• Re: Try This - Paul Barker 10:25:42 06/07/04 (4)
  In Reply to: Try This posted by Lynn Olson on June 07, 2004 at 00:18:18:

  How does this affect output impedance? For instance with an 80 ohme resistor is it increased by (mu +1)*80 in series with internal impedance and dc resistance of OPT?
  
  

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  • No Effect for a PP Circuit - Lynn Olson 00:06:00 06/08/04 (3)
    In Reply to: Re: Try This posted by Paul Barker on June 07, 2004 at 10:25:42:

    For a SE circuit, yes, there would be a rise in output impedance (damping factor), but this is Class A PP circuit, so increasing the common cathode impedance actually has no effect on output Z at all - although there are minor secondary effects related to clipping, etc. when comparing zero impedance (normal cap-bypass) to near-infinite (current source).

    The re-discovery of the WE Harmonic Equalizer, lost since 1936, is prompting a lot of off-line discussion amongst the PP triode folks. The reduction of 3rd and 5th harmonics is real, measurable, and exists over a range of signal levels from 5 watts down to a few milliwatts. (Measured on a $15,000 Audio Precision analyzer with a noise floor at -120dB, so we're not talking test-equipment limitations here.)
    
    
    

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    • Re: No Effect for a PP Circuit - Dave Davenport 05:38:35 06/08/04 (1)
      In Reply to: No Effect for a PP Circuit posted by Lynn Olson on June 08, 2004 at 00:06:00:

      Hi Lynn,

      Thanks for the article. It seems that this is a refinement of the Ultrapath idea. What is the advantage of using the series resistor to attenuate the grunge on the CT rather than to take the whole grunge to the cathode?

      Dave

      
      
      

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      • Independently Re-discovered - Lynn Olson 11:25:36 06/08/04 (0)
        In Reply to: Re: No Effect for a PP Circuit posted by Dave Davenport on June 08, 2004 at 05:38:35:

        By John Atwood of One Electron (see URL below). He's the one with the Audio Precision test set, a test amplifier, and links to the original WE articles.

        
        The actual test amplifier has independently biased triode-connected 6CK4's (each with its own adjustable current source), and a pair of 1000uF electrolytics in series between the two cathodes. The center-point of the two caps can be returned to ground, left open-circuit, or connected to a 500-ohm pot. If it is returned to ground, that is equivalent to normal cap-bypassed Class A PP amplifier; if it left open, that's equivalent to the Allen Wright current-source approach; and if the variable resistor is chosen, that's similar to the WE Harmonic Equalizer.

        Note the test amp is not designed to be listened to, but make a series of measurements. I expect John will re-test with battery-heated 2A3's at some point in the future, although these results are similar to what other people have seen with 300B's.

        Although there is no change in output Z with common cathode source impedance, the distortion nulling, although broad, has some sensitivity to the load the output tubes see. For a 2K pri impedance (very low), 20 ohms is about right; for 10K or higher, 100 ohms is about right. But the 3rd and 5th-harmonic nulls are quite broad, so a 2:1 or 3:1 mismatch error in resistor value is not significant.

        So it doesn't really make any difference if the 20 to 100 ohms series resistance is added to a conventional cathode-bypass capacitor, a WE-style shunt cap, or the cathode resistor is split in two (450 + 50 ohms, for example) and the cap connected to the center point of the two resistors.

        Close examination of the WE86 (visit the Nutshellhifi.com Library for PDF) schematic also discloses an unusual network attached to the grid circuit - we're still discussing what that does.
        

        • John Atwood - One Electron   (Open in New Window)

        
        

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    • Thanks - Paul Barker 00:43:21 06/08/04 (0)
      In Reply to: No Effect for a PP Circuit posted by Lynn Olson on June 08, 2004 at 00:06:00:

      nt
      
      

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• Aha, the second order question - IslandPink 05:48:17 06/07/04 (0)
  In Reply to: Try This posted by Lynn Olson on June 07, 2004 at 00:18:18:

  Ok Lynn,

  Thanks for that , I will print it out and take away to mull over on my business trip to Dundee .
  I was thinking , a while back , of putting up a message on TubeDIY , subject 'Do we need 2nd harmonic' , but thought it might be asking for trouble with the culture there at the moment. I might still do that though .

  More later ,

  Mark
  
  

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