level as it gets sucked up by grid capacitance.
  
  So what is the tube in between doing? It's the modulating voltage that maintains vo(+)=vo(-), or vo(+) + vpk + vo(-)(w/respect to virtual ground at B+) = B+ = constant. The current varies, the voltages vary, but there are those two equal voltages that must stay equal or the circuit isn't working. But this circuit can only do that by 100% NFB at vk (and hence vp w/ respect to B+).
  
  Vk is DC, vk is AC, BTW. So all this is an AC analysis. I hope this helps you get it.
  
  -Kurt
  - RE: Unbypassed cathode resistor and feedback - danlaudionut 17:32:46 10/26/09 (10)
    Kurt
    
    The problem is the term feedback
    being applied to this circuit
    because the output is not being
    fed back to the input.
    The grid is the input and the
    tube's output is not influencing
    the input just the tube's operation.
    The input IS the grid to ground signal
    and that is not being changed by the
    cathodes signal at all.
    The word "back" in feedback means that it must be
    fed back to a previous point which is not happening.
    
    DanL
    - The mind blowing "untouched" NFB of triode plates to grid. - kurt s 22:19:19 10/26/09 (7)
      Henry and I see this one differently and I know both are correct in how you see the models.
      
      First think of the pentode and electron beam theory. In fact think of the beam tetrode if you will. In this physical layout, it is specially designed to have all the attraction head straight toward the screen grid. The attraction is designed to be maximum there and it is "screening" out the the impact of the plate as much as can be. The electrons fly by if aligned well with the input grid 1 and suppressor grid 3. Now why is there something called a suppressor grid that is usually just connected to the cathode inside or set to cathode potential in the beam tetrode? This is at a lower voltage and it picks up secondary emissions lost from anywhere: the screen grid's secondary "multiplier electrons" and the plate's "impact splatter secondary emissions" of high energy electrons. The screen's primary job is to hold a high accelerating potential close to grid1 to start sending out a beam of the electrons past it to the plate. The plate and screen are too far apart for the plate to impact input grid1. The electrons are already accelerated toward the plate and increasing plate voltage does nothing to accelerate more electrons toward it. Again the screen is screening off the plate and its voltage matters for current acceleration.
      
      So we have flat plate curves for the pentode as the plate voltage goes up, the Ip remains flat and hence operating in the constant current region. So what? That means that plate secondary emissions are swept up and never sent back to the input grid and there is no contact for any real NFB inside there. Try NFB by adding a large plate resistor and it will not change its essential gm gain, and this is a real gm device. And what is the Rp of a good pentode? Essentially infinite. It's a current source, not a voltage source.
      
      Now take off the screen and suppressor and it's a triode now. What do we lose and maybe gain here? We lost plate isolation due to the loss of the screen, and we lost secondary emission clean-up by losing the suppressor. What does all this new dirty electrons running amok do for us now? Well now the plate is in close contact to the grid and cathode for attracting and accelerating electrons. And when we add more voltage to the plate, like we did with the screen, it will now increase electron current flow straight to the plate output. But this time a powerful electron bombardment does create a splash of plate electron secondary emission, and those negative ion clouds push back on more electrons running in as it isn't so positively charged anymore. So there is a new force slowing down an attempt to accelerate electrons to the output, and it operates in a way to negatively slow down more with ever more voltage and current hitting the plate. This is what I call the physics of a natural NFB inside. And the device is no longer a constant current, it now has internal plate resistance, Rp.
      
      But like most people see, the force is not pushing hard against the grid voltage, yet it counters your effect at the grid to push foward what was easy on the pentode, and hence has lower gain too. So now the plate voltage changes as well as current changes. This shifts the plate curves radically upward and no constant current region can be found. It's all triode region. If you increase plate current on the triode's plate by raising just plate voltage, it will rise fast and high at low Vgk and almost quite linear. In fact the plate current rises faster than the plate voltage. When you go past most published curves at very high plate currents that starts in on secondary emission and high red glowing plate electron bombardment you will start seeing the bend over where the plate can take no more current and is blasting off its plate a lot. This is increasing plate resistance fast. Some tubes are designed to handle it in that region.
      
      But then watch as you see the high gm tube work Ip vs Vp with higher Vgk levels. The grid is meant to control Ip as it is still a transconductance amp by Vgk. But at higher Vp it is having less and less control over this valve. The currents flatten out and Rp increases again. With lower allowed current by Vgk input, Vp itself is more hampered against getting Ip to move up. And that's what you want. But there will be no constant current region and there will be real plate resistance to see.
      
      Now set the load line for a vertical load, or a plate resistance real low. The lowering of Vgk from cutoff to all on will send it through only a few Vgk changes before it will burn up Ip as Vgk nears 0. Near cutoff it moves rapidly in changing Ip as it turns on, all completely nonlinear. The pentode would be fine with it and amplify well. To drop distortion rapidly, make the loadline horizontal, so that little plate current is altered as Vgk moves and gently moves Vpk. It's now much more linear. But we accomplish that at the plate side only! For the pentode, little difference in linearity for given plate loads. Built-in plate-grid NFB not there in a pentode, yet is in a triode.
      
      So with a variable Rp device called the triode, a non-touching NFB arises by increasing the plate load, and you can take it out to a full CCS. On a near infinite non-varying Rp device called the pentode, a CCS will cause disaster as two fighting CCS devices try to set the ONE current.
      
      So is there a PURE NFB inside a triode type device, or is there just the characteristics of a three terminal device? It hardly matters, but the triode is the only device made by man that can do that single trick, and is unique and not yet SS replaceable. BJT's might look like them, but their characteristics are current amps, not transimpedance, and the harmonic structure naturally is horrible. So only MASSIVE NFB makes them get linear looking.
      
      Many special devices were invented for unique characteristics wanted, but lost as they got replaced by mass production for alternative applications.
      
      Nothing replaces the mechanical line stretcher for microwave use, but it is a rarity item today as more electronic and more expensive equipment built uses alternative methods to encompass more measurement flexibility. But for one thing, that old line stretcher is what we look for sometimes and hope no one threw away. Triodes are on the list. It was the simplest and best for pure sound, but not for modern computers and most applications today where poor substitutes for cheaper solutions came into being. So it being first is an accident, and would not even be seen if the technology for SS was there in 1900.
      
      -kurt
      - I can't believe no one has commented on this post... - Tre' 14:20:11 10/27/09 (6)
        
        In Reply to: RE: The mind blowing "untouched" NFB of triode plates to grid. posted by kurt s on October 26, 2009 at 22:19:19
        
        but it is brilliant.
        
        Thank you.
        
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        RE: I can't believe no one has commented on this post... - Henry Pasternack 14:32:01 10/27/09 (5)
        
        In Reply to: RE: I can't believe no one has commented on this post... posted by Tre' on October 27, 2009 at 14:20:11
        
        Newsgroups: rec.audio.tubes
        From: "Henry Pasternack"
        Date: Fri, 22 Sep 2006 16:06:35 -0400
        Local: Fri, Sep 22 2006 4:06 pm
        Subject: Diodes, triodes, and negative feedback
        
        I noticed that the triode negative feedback argument is still going on. I'm a
        bit surprised because I don't think the issue is really all that complicated.
        Somewhat complicated, yes, but not beyond resolution.
        
        The answer to the question, "Does a triode have internal negative feedback?",
        is, "Yes, if you want it to, no if you don't." But I'll admit my bias and state
        from the start that I don't see much use to the triode feedback viewpoint.
        
        I'll explain what I mean. Negative feedback is a model. The system operates
        the same regardless of the model. You can imagine an operational amplifier
        with its inverting input connected to its output through a voltage divider. You
        can model the system in terms of its forward and reverse transfer functions
        and use the feedback equation to come up with the answer. Or you can just
        solve for the response as a single system of equations, and never bother to
        consider negative feedback. The amplifier doesn't care. It's up to you, the
        designer, to decide what form of analysis you find most convenient at the time.
        
        It's true that if you open and close the feedback loop, certain predictable
        changes occur. The gain, bandwidth, transient response, and input/output
        impedances change. You can say that the existence of these changes
        "proves" that feedback is at work. But this is circular reasoning. You could
        just as easily explain the difference by noting how the connection from
        output to input connection modifies the signal input voltage.
        
        With respect to triodes, you can model the internal behavior different ways.
        The traditional model views the cathode as an electron emitter where the
        probability of emission depends on the cathode temperature and the
        electric field at the cathode surface. It assumes the electrons come out
        with zero kinetic energy. The plate-cathode voltage establishes a boundary
        condition and the subsequent density, distribution, and velocity of electrons
        within the tube falls out from the solution of a relatively simple differential
        equation.
        
        The effect of the grid-cathode potential is to augment the plate-to-cathode
        electric field, modifyinng the boundary conditions of the differential equation
        and shifting the current-vs-voltage curve left or right according to the grid
        voltage. The plate resistance is simply the slope of this curve, and the
        effects of both plate and grid potentials are purely in the "forward" path.
        
        Taking the other position, you could view both the plate and the grid as inputs.
        Their voltages each produce an electric field at the cathode surface which,
        summed together, determine the space charge density around the cathode.
        The space change density, in turn, determines the plate current and, in
        conjunction with the plate resistor, this creates a signal voltage at the plate.
        The signal voltage "feeds back" to the cathode electric field, modulating the
        space charge density and therefore the resulting current flow.
        
        The second model is pretty complicated, and there are some serious
        objectsions to be made to it. I'll get to that, but first I want to talk about
        diodes and triodes.
        
        It's been stated on this forum that diodes, unlike triodes, have no internal
        negative feedback. Isn't this is a paradoxical claim? The grid in a triode
        biases the space charge density, but It doesn't fundamentally change how the
        tube operates. Indeed, if you connect the triode's grid to its cathode, the
        E-field component due to the grid is zero and the tube behaves for all intents
        and purposes, exactly like a diode. If a triode has internal negative feedback,
        then by the exact same argument, so must a diode. This is an extremely
        important point to understand.
        
        If you have an operational amplifier in the classic non-inverting configuration
        and you connect the input to ground, the negative feedback is still in control
        and the amplifier's output impedance stays the same. But grounding the
        non-inverting input takes that input out of the picture. Mathematically, it's
        as though it no longer exists. The three-terminal op-amp has turned into a
        two-terminal op-amp yet the negative feedback is unaffected. This is how a
        regulated power supply works, incidentally.
        
        The NFB argument says that the low output impedance of the triode is due
        to the negative feedback between the plate and cathode, which, if we were to
        put a screen grid into the tube, could be taken away, thereby significantly
        raising the output impedance. Now, the plate curves of the diode and the
        triode (with Vg=0) are exactly the same. You could put a screen grid in the
        diode, apply a fixed potential, and greatly increase its plate resistance, just
        like a triode. Overall, if you are comfortable saying that a triode is a pentode
        without a screen grid, then you should be equally comfortable saying that a
        diode is is a pentode without a screen grid or a control grid. Either they both
        have feedback, or they don't.
        
        Why is this important? One of the claims I have read on this newsgroup recently
        is that the plate resistance of a diode is just an "ordinary" non-linear resistance
        and has nothing to do with negative feedback. The mechanism that produces the
        3/2 power plate curve is exactly the same for the diode as it is for the triode. The
        Child-Langmuir law applies to both and the derivation is the same. If the plate
        resistance of a diode is just an "ordinary" resistance, then so is the plate
        resistance of a triode. The control grid is a complication, but it has no bearing
        on this mechanism.
        
        To understand whether or not a triode has negative feedback, we'd like to boil
        the problem down to its barest essentials so that the underlying principles
        are exposed clearly and without confusion. That's why it helps to show that
        the control grid is irrelevant to the question. It's hard to visualize negative
        feedback in a diode (or a resistor, for that matter, but more on that later)
        because a simpler, open-loop model is just cleaner and more intuitive. When
        you add a control grid in there, things get more complicated and it becomes
        tempting to start waving hands. We'd like to avoid that.
        
        Getting back to op-amps, it's very easy to separate out the forward path and
        the reverse path. The forward path is everything inside the little triangle,
        between the two input leads and the one output lead. The reverse path is the
        external network connecting the output terminal to the inverting input terminal.
        Being able to separate these two paths so cleanly makes it much easier to
        visualize how the operational amplifier operates as a feedback circuit.
        
        In the proposed diode NFB model, there is an inconvenient blurring between
        the forward and reverse paths. The plate-cathode potential has two functions.
        First, it establishes the electric field at the surface of the cathode, which in
        turn modulates the space charge density. Second, the very same field
        sweeps electrons out of the electron cloud and accelerates them to the
        plate, establishing plate current. The first of these two functions may be
        called feedback. The other determines the forward gain. To show that the
        tube has negative feedback, you have to separate these two paths out, but
        it's not such an easy thing to do. This is a problem for the feedback model.
        
        There are two ways that tube feedback modelers have solved this problem.
        The first is propose connecting the plate to a fixed DC potential. This lets
        the tube keep conducting but eliminates any E-field feedback from the plate.
        Fixing the plate potential raises the gain (transconductance) as predicted by
        feedback theory. This is the essence of an argument that one member of
        this forum put forth as "proof" of triode NFB.
        
        But there is a glaring hole in this argument. There is absolutlely nothing
        that external observation can prove conclusively about the internal operation
        of the tube. There could just as easily be a nonlinear resistor inside the tube,
        or little elves with voltmeters and a rheostat controlling the electron flow in
        real time. All you can determine from external observations are the external
        properties of the tube, and while these may suggest the action of negative
        feedback, they do not by any means prove it.
        
        The other, perhaps more convincing way that feedback modelers have tried
        to prove their point is by postulating the existence of a fictitious screen grid
        inside the tube. The screen grid takes away the feedback due to the varying
        plate E-field while allowing the plate voltage to vary. With this fictitious screen
        grid in place, the tube has higher gain, like a pentode.
        
        There are several rebuttals to this argument. The simpler one is that the
        fictitious screen grid is, indeed, a fiction. The triode is not a tetrode. It
        should suffice to model the tube without the artifice of the fictitious grid.
        Indeed, there is there is a perfectly good model that doesn't depend on
        this complication. But the real problem with the fictitious grid argument
        is that it pulls itself up by its own bootstraps. It says, a triode has NFB,
        and a pentode is a triode with the NFB taken away by the screen. So
        if we take a pentode and remove the screen, the feedback comes back
        and this proves the triode has NFB. This is a brilliant example of circular
        reasoning.
        
        The triode NFB argument depends on another fiction, the "virtual grid", which
        is the internal control point related to the electric field strength at the cathode
        and space charge density. The model says the actual input to the tube is
        this "virtual grid", which is related to but distinct from the external grid and
        plate connections. This is an extremely awkward model because it relies
        on an abstraction of the tube that is quite removed from its actual use in-circuit.
        Contrast this with the op-amp where the input, output, and feedback connections
        are quite clear and in the open.
        
        One of the points made against the triode feedback model is that there is no
        effect on the input impedance seen at the grid. In response, it's been noted
        that even with conventional shunt feedback applied to the cathode, there is no
        effect in the tube's input impedance. This isn't true. Shunt cathode feedback
        definitely raises the input impedance seen at the grid. It's just that the tube's
        input impedance is already so high and swamped by the bias resistor that we
        don't notice the difference.
        
        I mentioned resistors earlier, and I'd like to get back to that. Others have
        pointed out that you can use the same kind of argument for triode NFB to
        show that there is negative feedback in resistors, or automobiles, or electric
        motors. It's actually kind of fun to derive the formula for a voltage divider in
        terms of negative feedback, or even transmission line reflection coefficients.
        
        Consider a resistor as a cylinder of material with a metallic cathode
        bonded to one end and a metallic anode bonded to the other end. If you
        apply a positive potential between the anode and the cathode, an electric
        field will be established between the electrodes and electrons will begin to
        drift inside the cylinder, creating a current flow. If the power supply has
        significant output resistance, the supply voltage will drop, reducing the
        electric field within the body of the resistor and modulating the current. In
        other words, negative feedback. Or is it feedback?
        
        The difference between the diode feedback model and the resistor feedback
        model is that in the resistor you don't have the additional complication of
        the space space charge density. But the principle is the same. In either
        case, the current flow that results from the application of an external
        voltage is set by an equilibrium that develops within the device. You can
        find zillions of similar examples in nature and engineering.
        
        Parachutists are grateful for the negative feedback of air resistance that
        reduces the "gain" of gravity accelerating them to the ground. Airplane
        designers take pains to eliminate this feedback, maximizing the "gain" of
        their engines by installing aerodynamic screens on the aircraft. Yes,
        feedback is everywhere. And nowhere. It all depends on your point of
        view.
        
        Whether or not you choose to think of tube operation in terms of feedback
        is your choice. It causes no harm to anyone. But you should be careful
        to avoid insisting on the presence of feedback. Feedback is only as good
        as the value it brings to engineering analysis. In practical terms, when it
        comes to triodes the value isn't that great. I don't believe it gives us any
        greater insight than we get from the standard models. In particular, as we
        move from coarse generalities to much finer levels of detail, it remains to be
        shown that feedback helps to predict the second- and third-order nonlinear
        behavior of the tube.
        
        It's been said, and I agree, the tube doesn't give a damn how you model it,
        but goes about its business in blissful ignorance. Triode negative feedback
        is a great conversation starter, but there is a risk, in invoking negative feedback,
        of making things more confusing than they have to be. Out of confusion comes
        bullshit, and bullshit, we all can agree, is something to be avoided.
        
        Right?
        
        Have a nice day.
        
        -Henry
        
        I think my point of view and Henry's are still both correct... - kurt s 17:40:00 10/27/09 (4)
        
        In Reply to: RE: I can't believe no one has commented on this post... posted by Henry Pasternack on October 27, 2009 at 14:32:01
        
        where I take the "white box" or "clear box" testing paradigm and Henry takes the "black box" approach. But will yield same results where I am curious about the physics known inside and designed inside (but obviously don't know enough to be any expert there) to Henry's "so what how it does it, it just does it, take the simple external model."
        
        Correct me if I am wrong.
        
        -kurt
        
        RE: I think my point of view and Henry's are still both correct... - Henry Pasternack 17:58:27 10/27/09 (3)
        
        In Reply to: RE: I think my point of view and Henry's are still both correct... posted by kurt s on October 27, 2009 at 17:40:00
        
        Thanks for the reply, but I don't think you really got the point of my long article. (IMHO it's a brilliant essay, but I never expected anyone to read or understand it, not then, not now -- though I would be delighted if they did.)
        
        Here is a synopsis for the attention challenged: All the common triode circuit models are just abstractions of the tube that have no real bearing on the actual internal structure or operation. The internal NFB model is one such abstraction that does away with the series (Thevenin) or parallel (Norton) plate resistor and replaces it with an active control structure. It's no closer to the reality of how the tube works than the models it replaces. Furthermore, it gives no additional insight into externally observable behavior of the tube and its use in practical problems complicates circuit analysis. The model is based on circular logic and awkward assumptions. Finally, the underlying approach applied to other problems leads to feedback appearing in all kinds of devices where we don't normally think of it existing, like diodes, resistors, electric motors, and parachutes.
        
        I'm not actually that committed to the argument, not like I was three years ago, but I couldn't resist Tre's invitation to dredge up a few thousand words of archived historical drivel.
        
        -Henry
        
        RE: I think my point of view and Henry's are still both correct... - kurt s 21:22:33 10/27/09 (2)
        
        In Reply to: RE: I think my point of view and Henry's are still both correct... posted by Henry Pasternack on October 27, 2009 at 17:58:27
        
        I do understand all your points and maybe I am not conveying this understanding at all to you. And your essay was brilliant, Henry, and I mean that. I know I was certainly only one of the few who understood but pondered it over and said little about it for there are lingering doubts about what is knowable and what is important to know. Ya know?
        
        To access the POTENTIAL of its internal actions always requires external devices to complete the circuit that shows what that device is doing. If you don't have any way to predict what's inside as in Black Box.
        
        For the parachute, it is in a NFB situation when the application is showing the device's POTENTIAL. If it were used above the moon, it would never exhibit its POTENTIAL for any application, just as the feather and hammer simultaneous drop hides the properties inherent to a drop in atmosphere. External atmosphere differentiates the feather and the hammer and shows it's otherwise hidden basic property against wind resistance. Seeing it only on the moon would all make us think they were the same.
        
        You have to apply the device to see what the device's internal characteristics are. That can be done by black box analysis. Or if we do another analysis outside its normal application and peered inside instead to "measure" what's going on in there - the White Box case would tell us and predict to us what it will do in atmosphere and no atmosphere before we even begin applying it. Let's see: Broad light fabric attached to weight and we know F=ma and Force up will equal Force down. In resistive atmosphere Force up is real and we might even know it by calculation beforehand. In a vacuum Force up = 0 and nothing is even there to open it. So all forces are straight down to a parabolic descent.
        
        I see it as a viewpoint of believing you can predict the outcome of distortion in pentodes with varying plate loads without plate curves, but more basic physics that concludes calculable plate curves. And then do for structure analysis for triodes as well. And someone has written a computer program that will take triode structures and predict the plate curves from that and some other pre-known properties about the metals and coatings and what the heater is expected to see, etc., etc. And then we have insight into what triode curves are based on and what pentode curves are based on. No need to build the circuit to predict the basic outcome.
        
        An analogy is High Frequency Structure Simulator software, or HFSS. It allows you to draw a metal and dielectric full structure of however complicated you can get to in practical terms (hint, simplest geometries only, please). Then you can run the calculations and see what the EM fields do when sourced at one end through various frequency sweeps or just one point. This has helped many designers make some weird microwave accessories to better perfection, such as couplers. You now have a White Box to add into another application surrounding it and predict its basic performance, such as return loss, insertion loss, coupled loss, etc. Very helpful to add to further powerful microwave simulators of full circuits. But the circuits are only good as the designed couplers and those are based on known physical modeling and then making them as close as the model. So again, we see beforehand what kind of "in circuit" "NFB" will take place as we vary the circuit. Seeing it in action after the fact and modeling only after the build and measurement of the device and ignoring the structure simulator is a valid course of action and viewpoint on how you want to see that Black Box model. Just ignore the insides that do have loops and feedback all in there and how it became optimized by a more fundamental physical model. Or know that it was only this good because it "Clear Box" peered inside and did do that modeling work for a more precise design and a more predictable Black Box output spec.
        
        So this "new wave" is going on now. It's not enough to design a resistor by over processing with spirals as it will increase costs beyond the competitors and diminish performance. It can be designed and built by more stringent first pass controls and predictions from physical structural models. And then it really does matter that you know exactly how much NFB the device will cause in the circuit and what that is "internally" the "internal NFB" is, for lack of a better term.
        
        Now we need to differentiate those terms, like maybe the expected resistance, inductance, and capacitance along with what distortions are in it from nonlinear dielectrics in use from the device itself and what it will expect to do in a normal simulator of a full circuit it's used in.
        
        This is why I cannot dismiss the internals in knowledge and its own internal circuit that might have that wrongful by a tad "internal NFB" when placed in the circuit that shows what its properties are when applied for its intended use.
        
        There is just a new lower level of simulation that's happening, and has happened for many years actually. Parts need optimization by computer, too.
        
        Or am I still way off base here? I don't think so. Maybe I am not understood what I am trying to say. NFB requires more than one part and it needs stimulus, too. A device with "internal NFB" is assumed that the property of that poor term will come out in the circuit (environment) it was intended to be in.
        
        -kurt
        
        RE: I think my point of view and Henry's are still both correct... - Henry Pasternack 07:49:28 10/28/09 (1)
        
        In Reply to: RE: I think my point of view and Henry's are still both correct... posted by kurt s on October 27, 2009 at 21:22:33
        
        Interesting points, Kurt. This discussion has probably gotten too esoteric to be of interest to most readers, so I'll try not to go on too long.
        
        I agree that "white box" models based on direct internal testing or knowledge of device physics can help us do more accurate simulations and hopefully create better designs.
        
        I'm a bit hung up on whether the "triode internal NFB" model is really a white box model, or if it's just another black box model based on externally observable behavior that doesn't really tell us anything useful about how the triode works internally.
        
        The paper that was discussed on r.a.t., as I recall, wasn't derived from the physics equations (i.e., Child-Langmuir). I do know if you crack open a genuine triode and go looking for that hidden screen grid to do white box testing, you won't find it.
        
        So, let's not pretend the triode has a fictitious internally connected screen grid. Instead, let's treat it as a test probe that we use, in fact or in a thought experiment, to disconnect the hypothetical feedback loop inside the tube. I can't claim that this is really all that different from opening up a feedback amplifier and using a test probe to short out an internal feedback node for just the same purpose. Except, of course, in the case of the amplifier the internal feedback node is a tangible internal structure, whereas in the triode it's harder to visualize. But to a mathematician, I suppose, it's clear enough.
        
        Getting back to the introduction to my long essay, I can't argue that there is no way to model internal NFB in the triode. I just don't see what you gain from it versus, say, more detailed physics-based models. I know that internal negative feedback is a good way to model grid-plate capacitance, for instance, but I've never seen a practical problem where the kind of feedback model we're talking about here made the solution easier.
        
        I know that Mark Kelly, whom I respect as a math genius with a strong interest in triode modeling, looked at this question and came away unimpressed. Though I've had some strenuous arguments, notably with one fellow from Australia, nobody's ever framed it in such a way that I said to myself, "Holy moly, now I see da light!"
        
        In the end, I'm waiting for people to take this triode internal NFB thing and start solving problems with it that nobody's been able to solve by conventional methods. Or taking previously hard problems and making them simple. That will convince me.
        
        -Henry
        
        RE: I think my point of view and Henry's are still both correct... - PakProtector 03:49:07 10/29/09 (0)
        
        In Reply to: RE: I think my point of view and Henry's are still both correct... posted by Henry Pasternack on October 28, 2009 at 07:49:28
        
        After reading thourgh lots of the triode-has-internal-FB writings it seems for the most part to be a 'proof' that the pentode is bad.
        cheers,
        Douglas
        
        Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
    - RE: Unbypassed cathode resistor and feedback - Ray Moth 19:42:52 10/26/09 (1)
      "The input IS the grid to ground signal
      and that is not being changed by the
      cathodes signal at all."
      Dan,
      To understand what's going on, you need to consider grid-cathode as the input, not grid-ground, . Ground is not relevant to understanding the operation of a tube (in fact, if anything, it can be an obstacle). Ground is simply a reference point.
      Think of a grounded grid amplifier, such as the second stage of an LTP phase splitter or the upper stage of a cascode. The grid is grounded in both cases (as far as AC is concerned), yet both these have inputs, namely, cathode inputs.
      
      Edits: 10/26/09
      - RE: Unbypassed cathode resistor and feedback - danlaudionut 22:03:41 10/26/09 (0)
        
        In Reply to: RE: Unbypassed cathode resistor and feedback posted by Ray Moth on October 26, 2009 at 19:42:52
        
        Ray
        
        I realize that you need to have the cathode as
        the point of reference for your logic to hold water
        but everybody uses ground as the point of reference.
        To have you use a different point of reference to
        get your point believeable should ring some bells.
        You don't get to change the point of reference just
        to have your circuit theories make sense.
        I am talking about a circuit not the tube.
        I understand how the tube works, trust me.
        The tube works from grid to cathode BUT
        the input signal is grid to ground.
        Feedback by definition needs to have the output
        of the circuit(plate) be fed back to the input(grid).
        The input is not on the cathode on a grounded grid.
        The cathode signal is a byproduct of the tube's operation.
        It is not made by the plate current because the same current
        on the plate is the same current through the tube which
        is the same current through the cathode resistor.
        This current is set by the tubes interaction with
        the plate and cathode voltages and/or impedances.
        
        DanL
RE: Unbypassed cathode resistor and feedback - kurt s 15:52:35 10/26/09 (2)
In my day we called it current-series feedback. The four kinds were: a) a voltage amp using voltage-series feedback; b) a transconductance amp using current-series feedback; c) a current amp using current-shunt feedback, and; d) a transresistance amp with voltage-shunt feedback.

Terms change and probably new combinations were found so a new terminology got set up for simplicity.??

-Kurt
- RE: Unbypassed cathode resistor and feedback - Henry Pasternack 17:41:15 10/26/09 (0)
  I think you and I were trained in a similar era. I don't remember precisely what the terminology was back then, but from the materials I see on the Web today, it looks like your four kinds are now called:
  
  a) series-shunt feedback (a voltage amp using voltage-series feedback)
  b) series-series feedback (a transconductance amp using current-series feedback)
  c) shunt-series feedback (a current amp using current-shunt feedback)
  d) shunt-shunt feedback (a transresistance amp with voltage-shunt feedback)
  
  The first term of the pair refers to the type of input/summing (series = voltage, shunt = current) and the second term refers to the type of output/sampling (series = current, shunt = voltage).
  
  -Henry
- Always respect the still employed engineer who says "a 100 micro microfarad condenser." - kurt s 16:06:10 10/26/09 (0)
  And the ones who say "mega mega-cycles", or "Oersteds" or "hey rookie, get the new left handed spur sniffer."
  
  They usually know 4 more control variables than the rest to tweak out a design which were all published 30 years ago and never entered a textbook due to being too specific to include in a general text entitled
  "all about electronics, 101". It might, however, be seen in a post graduate class entitled "introduction to transformer theory."
  
  -Kurt
Can't argue with you on this... - Allen Wright 15:22:19 10/26/09 (19)
...but is it in reference to anything in particular?

Regards, Allen (Vacuum State)
- RE: Can't argue with you on this... - Henry Pasternack 15:29:54 10/26/09 (18)
  > ...but is it in reference to anything in particular?
  - http://www.audioasylum.com/forums/tubediy/messages/17/174720 (Open in New Window)
  - RE: Can't argue with you on this... - jon_s 17:08:16 10/26/09 (17)
    Oh boy, not this again... I tried to fight this good fight before, and decided that it's probably not worth it. Dan has been stubbornly unwilling to accept that the "input" to a simple triode amplifier is the grid-to-cathode voltage and not merely the grid voltage. As soon as he makes that connection, he'll hopefully have a "d'oh" moment and see the loop in the system.
    - RE: Can't argue with you on this... - danlaudionut 18:11:31 10/26/09 (14)
      Jon
      
      I understand what you're saying but nowhere else
      do you get to change your point of reference
      to suit you why should you be able to do it here?
      Do you measure the input to your amplifier
      referenced to the cathode of your first stage?
      No an input is referenced to ground -
      always was and always is.
      Well if you change the point of reference then
      the explanations make sense - Sorry no.
      
      DanL
      - RE: Can't argue with you on this... - Tom Bavis 06:17:50 10/27/09 (13)
        
        In Reply to: RE: Can't argue with you on this... posted by danlaudionut on October 26, 2009 at 18:11:31
        
        Think of it this way: the input is a voltage, but the output is a current. This current flows through the plate load AND the cathode resistor. It develops a voltage drop across the plate load (what you are calling the output) and also across the cathode resistor, which is in series with the input as well. The latter is subtracted from the input voltage, reducing the overall gain of the stage. This is CURRENT feedback, which will increase input and output impedance.
        
        Now if we make a different circuit with a resistor from plate to grid, I think you'd agree that feedback exists. But this is VOLTAGE feedback, which also decreases gain, but lowers input and output impedance.
        
        RE: Can't argue with you on this... - danlaudionut 07:18:41 10/27/09 (12)
        
        In Reply to: RE: Can't argue with you on this... posted by Tom Bavis on October 27, 2009 at 06:17:50
        
        Tom
        
        > > the output is a current.
        
        Incorrect the plate voltage referenced to ground
        IS the output if a CCS is the plate load then if
        what you say was true then no signal would be present.
        You have the textbook theory down pat.
        The cathode signal is subtracting from the grid input.
        Yes it is degenerative but you cannot try to
        change a given just to prove a point - Sorry.
        Plate current is not fed back to influence
        the grid to ground signal so no feedback.
        
        > > It develops a voltage drop across the plate load
        > > and the cathode resistor, which is in
        > > series with the input as well.
        
        Wrong again.
        The input is seperate from the circuit.
        The input is referenced to ground just
        like every other test point.
        Again you cannot change a given/point
        of reference just to prove a point.
        Signals always are refeenced to ground.
        You amp sensitivity is not referenced to
        the first stage's cathode voltage.
        No that would be silly.
        Somebody decided long ago to call it feedback
        and it was left to those after him to justify
        this to each generation after him.
        Nobody questions and everybody falls in line.
        This sounds familiar ... hmmm ...
        
        DanL
        
        RE: Can't argue with you on this... - Tom Bavis 10:41:47 10/27/09 (11)
        
        In Reply to: RE: Can't argue with you on this... posted by danlaudionut on October 27, 2009 at 07:18:41
        
        > > if a CCS is the plate load then if
        what you say was true then no signal would be present.
        
        Of course if the load is a CCS, then there is no current change and no signal develops across the cathode resistor. And there's no effect on stage gain, since there's no feedback.
        
        Feedback theory predicts the performance of a stage with an unbypassed cathode resistor. Call it a useful fiction if you like.
        
        RE: Can't argue with you on this... - PakProtector 13:18:44 10/27/09 (2)
        
        In Reply to: RE: Can't argue with you on this... posted by Tom Bavis on October 27, 2009 at 10:41:47
        
        ----Feedback theory predicts the performance of a stage with an unbypassed cathode resistor.
        
        I'd suggest adding that the condition must be met that current must change through the load. The current change in the load is the same current flowing through the cathode.
        cheers,
        Douglas
        
        Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
        
        Where is mutt and jeff in this thread - GEO 19:37:21 10/28/09 (0)
        
        In Reply to: RE: Can't argue with you on this... posted by PakProtector on October 27, 2009 at 13:18:44
        
        I suppose all this 'theory' and 'scientific engineering' has nothing to do with how an amp sounds...particularly if the right wire and other things are not used as per their specs......
        
        RE: Can't argue with you on this... - Tre' 14:16:11 10/27/09 (0)
        
        In Reply to: RE: Can't argue with you on this... posted by PakProtector on October 27, 2009 at 13:18:44
        
        Yes, there has to be current through the load otherwise nothing is being accomplished.
        But if we look at a 6sn7 with a CCS plate load and a 500K next stage grid resistor, the load on the tube is 500K. The load line for the tube would be 500K. Very close to horizontal.
        Yes there would be some local NFB if the cathode resistor is not bypassed but very little.
        So the output impedance would be increased because of this but very little.
        IF the tube was loaded with only the CCS and the CCS was truly constant current (I know, there is no such thing) then the gain and the output impedance would be unchanged, cathode resistor bypassed or not. (and, of course, no work would be done) If the stage is direct coupled to the grid of the next stage then only the Miller would be the load.
        Doug, I know you know all of this, I just wanted to write it down.
        BTW, I always bypass my cathode resistors even when the tube is loaded with a CCS.
        Tre'
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        Edits: 10/27/09 10/27/09
        
        RE: Can't argue with you on this... - danlaudionut 10:47:02 10/27/09 (7)
        
        In Reply to: RE: Can't argue with you on this... posted by Tom Bavis on October 27, 2009 at 10:41:47
        
        Tom
        
        > > Call it a useful fiction if you like.
        
        Well at least you admit it's fiction.
        
        DanL
        
        All circuit diagrams are fiction. - Henry Pasternack 12:18:29 10/27/09 (6)
        
        In Reply to: RE: Can't argue with you on this... posted by danlaudionut on October 27, 2009 at 10:47:02
        
        > Well at least you admit it's fiction.
        
        Dan, you're being stubborn and silly. Can you suggest any way that we might convince you to let go of your preconceptions and step up to the next level of understanding? Or maybe it doesn't matter to you...
        
        -Henry
        
        Just to keep the record straight ... - danlaudionut 17:15:41 10/27/09 (1)
        
        In Reply to: RE: All circuit diagrams are fiction. posted by Henry Pasternack on October 27, 2009 at 12:18:29
        
        I am not the one that keeps
        bringing this topic up.
        Ray thought he me in a "gotcha"
        then you started a new thread
        and I responded to them..
        
        DanL
        
        RE: Just to keep the record straight ... - PakProtector 17:37:50 10/27/09 (0)
        
        In Reply to: RE: Just to keep the record straight ... posted by danlaudionut on October 27, 2009 at 17:15:41
        
        But it is an interesting puzzle: how to explain it so that you get it. It is nothing new...took a few posts to get a good write up explaining just exactly why a PP, class A output stage loads the tubes with a-a/2, and still there were folks who argued very passionately otherwise.
        cheers,
        Douglas
        
        Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
        
        RE: All circuit diagrams are fiction. - danlaudionut 17:00:29 10/27/09 (3)
        
        In Reply to: RE: All circuit diagrams are fiction. posted by Henry Pasternack on October 27, 2009 at 12:18:29
        
        Henry
        
        The "up to the next level of understanding" seems
        to be change the point of reference which is
        the same as change the given in the analysis
        so that it can be seen as feedback.
        OR keep expanding the category of feedback so
        that this circuit falls with in it's perameters.
        I suggest you are the stubborn ones that
        cannot/will not see the truth because that
        is not what is the the "BOOKS".
        Independent thought has a tendency to be killed
        or at least disabled with formal education.
        I have seen it many times with engineers.
        The look into the book for all the answers
        and/or if it isn't in the book then
        it ain't so mentalities.
        
        It is real simple.
        GIVEN the input is the grid to ground signal.
        GIVEN the output is the plate to ground signal.
        Definition feedback is the output fed back to the input
        either to aid(positive) or oppose(negative) the input.
        So far I have been told to -
        Change the input to grid to cathode voltage.
        Change the output to the current on the plate.
        Expand my narrow vision of what feedback means.
        You can't change GIVENS to solve your proof.
        You can't change a definition just because
        you want it to fit something that it doesn't.
        Just because it is written in the "BOOK"
        doesn't mean it is right.
        I know heresy.
        
        DanL
        
        RE: All circuit diagrams are fiction. - Tre' 18:36:21 10/27/09 (0)
        
        In Reply to: RE: All circuit diagrams are fiction. posted by danlaudionut on October 27, 2009 at 17:00:29
        
        The tube does not respond to the grid to ground signal, it responds to the the grid to cathode signal. If the cathode is held tight (cathode resistor bypassed) then the cathode is at ground potential, AC wise.
        When the cathode resistor is not held tight the tube is not responding to the grid to ground signal but the grid to cathode signal.
        You said it yourself, the signal at the cathode will be subtracted from the signal applied to the grid. We are saying the same thing here.
        A cathode follower's output is not plate to ground but cathode to ground.
        In a parafeed circuit, where the output transformer is connected to the plate through a cap with the bottom end of the primary connected to the cathode not ground, the output is not plate to ground but plate to cathode.
        So your "Givens" have holes. :-)
        "Holy Givens" Someone should Google that!
        Tre'
        
        Have Fun and Enjoy the Music
        "Still Working the Problem"
        
        Edits: 10/27/09
        
        RE: All circuit diagrams are fiction. - Henry Pasternack 17:36:28 10/27/09 (1)
        
        In Reply to: RE: All circuit diagrams are fiction. posted by danlaudionut on October 27, 2009 at 17:00:29
        
        > GIVEN the input is the grid to ground signal.
        > GIVEN the output is the plate to ground signal.
        > Definition feedback is the output fed back to the input
        
        This is an easy one. You've proven there is no external feedback around the circuit as a whole. It says nothing about how it's implemented internally.
        
        The forward gain element in the circuit is the triode. It has two input terminals -- the grid and the cathode -- and two output terminals -- the plate and the cathode. Since your argument begins with a fallacy, it ends with one as well.
        
        There's no mystery or conspiracy here.
        
        -Henry
        
        That clears it up. - Sr5 06:33:56 10/28/09 (0)
        
        In Reply to: RE: All circuit diagrams are fiction. posted by Henry Pasternack on October 27, 2009 at 17:36:28
        
        Very interesting discussion. I'm glad it's finally over.
    - RE: Can't argue with you on this... - Henry Pasternack 17:44:12 10/26/09 (1)
      I guess I'm a glutton for punishment. The more the merrier. Anyway, Dan has all the information he needs here to undo his confusion.
      
      Oh, I forgot to mention before that by Dan's definition, the Williamson amplifier has no NFB either, because the feedback resistor doesn't connect to the input grid.
      
      I guess that's all I have to say on the subject.
      
      -Henry
      - RE: Can't argue with you on this... - danlaudionut 18:03:39 10/26/09 (0)
        
        In Reply to: RE: Can't argue with you on this... posted by Henry Pasternack on October 26, 2009 at 17:44:12
        
        Henry
        
        The Williamson amplifier feeds signal back.
        This means to a previous point in the circuit.
        That has come up before.
        
        DanL

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