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previous: Bruce Other then a constant current sourc... -- 5/11/1999 1:32 AM View Thread

Re: Rectification Effects

5/11/1999 8:33 AM
Ken Gilbert
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Re: Rectification Effects
Bruce asks:  
 
quote:
"Other then a constant current source, is there a cure for that?"
 
 
Not that I'm aware of, Bruce. In cathode biased amps, you can minimize the effect of the changing plate current by bypassing the cathode resistor with a LARGE cap. This serves to keep the cathodes at a reasonably steady potential. You should make the reactance of the cap 1/10th of the value of the cathode resistor at the lowest reproduced frequency for best effect.  
 
You can also try to optimize the operating point according to that equation I spouted out... If you look at it, you can see that if you make Imax + Imin equal to 2Iq you'd reduce the rectification effect to zero. Of course, you can't really do this in practice, but you can try to approach that ideal.  
 
Incidentally, that equation from the other post is the same one for 2nd harmonic distortion (so sayeth the RDH4).  
 
Rectification effects are taken to an extreme as the tube is biased colder. This effect is easy to visualize if the equation is kept in mind: In a class B amp, for example, the Iq is very small (if not zero), the Imax is very large, and the Imin is zero. Plugging those values in nets a huge dIp, which is indeed one characteristic of the class B amp. Class AB is somewhere in between the two, natch.  
 
~KG~