A question re: Negative feedback.

[caps]

One thing continues to baffle me somewhat:

With a UNbypassed cathode resistor, we see that as the grid voltage increases so does the voltage drop across the cathode resistor.

So with conduction through the tube, we have a negative going plate and a positive going cathode. This is said to decrease the conduction through the tube. I think I got that right, I hope.

Now, how exatly does it decrease? A negative going plate, a positive going cathode.....I want to understand how the electrostatic field actually decreases, the nuts and bolts of it so to speak.

Additionally, how does one calculate the amount of NFB? I see references to many designs using X amount of NFB, so there must be a way to measure it ?

Thanks very much.

 

[caps]

Ok, after some further thinking...

Im thinking the , lets say, a larger voltage drop across the cathode resistor makes the grid less positive ( in relation to the cathode), hence this reduces the plate current? Of xourse the same theory would apply inversely.

hehe, I think I answered my own question, emphasis on "I think"! :shock:


However, how does one control it , is a question that still remains. Just changing the cathode resistor value couldnt be it, youd stuff your bias point...

So, there has to be a way to control it and measure it no??

I seem to remember something about increasing a plate resistor value will lessen the effect of NFB. Maybe this is the way? Of course, this would also increase the amplification...hmmmmmmmmmmmmm.....

 

[PRR]

> how does one calculate the amount of NFB? I see references to many designs using X amount of NFB, so there must be a way to measure it?

Measure gain. Apply feedback. Measure gain. Difference is amount of NFB.

In this case: with and without cathode bypass cap.

And I'll tell you the answer: for most happy tube amps: 4dB to 6dB NFB by omitting the cathode cap. As you say, any attempt to get a very different answer "stuffs your bias", unless you go to some much more complicated plan.

 

[caps]

Once again, your the man PRR! Thanks mate . :thumb: :thumb: