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In Reply to: RE: update posted by vinnie2 on October 19, 2016 at 04:58:54
I'm not really sure what you are looking at.
The linearity of a tube can be judged by looking the how much the plate voltage will move as the grid bias is changed by the input signal.
Once you pick your operating point and plot your load line, view the plate voltage change along the load line.
If the plate voltage changes the same amount as you move along the load line to the right of the idle point vs. as you move to the left along the load line.
Even spacing of the grid lines along the load line is a visual indication of linearity.
Just for an easy example let's say your idle point is 420v, 60ma, -50v bias and your load line is horizontal.
With an input signal of 100 volts peak to peak the positive peak will change the grid bias from -50 to 0 and drive the plate voltage along the load line from 420 down to 140. That is a change of 280.
The negative peak of the input signal will change the grid bias from -50 to -100 and drive the plate voltage along the load line from 420 up to 690. That is a change of 270.
If the tube was perfectly linear the plate voltage change would be exactly the same. With this example we have a change moving to the left of the idle point of 280 and a change moving to the right of the idle point of 270. Close but not perfect. If it was perfect and the plate voltage change was exactly the same there would be no harmonic distortion.
Now this was just an example, you can't have a horizontal load line with a output tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Oh, going down the wrong track I was. So what does the curvature of each plate curve represent? On some tubes the curves are straighter than others. I thought that was meant by being in a linear region. Apparently not.
Another page for the note book.
Edits: 10/19/16
that might help you understand. If the tube and operating point were perfect then the output current and output voltage would be symmetrical just like the input signal but their not. One side is larger than the other. That's harmonic distortion.It's caused by the spacing between the grid lines not being even along the load line. The lines to the right of the idle point are closer together than the lines to the left of the idle point.
If you want linearity (low distortion) you want to look for even spacing between the grid lines along the load line.
Most tubes have grid lines that are close together to the right and further apart to the left. Only a few tube have grid line spacing that is even all the way across.
A 6sn7 at 200 volts plate and 14ma idle current is pretty linear.
The 45, 2a3, 300b, 845 and 211 are pretty linear.
A 12au7 is not. The grid line spacing starts wide on the left and get narrower and narrower and narrower all the way across and there is no area where the spacing is even.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/19/16 10/19/16 10/19/16
If the "applied grid signal voltage swing" is AC voltage from the input to the amp, then I think I understand. If not I am still not able to put all the pieces together for an "aha" moment. If not that, what is causing the applied grid signal voltage swing?
Yes, the "applied grid signal voltage swing" for the first tube is the input signal to the amp.The "applied grid signal voltage swing" for the second tube in the amp is the output signal of the first tube and so on.
In other words, the "applied grid signal voltage swing" is the input signal to the grid of the tube and the graph is describing what happens to the tube (in terms of current and voltage changes in the tube) because of that signal being applied to the grid of that tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/19/16
Hurrah! By Jove I think I've got it! At least this one tiny little piece. All this is definitely going in the note book. Thanks for taking the time to explain it, Tre', it really helped!
.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
There is a simple formula, starting at the bottom of page 2, that will give an approximation of the 2nd HD based on how much one half of the output wave form differs from the other half of the output wave form in terms of amplitude.
This is an article written by Norman Chowhurst.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thanks Tre - I will save that with my others. I own a few relevant texts and articles but not read all of them extensively; of those I have read it has been a while...
What I liked about your contribution was the discussion (or Q and A) and relevant information provided in a way that was easily digestible (and without ego). Great stuff.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I vote for a weekly post called "Tre's Tips" posted in the same style!
.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
You're welcome Vinnie.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Changing plate resistance.
The more vertical, the lower the plate resistance.
Even within one tube, the lines more to the right show higher plate resistance (they lay over sideways) while the ones to the left show lower plate resistance (they are more up and down).
Plate resistance (just like gain) is not constant and is also a source for distortion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I can understand each of these things as an individual concept; but when I try to pull them all together my head starts to hurt and my eyes cross. : )
You'll get used to it. You might even start to like it after a while. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think I would have to learn like pain first! : )
Actually, I think as long I can learn theory while I am doing something, I am ok with it. It's just if I have to sit down and study it for hours my brain melts down. So I really appreciate you taking the time on something like this one, and now that it is in my notebook I will be able to refer to it whenever I need to. Who knows, in 10 or 20 years I might get the whole picture this way. : )
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