• Re: Clear as Mud! - bill.ramsay 00:41:51 04/26/02 (4)
  In Reply to: Re: Clear as Mud! posted by tre on April 25, 2002 at 16:59:41:

  i think that you are putting too much into this. your statement

  But Class A, as used in a single ended amplifier, means more than that. In a single ended amplifier the current needs to increase and decrease in a linear, symmetrical way. Otherwise the output waveform will be distorted. This is described on page 604 of the AudioCyclopedia "little distortion exists because the grid signal operates in the most linear portion of the dynamic characteristic"
  To put that in plain english, the tube is only allowed to operate in the linear part of the operating curve.

  you can make the tube operate in any part of the curves that you want, provided that you do not exceed power dissipation, max anode voltage, anode current etc.

  operating in a nice linear way is definately the best way to go, but it is not always possible. when you look at single ended designs, and lift the data from the data books, power output is normally listed at 5% distortion. live with it.

  I am building an 811 SE amp, without feedback the distortion would be awful. have a look at the curves, you will see that they are not equidistant.

  kind regards

  bill ramsay
  
  

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  • Re: Clear as Mud! - tre 08:33:43 04/26/02 (3)
    In Reply to: Re: Clear as Mud! posted by bill.ramsay on April 26, 2002 at 00:41:51:

    5% I could live with.
    The reason I brought All this up is;
    Some push-pull amplifiers that are being called "Pure Class A" use a tube that is remote cutoff. The tube is driven into a very non-linear part of it's curve. Example- For the amp to reach it's rated power, the current in each triode must rise from it's idle current of 70ma. to 500ma. At the same time it falls to only 4ma. Now that's alot more than 5% distortion, but this amp. is still advertised as "Pure Class A" IMHO this amp. is behaving much more like a Class AB amplifier than a "Pure Class A" amplifier. This is really my only point.

    
    
    

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    • sorry i thought that you were particularly talking about SE - bill ramsay 15:13:20 04/26/02 (2)
      In Reply to: Re: Clear as Mud! posted by tre on April 26, 2002 at 08:33:43:

      amps in the bit i referred to.

      if you take the curves for this type of tube, then make a second copy and turn it upside down, then join them at the bias points, how does the composite curve look?

      for that is the essence of the argument, if the composite curve is ok, then it does not really matter what the individual tube looks like.

      kind regards

      bill
      
      
      

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      • Re: sorry i thought that you were particularly talking about SE - tre 15:32:07 04/26/02 (1)
        In Reply to: sorry i thought that you were particularly talking about SE posted by bill ramsay on April 26, 2002 at 15:13:20:

        if you take the curves for this type of tube, then make a second copy and turn it upside down, then join them at the
        bias points, how does the composite curve look?

        Yes, I completely understand. It looks fine.

        for that is the essence of the argument, if the composite curve is ok, then it does not really matter what the
        individual tube looks like.

        I agree. that is why Class AB PP and even some Class B PP amplifiers work just fine. But wouldn't you agree that in an amplifier, any amplifier, that is being advertised as "Pure Class A" all the tubes should be running in a somewhat linear way?
        Thanks Tre'
        
        
        

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        • Re: sorry i thought that you were particularly talking about SE - bill ramsay 00:31:06 04/27/02 (0)
          In Reply to: Re: sorry i thought that you were particularly talking about SE posted by tre on April 26, 2002 at 15:32:07:

          no: as long as the net result is ok.

          kind regards

          bill
          
          
          

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• Still clear as Mud! - Ryder 18:24:30 04/25/02 (1)
  In Reply to: Re: Clear as Mud! posted by tre on April 25, 2002 at 16:59:41:

  Hi tre:
  
  Thanks for the response.

  Is it the most linear? What is "linear". Non linearity is just the addition of anything to the signal, no? Isn't there generally some difference in plate swings on opposite grid swings and the creation of 2nd order harmonic distortion with sets too? (I'm taking this point from "Designing Your Own Amplifier Part 1: Voltage Amplifier Stages" by Norman Crowhurst, p 2, Available at the link below.) If not, where does all the 2nd order distortion you hear about with SETs come from? And don't they sound good for pyscho acoustic reasons or because the 2nd order can be cancelled out if you have correct phase with your speakers ( See Eduardo de Lima's very interesting Article "Why Single Ended Amplifieres @ http://usuarios.unisys.com.br/~edelima/reasons.htm for more on 2nd order cancellation between speakers and SETS) or some other unknown reason?(unknown at least too me)

  In any event, to me, one of the mysteries about SETS, is why they sound so good when they are NOT linear, that is, they do not measure all that well. (If we went by measurements we would all be listening to SS.) One of the strange things about SETS, too me, is that what we measure and hear - do not jive.

  Again, IMHO. I am a beginner and just asking to learn.

  Cheers,
  Ryder

  
  

  • AudioXpress Classroom Series.

  
  

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  • Re: Still clear as Mud! - tre 20:37:14 04/25/02 (0)
    In Reply to: Still clear as Mud! posted by Ryder on April 25, 2002 at 18:24:30:

    What is linear? Usually considered an operating curve that will produce less than 1% distortion.

    "Isn't there
    generally some difference in plate swings on opposite grid swings and the creation of 2nd order harmonic
    distortion with sets too?"
    That depends on the tube. Some plate curves are very straight. A 12ax7 is almost perfect. Set up right, What goes in is what comes out. Almost no distortion of any kind. Now output tubes are a different story. I don't think you will find a power tube with that straight of a curve.

    ".....is why they sound so good when they are NOT linear"
    Good sound and good measurements don't jive. The very same circuit tricks (neg. feedback, push-pull Class B {for effiviency}etc....) that make an amplifier measure good, make an amplifier sound bad.

    I'm talking about what is happening in terms of the output tube only.
    The linearity of an amplifier, as a whole, will have to do with many things in combination.

    A SET with no neg. feedback will test poorer than other designs.
    designs with feedback. Designs that are push-pull, where there is cancellation of even ordered harmonics, etc....

    Linearity is a relative term. When I talk about "the linear part of a tube curve" I mean for the tube being used, the part that is relatively straight. If you look at a few plate curves it will be obvious. At the top (saturation) and at the bottom (approaching cutoff) the line get very curved. I mean that a Class A amplifier does not operate in those parts of the curve.
    A Class AB or B amplifier does. The reason the output is not highly distorted is because AB or B amplifiers are always push-pull.
    Now, if you followed all that. A push-pull amplifier can be operated Class A (the tubes are only allowed to operate in the relatively straight part of the curve). The advantage is subtle. Because the amplifier is push-pull many distortions are canceled anyway, but because the distortions never were generated in the first place, I believe a push-pull Class A amplifier will sound better than a push-pull Class AB or Class B amplifier.

    
    
    
    

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