Bias Adjustment

[JammyDodger]

Hey All,

I am continuing to tweak on my 100W ODS-101-HRM. I have played the amp for about a year and am 90% happy with it. I have pulled it out of the cabinet and looked at my biasing. It is running 49V and 50mA. This seems a bit high to me put the trimmer pot is at the extreme of it's rotation. I have a 10l pot and a 15k Resistor to ground. I know some designs are using a 27k. What is the effect of raising this resistance. Will it take me in the right direction. I should be around 35mA I believe.

Cheers, Mike

[talbany]

Mike


Raising the tail resistor(sets the range) adds more -voltage...So if you want to drop the bias you raise the resistor...EL-34's require more -voltage than 6l's..With the tubes out your range should read between 0-60-70 neg volt should be enough to do both..Good luck



Tony VVT

[fp2000]

I used a 10K tail resistor and it worked better for me. I use 6v6 and 6l6, I get a better range to work with.

[heisthl]

You can also drop the input resistor to 1k to get more range.

[ayan]

Mike

Raising the tail resistor(sets the range) adds more -voltage...So if you want to drop the bias you raise the resistor...EL-34's require more -voltage than 6l's..With the tubes out your range should read between 0-60-70 neg volt should be enough to do both..Good luck

Tony VVT

Hey Tony, maybe we're just getting our terms mangled up... I know you know your stuff. However, EL34s definitely require less - (negative) bias voltage to bias properly, for a given plate voltage. Thus if 6L6s may like to see say -52 VDC (old Fender Twin spec), EL34s for the same plate voltage would want to see maybe -44 VDC or so. On a personal note, ideally the bias range would never go to 0 VDC since that setting would, in every case, result in the tube burning itself very quickly. In the amps I built for multi tube purpose, I use a range of maybe -35 to -65 VDC.

Cheers,

Gil

[JammyDodger]

To All,

I put in the 27k resistor and I now get plenty of adjustment room. I am running -54.4V to get 35mA on the tubes.

Seemed to work!

Cheers, Mike

[talbany]

Hey Tony, maybe we're just getting our terms mangled up... I know you know your stuff. However, EL34s definitely require less - (negative) bias voltage to bias properly, for a given plate voltage. Thus if 6L6s may like to see say -52 VDC (old Fender Twin spec), EL34s for the same plate voltage would want to see maybe -44 VDC or so. On a personal note, ideally the bias range would never go to 0 VDC since that setting would, in every case, result in the tube burning itself very quickly. In the amps I built for multi tube purpose, I use a range of maybe -35 to -65 VDC.

Cheers,

Gil

Gil
Thanks for the correction.. you are right.. I was thinking backwards with respect to the 34's Thanks... Also with the tail resistor there you will never take it completely to 0..Brain fart.. I stand humbly corrected!!

Tony VVT

[Funkalicousgroove]

in the past I've actually wired up a small trimpot in place of that tail resistor so that The range is always adjustable, a 25-50K trimmer will allow you to adjust your range on the 10K pot for any tubes, You can also just use a larger pot and smaller tail for similar results.