Cutoff and saturation shown on an oscilloscope

Discussion in 'Amp Tech Center' started by peteb, Nov 23, 2019.

  1. peteb

    peteb Friend of Leo's

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    Wave right before it reaches saturation






    98954C93-0C3C-4E10-A869-A76BA9BF81BB.jpeg

    Wave reaches saturation





    607B17FA-D5D6-4DF3-9194-9AD878E21D75.jpeg



    Wave reaches cut off

    3ADB2034-37F5-40BE-AE49-3F8653465BB7.jpeg




    You can see that the wave will not advance above a certain line. This must be saturation in a tube.

    The bottom looks different, it must be cut off. On class AB amps the top and the bottom both look the upper half of this class A amp. The class AB amp will develop cross over distortion instead.






    Signal is 0.05 VAC RMS at 440 Hz.

    Treble and bass set on 5

    The clip starts when the volume on the amp reaches 7.5



    O scope set at 2 volts / cm


    I see it max out at 6 peak volts if that


    6 peak volts / 1.4 is 4.25 rms on a 5 ohm dummy load, amp expects a 4 ohm load




    4.25V * 4.25 V / 5 ohms = 3.6 Watts


    I’ve seen this amp show as high as 4.8 Watts. It is on the low side, but I am not too concerned as it is my low powered amp.
     
    Last edited: Nov 23, 2019
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  2. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    Then it did this.....................and stopped working;

    [​IMG]
     
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  3. The Ballzz

    The Ballzz Tele-Afflicted

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    Ah yes! Harlan Ellison was such a visionary of speculative thought! And oh yeah, it is my considered and informed opinion that he was absolutely "Nuckin' Futs" in a most special way!
    One Of My Faves!
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    Last edited: Nov 26, 2019
  4. D'tar

    D'tar Friend of Leo's

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    Ahh.... Be nice now...

    Fun with Oscilloscopes! Pete, glad to see you are using your scope!

    Tech or guitar player definitions? Don't quote me here now....

    Saturation? unrestricted electron flow, cathode to plate

    cut off? No flow of electrons from cathode to plate

    Clipping.... Ok

    Who is the patient. perhaps we have some grid clipping happening. Watch/scope your grid as this occurs.:)
     
  5. peteb

    peteb Friend of Leo's

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    Thank Dtar,


    the amp is an AA764 champ. I forgot to mention that.

    maybe it would be more correct to say that when the top of the wave begins to clip, the tube is approaching saturation. I dont think saturation is unlimited electron flow. I think it means that the current can go no higher. after the initial clip, more current will flow, I believe, as the wave fattens out and makes a complete square wave. As the wave fattens out and becomes squarer, the rms voltage continues to rise even though the peak voltage does not. (thank you Elpico for showingthat to us)

    being a single ended amp, the clipping at the bottom of the wave must indicate that cutoff, no electron flow, is getting near.



    what is grid clipping?



    part of the mystery is do I know if the power tube is clipping or is it the 2nd pre amp stage clipping. I viewed the signal on the grid of the power tube and I did see the clip there, but I am not convinced that the clip originated before the power tube stage.




    I did a small assessment and decided that the signal on the grid of the 2nd pre amp stage was not enough to cause that stage to clip.


    my brief analysis: the 2nd pre amp tube stage is biased at 1.7 volts and a large signal on the grid is around 0.4 volts, and my reasoning is that 0.4 volt signal will not clip when the bias is 1.7 volts. Is this a wrong conclusion?
     
  6. D'tar

    D'tar Friend of Leo's

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    From Robs site...

    Grid Clipping
    Grid clipping occurs when grid current flows into the input circuit and loads down the positive lobe of the input signal. Grid clipping distorts the input signal. When grid current begins to flow the grid current caps the grid's maximum positive voltage which clips the positive lobe of the input signal on the grid. A larger positive signal voltage simply causes more grid current to flow and does not add to the flow of electrons to the tube's plate so the output positive lobe is clipped.

    The tube will then faithfully amplify the clipped input signal and send it out the plate. In other words when grid current begins to flow it greatly reduces the tube's input impedance and the positive half of the input signal is loaded down to the point of clipping.
     
  7. Speedy454

    Speedy454 Tele-Holic

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    I saw the exact waveform when messing with my Valve Jr.
    Thanks for the explanation.
    I am 99% more familiar with solid state amp designs, mostly with bipolar power supplies. They almost always clip equally on the positive and negative cycle.
    And that clipping is not nice to our ears.
     
  8. peteb

    peteb Friend of Leo's

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    the valve jr is single ended like the champ. It makes sense that the top and the bottom of the wave should be similar.

    I also scoped a push pull tube amp. in that case, the top and bottom should and do look the same. they look like the top of the wave shown above.
     
  9. peteb

    peteb Friend of Leo's

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    This also from Rob's website:

    When a large input signal voltage hits the control grid the positive lobe of the signal brings the grid toward a positive voltage.
    As the grid becomes more positively charged the more negative charged space charge electrons will be attracted to it in the same way space charge electrons are attracted to the positive plate. Remember, a positive voltage is a scarcity of electrons so a positive guitar signal on the grid pulls electrons off the grid. If the pull is too hard it will actually start pulling space charge electrons in the tube onto the grid and into the input circuit. This is an important
     
  10. peteb

    peteb Friend of Leo's

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    I do not think grid clipping is occurring in the pictures above.



    I quoted the part of Rob's description of grid clipping to emphasize the part about "large input signal".




    this should only happen in extreme cases of large signal.





    when the current begins flowing out of the grid towards the inputs, I am fairly certain that it is the case of the signal exceeding the bias level on the grid. that is also defined as class A1 becoming class A2, or class AB1 becoming class AB2.





    I have my other thread looking at the relationship between signal on the grid and bias at during full measured output test.




    What I have found is that when the signal begins to clip at the speaker in a full power test, the signal on the grid is not real big, it is more like half of the bias, not the full bias.
     
  11. D'tar

    D'tar Friend of Leo's

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    Your signal is large enough to overdrive the tube and start to clip in your second image. Keep turning up the volume and the clipping continues in the third image. The negative half of your wave is growing as the positive is being squashed. Keep going and it would be so negative that all flow would stop... Cut off reached at that point. Isn't this how cathode bias amps are biased 100% at idle. The tube is closer to true saturation without a signal.
     
  12. peteb

    peteb Friend of Leo's

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    Dtar,

    This is what I think.

    The top of the wave is saturation, even if it is not total saturation. Total saturation would probably look very similar.


    The bottom of the wave is cutoff, and it is most likely not total cutoff.


    When setting up the operating point of a class A amp, the bias is to be set equally in between cut off and saturation, to achieve maximum clean signal amplification. This amp is probably not exactly biased in the middle, but we can see it is close. The saturation sets in before the cutoff.


    What does total cutoff look like?



    Like this?






    112F7926-5932-4A2C-8821-79336896EE3B.jpeg
     
  13. D'tar

    D'tar Friend of Leo's

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    Pete, can you measure dc voltage on the grid in the above condition, image 3 of post 1. I think you will find negative voltage accumulation causing your sine wave distortion right at the grid.
     
    Last edited: Nov 26, 2019
  14. peteb

    peteb Friend of Leo's

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    Dtar, I meant to show voltages, here they are.

    This is an AA764 camp amp.

    Bias voltage: 22.8 VDC


    Grid signal at clip: 9.25 VAC RMS
    Cathode signal at clip: 0.4 VAC RMS
    Grid minus cathode: 8.85 VAC RMS



    Comparing grid signal at clip in VAC RMS to bias voltage in DC:

    8.85/22.8 = 39%



    Comparing grid signal at clip in peak voltage to bias voltage:

    12.5 / 22.8 = 55%





    I’m not sure what you mean about negative voltage accumulation?



    The grid stays negative, the grid leak resistor is 220K, it’s not like grid leak bias?
     
  15. D'tar

    D'tar Friend of Leo's

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    Are you sure your bias voltage is still 22.8vdc while clipping?

    When the input starts to clip, your grid is ac positive enough to allow negative electron flow back into the input circuit.... grid current....measure as vdc. This will squash the positive hump in the sine wave... scope at the grid. You have seen this already as stated in your postings earlier.
     
  16. D'tar

    D'tar Friend of Leo's

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    Yes the grid stays negative in relation to the cathode and continues to become more negative as grid current flows. If you were not limited by the circuit you could continue this negativity until all conduction stops..... cut off
     
  17. peteb

    peteb Friend of Leo's

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    Dtar, good questions and it is good discussing this with you,


    In my experience of measuring amp circuits, the current draw in class A amps does not change. I have even monitored the plate voltage on a champ amp under extreme playing conditions. no change. at all.


    when the clipping occurs in the power test, I don't consider it extreme conditions, although it is getting close.



    i think the voltages I posted in post 14 show the grid staying negative, by quite a bit.


    I did scope the grip when the amp clipped. this is the part I am the most unsure of. I saw the clip on the grid, but I am not convinced that means the clipping occurred in a previous stage, although I understand that it might mean that it did.


    AS I understand it, the grid leak resistor would keep negative charge from accumulating at the grid.


    thanks
     
    Last edited: Nov 27, 2019
  18. peteb

    peteb Friend of Leo's

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    Dtar, I think you make a good point that the wave is not complete cut off or saturation. That makes sense because single ended amps are biased so that they don’t go into cutoff. These images must be the effects of saturation and cutoff before they are complete saturation and cutoff. If there is cutoff, then it is operating class AB. Although the true definition of class AB is that the tube goes into cutoff for a (significant?) part of the cycle.




    Here is a sketch of how cut off looks in class AB. If the darkened wave on the left were combined with the darkened wave on the right, it would look like one continuos wave, like class AB looks like on the scope. If the two half waves were looked at separately, I believe it would look like these two parts.





    This brings up the complexity of making sense of oscilloscope images. I’m fairly certain that if you looked at the oscilloscope trace of one plate of a pair of push pull power tubes, that you would see one continuous wave, and not these pieces shown below, and I don’t know why.




    D12BF39C-8D93-4E6C-8D8A-EC490BB0C159.jpeg
     
  19. CheesyCucumber

    CheesyCucumber NEW MEMBER!

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    You'll be seeing the output from the other tube. The 2 plates are after all electrically connected. You could try removing one of the p-p pair but I wouldn't recommend it :)

    I'm new here, so forgive me for asking but is the aim of this thread to experimentally prove what the myriad of books say on the subject of tube amplification?
     
  20. peteb

    peteb Friend of Leo's

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    Thanks cucumber, that makes sense that what’s on the one plate would be visible on the other, and that removing one pp pair should solve that problem, and that it is not worth doing.


    In lieu of showing it on the scope, do you agree with my sketch in post 18 showing what complete cutoff would look like if one could view it?





    I’m not sure what this means. What the thread is about is showing scope images and trying to positively identify what tube operations or conditions are present on the images.








    I don’t know. What do the books say?

    I guess I’m more of a ‘hands on’ person.


    If you are familiar with what the books say, would you say my conclusions are in line with the books or not?



    As far as I am aware there is not one place to go and see various scoped images of signals in different but common conditions, with an explanation.


    Is there?
     
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