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Old 8th April 2011, 03:11 AM   #1
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Default Need help modifying tube bias circuit


I need help modifying the bias reference point in my Beard P100 tube power amp. The amplifier uses a LED biasing scheme where you turn the pot until the bias led turns off. Currently the circuit is calibrated for ~40ma bias current. I want to raise this to 55mA bias and make it adjustable so that I can calibrate and change the reference point for all power tubes with one adjustment.
In the attached schematic how is this best achieved? I assume I need to change the reference voltage to something other than the set 0.5v but I am not sure how to do this.
Any help will be greatly appreciated.

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Old 8th April 2011, 05:33 AM   #2
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My suggestion would be to add a 100 ohm pot in series with the 560 ohm resistor. This would allow a small adjustment to the bias reference voltage e.g. 0.5v to 0.6v.
You could, alternatively, reduce the value of the 4k7 with a parallel high value resistor. Start with 100k and work down the values till you get the bias you're looking for.
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Old 8th April 2011, 01:55 PM   #3
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Thanks, but these two suggestions seem to conflict as adding the 100 ohm resistor in series will raise the overall resistance and adding a 100K resistor in parallel with the 4K7 resistor will lower the overall resistance.

Am I making sense?
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