5F6A Adjustable Balanced Bias ala Hoffman Adjustable bias circuit

[Bangy]

Love Dougs bias circuit for the 5F6A, and would like to attempt to implement a 2 pot Balanced version so I can bias each tube individually. I need help with how to do so and values.

Do you think this would work?  Im hoping to give each output tubes its own bias supply. Any suggestions are welcomed.

[sluckey]

I don't think that's gonna work.

[Bangy]

Any recommendations ?

[PRR]

I'm going cross-eyed looking at the layout.

But it is clear both grids get bias from the same wire.

[Bangy]

Is it possible adjust bias each out tube separately with and amendment/addition to Dougs circuit?

[sluckey]

Is it possible adjust bias each out tube separately with and amendment/addition to Dougs circuit?

Yes. Modify the circuit using this schematic as a guide...

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

[Bangy]

Cheers Sluckey.

[Bangy]

Mr. Sluckey, anybody--How's this look?

[sluckey]

That's basically it. You may need to experiment with the values of the fixed resistors to get the voltage you need. Especially that 1K/2W. It may need to be much bigger depending on the amount of AC voltage feeding the circuit. But if you are already using a 1K on your circuit now that's a very good starting point. For 6L6s, I'd choose resistors that will give approximately an adjustment swing of -35v to about -55v. You can set all this up with not output tubes plugged in.

How much negative bias voltage swing do you have with the stock Hoffman circuit?

[Bangy]

Cool.

At 120 VAC 20mA difference with the Hoffman Bias circuit.

In another bassman build I bought completed. I was using EL34 (LOVE THEM) and I had no prob biasing them up to 30aM.
Same tubes in the Hoffman Bias circuit I cant get the tubes past 20mA.  I need to make a resister change to move the range higher. Can you walk me through that?

Thanks for your help Sluckey.  Very much appreciated..

Bang

[HotBluePlates]

... I was using EL34 ... I cant get the tubes past 20mA.  ...

Your EL34's are idling cooler than you want; that means they have too much negative voltage on the grid.  Less negative voltage will idle them hotter.

If the 1kΩ resistor is made larger (maybe 2kΩ?  You'll have to experiment), the bias supply will start with less voltage in, and generate less negative bias voltage.

You're finding out firsthand that EL34's bias in a different range than 6L6's.  That's because of the difference in transconductance.  What works well for one doesn't necessarily work well for the other.

[Bangy]

Thanks HBP!