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Can't argue with you on this...

87.245.73.97

Allen Wright

In Reply to: RE: Unbypassed cathode resistor and feedback posted by Henry Pasternack on October 26, 2009 at 15:19:28

...but is it in reference to anything in particular?

Regards, Allen (Vacuum State)

Follow Ups:

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)
      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)
      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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