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Old 27th April 2011, 10:08 PM   #1
martyh is offline martyh  United States
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Default Can I balance a diff stage like this?

Possibly another really silly question by yours truly but…
Can I balance DC in a stage like this by making Bias 1 and Bias 2 adjustable assuming that there will be enough voltage across the CCS to make it work?
Thanks,
Marty
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Old 27th April 2011, 10:24 PM   #2
DF96 is offline DF96  England
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Yes. Better to have one adjustment, but you can do it with two.
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Old 27th April 2011, 10:25 PM   #3
lineup is offline lineup  Sweden
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Usually you put a suitable linear potentiometer between cathodes.
And the wiper connects to the current sources.
This will divide the current to the triodes according to potentiometer turn.
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Old 27th April 2011, 10:27 PM   #4
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Quote:
Originally Posted by martyh View Post
Can I balance DC in a stage like this by making Bias 1 and Bias 2 adjustable assuming that there will be enough voltage across the CCS to make it work?
Yeah, but why would you want to? It's a differential, and so there is no need to adjust bias on both sides. One side will do (see attached). In this preliminary design, DC balance is provided by a small +/- supply.
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Old 27th April 2011, 11:52 PM   #5
martyh is offline martyh  United States
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Thanks again to all of you.

Miles and DF, Making only one side adjustable makes sense. Thanks for the suggestion. As a follow up any thoughts on how much adjustment I would need for a typical medium mu twin triode? Would say setting the fixed side to get 20% of the required bias and setting up the adjustable side to do between 30% and 10% be a reasonable starting point?

Lineup, I proposed something like that a while ago but as I understood the replies using resistors on the cathode side would screw up the AC balance. Maybe bypassing from the wiper to each cathode would make that work but I’m probably the worst guy in the whole world at figuring stuff like that out.

Marty
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