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3rd April 2012, 08:15 PM  #41  
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I suspect that in a genuine balanced SRPP (i.e. same top and bottom, no cathode bypass) the 12AU7/ECC82 would work quite well as there is some cancellation. For an unbalanced one, including mu follower, there is much less cancellation so you have to rely on the valve's intrinsic linearity  other valves can beat the ECC82. 

3rd April 2012, 08:30 PM  #42 
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The good thing is, you can cancel 2'nd order distortions decreasing THD. The bad thing is, canceling it you increase distortions of higher order.
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3rd April 2012, 09:15 PM  #43  
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Many tube data sheets include “Average Transfer Characteristic” curves. The slope of the curve is gm. From low grid voltage on the left to less negative grid voltage on the right the tube data sheets show increasing slope (gm). The difference between straight line constant slope and the tube data sheet is the source of the 3rd harmonic. Now add a cathode resistor to the average tube in the data sheet, with an increasing cathode resistor value the Transfer Characteristic” curve begins to slope in the opposite direction minimizing the 3rd harmonic as the composite curve approaches a straight line. So yes, on a good day you can tune a circuit’s composite “Transfer Characteristic” curve to minimize both even and odd harmonics. Sometimes they do not tell you everything in class. DT 

3rd April 2012, 09:20 PM  #44 
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"increase distortions of higher order"
I thought you only change the ratio ,low to high order.
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3rd April 2012, 10:41 PM  #45  
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4th April 2012, 01:44 AM  #46  
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Page 5 of the linked 12AU7 data sheet (real tube data) shows a positive rate of change of plate current as grid voltage becomes less negative (this curve is without a cathode resistor). Add increasing values of cathode resistor and the rate of change goes from positive to zero and then negative. The added 3rd due to the cathode resistor harmonic is opposite in sign to the original (I believe this is how you put it). http://www.mif.pg.gda.pl/homepages/f...3/1/12AU7A.pdf DT 

4th April 2012, 10:33 AM  #47 
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Because of the sign of the secondorder distortion, cathode degeneration will always introduce compressive 3rdorder. Thus for 3rdorder cancellation you need intrinsic expansive 3rdorder. The 12AU7A data sheet (on page 5) appears to show compressive 3rdorder for high currents, as the curves approach a straight line, but I accept that they may be expansive at low currents. The risk is that in order to cancel some 3rdorder you end up accepting higher amounts of other orders because you are operating in a nonlinear region of the valve characteristic. However, the 12AU7 is unusual in that it is almost squarelaw in places (this is why it makes a good receiver mixer). Of course in a real circuit there is also the effect of varying mu to complicate things (see page 3 of the linked data sheet).
I worked out some of the maths here, in the context of receiver performance. Note the difference in sign of the thirdorder term when comparing 3/2 with exponential responses. I think we can agree that a little cathode degeneration may sometimes reduce 3rdorder. The wrong amount, or at the wrong bias, or with the wrong valve, could make things worse. 
4th April 2012, 11:10 AM  #48 
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I have just thought of an explanation. At low currents, where island effect is present, many valves follow a 5/2 law. This would give expansive 3rd, so a little cathode degeneration will cancel it.
It is possible that a "3/2" model may include some 5/2 at low currents, or achieve the same effect in a different way such as reducing mu. 
5th April 2012, 01:24 AM  #49 
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DF96,
Thanks for sharing the IP3 theory and Maths. Also thanks for sharing your thoughts plus explanation. I like your solution oriented approach to see how the data fits the models. A couple of years ago I looked for and did not find SPICE models for 5670 and 12B4. I adjusted similar models to match the performance tested on the bench. This worked pretty well above the knee of the curves. I found a set of plate curves on line for a triode connected 6BQ6, not your ideal 3/2’s law triode. The plan is to tweak the value of the cathode resistor plus varying degrees of cathode resistor bypass by adjusting the value of a resistor placed in series with the bypass capacitor. It will be interesting to see what happens to with the FFT and how clean I can get a few watts of PushPull output to be. Just for fun! DT 
5th April 2012, 02:35 AM  #50 
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I mentioned "cancellation" by another device with similar curve, like in SRPP example, so it is always true in this case. The main advantage of SRPP is not cancelation. It is dynamic load of bottom tube and using the top one as cathode follower as well, i.e. as the result the bottom tube sees higher load resistance, that is the source of lower distortions.
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The Devil is not so terrible as his math model is! Last edited by Wavebourn; 5th April 2012 at 02:42 AM. 
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