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RE: update

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'


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