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Old 4th January 2012, 06:20 PM   #1
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Default Fixed Bias question

I have a voltage divider on my B+ which gives me 70 volts.Could I use this to make a negative bias supply and if so, has anyone got a basic schematic or explanation on how to do it.
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Old 5th January 2012, 01:44 AM   #2
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Not without making some sort of DC/DC converter.

Easier to tap off the B+ AC supply and make bias from that if there is no bias winding.
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Old 5th January 2012, 08:14 AM   #3
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Quote:
Originally Posted by Enzo View Post
Not without making some sort of DC/DC converter.

Easier to tap off the B+ AC supply and make bias from that if there is no bias winding.
Here it is:
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Old 5th January 2012, 10:23 AM   #4
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Quote:
Originally Posted by kianbach View Post
I have a voltage divider on my B+ which gives me 70 volts.Could I use this to make a negative bias supply and if so, has anyone got a basic schematic or explanation on how to do it.
You need a negative voltage for the bias supply rather than positive you could get out of your B+.
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Old 5th January 2012, 06:02 PM   #5
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Thanks for the help...I also have a small toroidal transformer that gives me 22vct.Could this be used to build a negative supply?
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Old 5th January 2012, 06:45 PM   #6
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Quote:
Originally Posted by kianbach View Post
Thanks for the help...I also have a small toroidal transformer that gives me 22vct.Could this be used to build a negative supply?
You could try it. If the negative voltage isn't high enough, you can use 0.5K/7W resistors at the power tubes cathodes (with some 220uF caps across them). This way you could have half fixed and half cathode biased amplifier.

However, I' d make the bias supply the same as in the picture above.
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Old 5th January 2012, 11:39 PM   #7
Enzo is offline Enzo  United States
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Current demand on bias is tiny, so if your 22v transformer is not enough, you could make a voltage doubler for it.
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