Never Get Old,
No negative feedback allowed for this discussion, except for a comparison at (*) below.
Damping Factor is a function of plate impedance, rp, versus output transformer primary impedance.
1. Single Ended:
Draw a load line on a graph of plate curves. 2A3, 6CK4, your favorite triode, or a Triode Wired KT66, etc.
Look at the slope of the plate curves:
Steep slope at high plate current when the grid is just a few volts negative.
Flatter slope at high plate voltages, when the plate current is low, and the grid is many volts negative.
The plate impedance, rp, depends on the slope of the plate curves.
High slope is low impedance.
Flat slope is high impedance.
The Varying plate impedance, rp, versus signal, drives the output transformer primary.
That causes Un-symmetrical damping factor.
A triode rp that is 700 Ohms at 60mA Quiescent, might be 600 Ohms at 110mA (50mA extra with signal),
And rp that is 1200 Ohms at 10mA (The opposite signal polarity causes 50mA less than the quiescent 60mA).
And, one direction (polarity) of output signal is higher in amplitude;
While the other direction (polarity) of output signal is lower in amplitude.
We notice the difference: it is called 2nd Harmonic distortion.
(*) Some will say that negative feedback will fix the second harmonic distortion; to some degree it does.
But for a certain amount of negative feedback, there will still be a difference in the low rp, versus the high rp regions on the set of plate curves:
2. Push Pull:
When output tube # 1, rp goes to 1200 Ohms, output tube # 2 goes to 600 Ohms.
Then when the signal polarity reverses, the two tube's rp values "swap":
Output tube #1 rp goes to 600 Ohms, while output tube # 2 goes to 1200 Ohms.
The graphical analysis of push pull requires 2 identical graphs of plate curves, But one is rotated 180 degrees, and is placed on top of the other graph of plate curves.
That is as symmetrical as it gets versus signal polarity.
When one tube rp goes from 700 Ohms quiescent, to 600 Ohms, the other tube rp goes from 700 Ohms quiescent, to 1200 Ohms;
and the reverse happens (swaps) when the signal polarity changes.
That means that at both extreme output signal polarities, the peaks are compressed somewhat.
That is called 3rd harmonic distortion.
As Paul Harvey used to say, "Now you know the rest of the story".
I hope that explains it.
Keep designing, building, modifying, and listening!
No negative feedback allowed for this discussion, except for a comparison at (*) below.
Damping Factor is a function of plate impedance, rp, versus output transformer primary impedance.
1. Single Ended:
Draw a load line on a graph of plate curves. 2A3, 6CK4, your favorite triode, or a Triode Wired KT66, etc.
Look at the slope of the plate curves:
Steep slope at high plate current when the grid is just a few volts negative.
Flatter slope at high plate voltages, when the plate current is low, and the grid is many volts negative.
The plate impedance, rp, depends on the slope of the plate curves.
High slope is low impedance.
Flat slope is high impedance.
The Varying plate impedance, rp, versus signal, drives the output transformer primary.
That causes Un-symmetrical damping factor.
A triode rp that is 700 Ohms at 60mA Quiescent, might be 600 Ohms at 110mA (50mA extra with signal),
And rp that is 1200 Ohms at 10mA (The opposite signal polarity causes 50mA less than the quiescent 60mA).
And, one direction (polarity) of output signal is higher in amplitude;
While the other direction (polarity) of output signal is lower in amplitude.
We notice the difference: it is called 2nd Harmonic distortion.
(*) Some will say that negative feedback will fix the second harmonic distortion; to some degree it does.
But for a certain amount of negative feedback, there will still be a difference in the low rp, versus the high rp regions on the set of plate curves:
2. Push Pull:
When output tube # 1, rp goes to 1200 Ohms, output tube # 2 goes to 600 Ohms.
Then when the signal polarity reverses, the two tube's rp values "swap":
Output tube #1 rp goes to 600 Ohms, while output tube # 2 goes to 1200 Ohms.
The graphical analysis of push pull requires 2 identical graphs of plate curves, But one is rotated 180 degrees, and is placed on top of the other graph of plate curves.
That is as symmetrical as it gets versus signal polarity.
When one tube rp goes from 700 Ohms quiescent, to 600 Ohms, the other tube rp goes from 700 Ohms quiescent, to 1200 Ohms;
and the reverse happens (swaps) when the signal polarity changes.
That means that at both extreme output signal polarities, the peaks are compressed somewhat.
That is called 3rd harmonic distortion.
As Paul Harvey used to say, "Now you know the rest of the story".
I hope that explains it.
Keep designing, building, modifying, and listening!
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