Dual bias question

C Moore · Post by **C Moore** » Mon Aug 20, 2012 2:50 am

So, I am building this little amp.
It has dual bias and bias vary Trem.
I have never done this before, and have never really experienced an amp with this.
As far as I can see, I have built it the way I have the schem drawn (STILL learning J Schem...sorry).

With just the rectifier tube, I have 420 VDC on Pin 8.
My bias goes from about -10.66 to -13.66 on each side.
But BOTH bias pots have an effect on both grids of the power tubes. Not sure if I screwed up somewhere, and I am not sure how the Trem effects any of this.

But with dual bias, the idea is to have control of each grids negative bias.....separate from each other.....right.?
That is...when I turn a bias pot, it should only change the negative voltage on ONE grid, not both.
I must have a problem somewhere.?
Thank You

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JazzGuitarGimp · Post by **JazzGuitarGimp** » Mon Aug 20, 2012 3:48 am

I suspect the issue stems from the 330K resistor in the head of the bias supply - this makes the impedance of the bias supply relatively high, so the more current you pull from one leg, the lower the overall voltage to both legs. You might try placing the diode before the 330K, change the 330K to 470K (using it for only one of the bias supplies), and add another 470K from the anode of the diode for the other bias supply. What is the value of the pots?

Firestorm · Post by **Firestorm** » Mon Aug 20, 2012 5:26 am

As I see it, you've designed the bias supply with variable loads rather than voltage dividers so that either side can load down the supply, thus affecting the other side. There should be one common load with the pots wired as voltage dividers (with the individual bias feeds coming off the wiper of the adjustment pot). You can add a 100K-220K resistor from the wiper to the "input" side of each pot to guard against the wiper opening.

martin manning · Post by **martin manning** » Mon Aug 20, 2012 11:08 am

JGG, if the supplies are split further upstream I think another electrolytic would be needed as well as the additional resistor, no? Firestorm's solution would only require some changes to existing values, and perhaps the safety resistors on the pots.

JazzGuitarGimp · Post by **JazzGuitarGimp** » Mon Aug 20, 2012 12:48 pm

Hi Martin,

I was actually thinking the first lytic could be deleted, but failed to mention it. Each supply leg already has a 10uF filter, and since there is no current-requirement (at the 6K6's) to speak of, the first cap isn't really needed.

I haven't seen a bias supply done as a variable shunt like this, but then I've only been dabbling in valve design for a few years. Firestorm's suggestion to take the design to a more traditional approach is certainly valid and on-point. I just thought the existing design looked interesting.

Cheers,
JGG

martin manning · Post by **martin manning** » Mon Aug 20, 2012 1:12 pm

The advantage of the variable shunt is that it fails "cold" if the wiper loses contact. You'll see this design on classic Marshalls and the Ken Fischer amps. That first 10uF cap will take the last 1-2% of ripple out, which may or may not be important given the common mode cancellation in PP.

JazzGuitarGimp · Post by **JazzGuitarGimp** » Mon Aug 20, 2012 1:26 pm

When I edited my last post to add "(at the 6K6's)", I had a feeling I'd be taken back to school!

There is considerably more current flowing in the two output legs of the power supply than at the 6K6 grids. Thanks, Martin!

Structo · Post by **Structo** » Mon Aug 20, 2012 3:12 pm

I installed dual bias pots in my D'ite so that I could balance tubes that were not closely matched.

On my amp the red wire that is connected to the left lug is the power from the bias supply.

The middle lugs go to the respective 220K resistors on the PI.

The right lugs have a 15K resistor to ground.

You can really zero the amp in with no hum with this setup.

JazzGuitarGimp · Post by **JazzGuitarGimp** » Mon Aug 20, 2012 4:31 pm

The only addition I'd make to Structo's circuit is a 470K resistor from the wiper to the bias supply on each of the pots. In the event the wiper of a pot goes open, the bias voltage will be pulled to the most negative voltage, rather than left floating (which will likely cause red-plaiting and destroy the tube). The chances of a pot-wiper failure is very slim, but for the price of two additional resistors, it could save you the cost of an output tube.

C Moore · Post by **C Moore** » Mon Aug 20, 2012 4:36 pm

I am just reading all the responses now.
It dawns on me that I did not include my Trem circuit.
I edited my OP to include a schem of that circuit also.
Thank You

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