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previous: Ken Gilbert q{Out of curiosity, when do you swi... -- 2/29/2000 11:48 PM View Thread

Re: Stupid Question

3/1/2000 4:10 PM
Re: Stupid Question
Re. does adding a cap bypass to the cathode resistor diminish distortion:  
1. Local degeneration caused by the cathode resistor does cause overall diminishment of the distortion through the stage, as you pointed out, but the effect of bypassing the cathode R makes the tube itself behave more linearly for the following reason: (see Radiotron tube book, vol 4, on p. 552, or so.)  
2. The tube is the least nonlinear when it has a perfect current source feeding the plate and cathode. This is why folks go to the trouble of active plate loads, (althogh I don't like 'em). A 1500 0r 820 ohm resistor ain't squat when it comes to acting like a current source. The current and voltage at the cathode will change (distort) within the cycle. As the grid voltage varies the current will not track it. There is now an extra pole of the imperfect current source added.  
so thats why people bypass cathodes to diminish distortion.

GFR Bypassing a resistor with a cap can... -- 3/1/2000 4:23 PM
Ken Gilbert bypassing rk's and effects on thd -- 3/1/2000 10:05 PM