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Re: Sweetness VS Load Lines

Posted by Naz (A) on August 9, 2006 at 21:17:40

In Reply to: Re: Sweetness VS Load Lines posted by Tre' on August 9, 2006 at 08:48:04:

>>First I agree with you as to load lines. I have all my stages (pre amp and amp) CCS loaded and have increased the value of the grid resistors as much as is reasonable.

Good, but don't forget that the input impedance of the following stage is NOT just the value of the grid resistors to ground. These are in parallel with the input impedance of the tube itself eg a CCS loaded 6SN7 will typically have an input impedance of approx 100K.

>>OK, one step at a time. We have a pure 1K sinewave going into the first stage, forward going (first half of waveform positive). The first stages output will be a negative going 1K waveform and a negative going 2K waveform (distortion).

Cool!

>>These negative waveforms feed into the second stage. The output of the second stage, not counting distortion, will be a positive going 1K waveform and a positive going 2K waveform.

>>Then we have the distortion of the second stage which is positive going 2K waveform and a postive going 4K waveform (distortion of 2K).

>>The two positive going 2K wave forms do not cancel but add. They are in phase with each other.

Here's where we are going wrong and it's all a matter of how it's viewed. I can see where you are coming from and it can be extremely confusing if you look at it this way. The simple answer is that the 2ndH produced by the tube in the second stage is always in antiphase with the signal at its input and therefore the two are cancelled within the tube.

In effect the 1kHz fundamental is fed into the second stage along with the distortion components produced by the first stage. In this case the 2kHz 2ndH distortion that the second stage tube produces will be mixed within the tube with the 2kHz incoming distortion signal from the first stage and therefore cancel with it (before it gets out so to speak).

If the levels were such that it cancelled all of it no further harmonics (eg 2kHz, 4kHz) would be produced, leaving only the 1kHz fundamental. If there were no distortion signal present in the input signal to cancel with (ie no previous stage) then both the fundamental and the distortion components will be output.

Does this help?

Naz

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