Brown Box and Biasing

[chris1954]

Hi - I have a Brown Box ( https://www.amprx.net ) that I use to tame the varying voltage swings from venue to venue. I noticed when re biasing my Blues Jr, varying the line voltage varies the bias voltage. My target voltage is around 110 v to the amp. My question is I'm guessing I should bias the amp whilst it's at the 100volts - right? Not at 120+ that my house puts out...

Thanks - Chris

[The Dude]

If you're going to play the amp using the Brown Box, why wouldn't you bias it that way?

[g1]

I think you meant 110, not 100V.
Yes, if you are going to be using the amp off 110V, then bias it at that voltage.

[pdf64]

I suggest to set the bias / idle current at normal mains voltage.
Then as its mains voltage is reduced, the bias should broadly track the HT, thereby maintaining the same conduction angle.
Plate dissipation is a limit not a characteristic.
It makes no sense to increase the conduction angle as HT is lowered. Biasing to a idle plate dissipation would seem to put the cart before the horse.

[g1]

Good point that I can't really argue with. But I was thinking along the same line that bias voltage would track mains voltage and you would end up with roughly the same anyway.
(in this case I believe bias is set across a cathode resistor to a number, and plate voltage/% diss. does not factor in)