bypass cap?

[shooter]

I'm reading this;
http://www.angelfire.com/electronic/funwithtubes/Amp_Design.html

excerpt;

The best way to lower the gain of a stage is to introduce some cathode degeneration or negative feed back by leaving off the bypass capacitor on the cathode resistor. This will also lower the amount of distortion in the stage.

Is the distortion "caused" by the cap, or distortion is less because there's now nfb cancelling inherent noise?

thanks
dave

[sluckey]

The distortion is not caused by the cap. The degenerative feedback due to an unbypassed cathode resistor causes less gain, less distortion, higher fidelity, etc. I don't like to think about it any deeper than that.   

Here's a little more discussion but it probably doesn't answer your question any better than my attempt...

     http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

Jump to page 25.

[PRR]

The tube distorts. Tube plus large cathode cap distorts the same. The tube is a non-linear resistor, the resistor is (almost perfectly) linear. The tube plus an unbypassed cathode resistor distorts less, about half as much.

[shooter]

I don't know which hurts more, my body from beating on a spline shaft wheel bearing for hours or my brain from trying to "get it" 

going with my body,  my brain just has to remember no bypass cap = 1/2 distortion!

thanks gentlemen

dave

[jjasilli]

Looking at this another way:  there is nothing about the cap, in and of itself, which causes distortion.  Without the cap there is inherent, internal NFB in the tube, which reduces gain.  The presence of NFB lowers distortion.  Less gain means less distortion.  So if we find any way at all to reduce NFB, and/or to increase gain, distortion will increase.

[pdf64]

I think that a CC stage is actually noisier if its cathode is unbypassed!
The noise generated by the cathode resistor (though the resistance value is small, there's 'a lot' of dc compared to the grid stoppers / leak resistors) is no longer bypassed to 0V, so it adds to noise at the grid.
Plus an unbypassed cathode is way more susceptible to picking up hum from its heater.
Hence unbypassed CC stages are best put later in the signal chain, ie not used for the input stage (of high gain systems).

[dennyg]

Shooter - since you've given me great advice on here, I'm humbled to comment on your thread.  You use 'distortion' and 'noise' potentially interchangeably, and you received legit responses to both.  As pdf implied, noise should not be an issue since you would never (?) have an unbypassed first stage as the objective is to maximize gain/amplification with any first stage, hence later unbypassed stages would introduce negligible noise due to much greater S/N ratios.  However distortion is potentially reduced (and fairly dramatically depending on the operating points) in an unbypassed vs bypassed stage, since bypassing would increase the potential for clipping on both ends of the wave form since the tube is amplifying the delta between grid and cathode. 

[shooter]

the objective is to maximize gain/amplification with any first stage,

thanks DennyG, in this case I'm not concerned with maximizing gain.  I'm interested in ZERO "induced" distortion, and minimizing inherent distortion. So for an Audio build, "gain" is easy, keeping it "clean and shiny" is the hard part

[dennyg]

Ah - then leave that bypass cap in the drawer - although I will add that I did some experimentation with caps on the first stage and found unbypassed to yield a rather lifeless, dull tone to be amplified downstream as louder lifeless dull tone 

[shooter]

yield a rather lifeless, dull tone

for a guitar amp, I might agree, for an audio amp...........

[jjasilli]

Shooter - since you've given me great advice on here, I'm humbled to comment on your thread.  You use 'distortion' and 'noise' potentially interchangeably, and you received legit responses to both.  As pdf implied, noise should not be an issue since you would never (?) have an unbypassed first stage as the objective is to maximize gain/amplification with any first stage, hence later unbypassed stages would introduce negligible noise due to much greater S/N ratios.  However distortion is potentially reduced (and fairly dramatically depending on the operating points) in an unbypassed vs bypassed stage, since bypassing would increase the potential for clipping on both ends of the wave form since the tube is amplifying the delta between grid and cathode.

Lots of guitar amps thru the years have un-bypassed 1st stages. 


I agree that the term distortion is ambiguous.  It could mean "non-linearity" in the sense that the output waveform deviates form the input waveform = harmonic distortion. Or, it could could mean THD (Total Harmonic Distortion) which does include noise -- such as hiss, hum, clipping, bent waveform.


Higher gain generally means more non-linearity, i.e., more harmonic distortion; even if other "defects" are not apparent.  I understood this to be the focus of Shooter's question.

[shooter]

the focus of Shooter's question

I basically didn't spend enough brain power that day thinking why removing the cap lowered distortion.

In my old life SNR was a big deal, I can't control inherent noise, but I can try and keep the signal linear as possible, phase shift to a minimum (ya, I know  ), also filtering to help keep the signal big enough while I keeping the noise low enough to satisfy me and maybe even an unsuspecting client 

[Willabe]

the focus of Shooter's question

I basically didn't spend enough brain power that day thinking why removing the cap lowered distortion.

........ to satisfy me and maybe even an unsuspecting client 

 

[pdf64]

Bear in mind that an CC stage with unbypassed cathode will have a somewhat higher output impedance than if it was bypassed. Hence if the load applied to the stage varies with frequency (even just due to the inevitable parasitic capacitances etc), its output voltage will vary more with frequency.