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previous: dpcoyle Re: Stupid Question -- 3/1/2000 4:10 PM View Thread

bypassing rk's and effects on thd

3/1/2000 10:05 PM
Ken Gilbert
bypassing rk's and effects on thd
"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.)"
I am not sure what to make of your above statements... it seems on the one hand you agree that the degeneration in the cathode circuit does indeed lower the distortion contributed by the stage itself. Then on the other, you refute it, giving a reference to RDH4.  
Well, I am safely again at home, and I have looked up the reference. One discrepancy here is that page 552 is in chapter 13, and chapter 13 is on POWER AMPLIFIER circuits. Now, most of the rules of small signal stages do indeed apply equally to power amp circuits, but some do not.  
Instead I point you to p.327, the section on the cathode degenerative amplifier, which is--as far as I know--what we were last taking about. Particularly, on page 329:  
""current feedback is undesirable in transformer loaded amplifier for it tends to stabilize the output transformer's magnetizing current (i.e. make it sinusoidal) and thus produce a distorted output voltage.""
This quote explains why, on page 554, in the POWER AMPLIFIER section, it states:  
""it is important for the by-pass condenser to be sufficiently large to maintain the bias voltage constant over each cycle--any fluctuation leads to increased distortion and loss of power output.""
We are assuming here that the stage is loaded by a transformer, since it is a power amplifier.  
You then stated:  
"There is now an extra pole of the imperfect current source added."
Sorry, but I am not sure I understand your use of the word "pole" here.  
"so thats why people bypass cathodes to diminish distortion."
That may be why YOU bypass your cathodes, but I assure you that an unbypassed Rk, all other things equal*, produces LESS, not MORE total harmonic distortion.  
We seem to agree that an unbypassed Rk produces negative current feedback. See page 315:  
"negative current feedback:  
decreases gain  
decreases harmonic distortion  
increases frequency response"
*Principally, the one caveat I make here is that since the unbypassed cathode resistance increases the effective output impedance of the stage, it may come to be that the stage is now presented with a lower than optimal load impedance. This _may_ induce greater THD from the stage in question, but only if the following stage is of sufficiently low impedance to produce unwanted loading effects. Taken as a single stage, this is not an issue.  
I do not mean to sound arguementative, but I am really quite very sure that bypassing cathodes doesn't decrease THD. I have never heard that claim voiced by anyone before. Less noise, less hum, more gain, less output impedance... sure! But _not_ less THD.  

dpcoyle Ken, You are not at all arg... -- 3/2/2000 4:09 PM
dpcoyle I'm glad to have found Amp Page. -- 3/3/2000 5:04 PM