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Question: Acceptable DC current in push-pull output transformer due to tube mismatch?

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#41
trobbins said:
If the PP tubes have exactly the same idle dc current, but different conductance performance, then there will be an effective dc offset over a waveform cycle.

With respect to BH curve locus, one side of the excursion will move waveform peaks in to worsening linearity region before the other side - which is the same distortion related mechanism that is being discussed when there is a dc idle unbalance but otherwise perfectly matched valves.
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This works quite differently to a standing DC balance since it is proportional to the output signal rather than been fixed in the case of the DC imbalance. That means DC imbalance produces a fixed quantity of relatively high distortion, but AC imbalance produces a small to negligible amount at normal levels - only rising to levels comparable to DC imbalance at large signal excursions.

It a question of choices really - but the choices are not really equivalent.

Shoog
 
#44
jazbo8 said:
When does AC imbalance become objectionable? How often does it to be adjusted?
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Exactly my position, my experience is that it doesn't become significantly objectionable right up until you have to replace the valves.
I have to wonder how the DC bias point and AC amplification behaviour are related. It seems to me that they must track to a large degree and so if you correct one over time you also help to correct the other. Am I wrong one this, and if so where can I find the demonstration of my wrongness.

Shoog
 
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