Bias help

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Bonsaimaster
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Bias help

Post by Bonsaimaster » Sat Jan 06, 2018 2:02 am

I have built this power amp for my preamp and having an issue with biasing. I can get one tube biased to 10mA but the other tube only to 3mA. I need to increase the sweep of the bias control. Do I change the 180K resistor or the 10K resistor to achieve this and are they higher or lower values.

I read this:
You can't get the current any higher than 20MA for each power tube and the bias pot is turned all the way up.
You have too much negative voltage in the bias circuit.
Make the bias range resistor larger in value and try again.
Keep adjusting the bias range resistor until you can get the bias pot to sweep higher and lower than the desired voltage.
If you want to be able to bias each power tube at 35ma of current, adjust the bias range resistor until you can get more than 35ma when the bias pot is turned all the way one way and less than 35ma when the bias pot is turned all the way the other way.

I replaced the 180K resistor with a 470K and that still did not get enough range. Any suggestions

Thanks in advance

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neikeel
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Location: Suffolk, England

Re: Bias help

Post by neikeel » Sat Jan 06, 2018 1:57 pm

It really depends on what circuit you have (centre tap/no centre tap, series stop resistor or not etc.

Post a schem and someone will chip in with easiest fix 8)
Neil

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