Fixed Bias question

[kianbach]

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.

[Enzo]

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.

[Wieslaw Lipowsk]

Originally Posted by Enzo

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:

[Wieslaw Lipowsk]

Originally Posted by kianbach

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+.

[kianbach]

Thanks for the help...I also have a small toroidal transformer that gives me 22vct.Could this be used to build a negative supply?

[Wieslaw Lipowsk]

Originally Posted by kianbach

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.

[Enzo]

Current demand on bias is tiny, so if your 22v transformer is not enough, you could make a voltage doubler for it.