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My thoughts on cross over distortion in a guitar amp, millions of amps sold and enjoyed.
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Interesting discussion and more than a tad over my head.
I note the OP refers to "idealised" plate characteristics, does this also imply a constant screen voltage ?
I was thinking,
In typical PP guitar amp circuits that utilise separate screen resistors on each tube, that when the current in the class B tube doubles, its screen current would also double.
Ergo there would be a reduction in screen voltage.
Is the screen voltage drop likely to lessen the effect of the GM doubling ?
I ask this in part because over the years, my ears tell me that shared and separate screen resistors seem to change the feel/response of amplifiersComment
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Yes, interesting point. An ‘ideal’ pentode has plate curves which are equally spaced, horizontal, straight lines. So that would also imply a constant screen voltage.
In a real pentode, the screen current goes up dramatically, in the ‘class B’ tube, when it reaches the highest plate current on its load line (and the plate voltage reaches its lowest point). If there is a screen resistor, the increased screen current (and hence reduced screen voltage) causes all the plate curves to drop to a lower level, which means earlier clipping. If the screen resistor is shared between push and pull sides, as the ‘class B’ tube current increases, the compression of the plate curves for the other tube might cause it to enter cut-off more abruptly.
The Gm doubling effect is a change in slope (by a factor of two) of the overall transfer characteristic, at the transition point between class A and class B. I think a shared screen resistor might make that change in slope a bit more abrupt.
As you say, the screen compression effect would make it more difficult to double the plate current in practice.Last edited by Malcolm Irving; 02-09-2016, 02:15 PM.Comment
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It's been a while since I've read that page. Some great nuggets in there.Originally posted by Jazz P Bass View PostComment
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