adding an adjustable bias pot

[Randall]

I am refurbishing an old Ampeg Jupiter, and wish to make the bias voltage adjustable. Would replacing the 39K R38 with a 50K pot do the trick?

Also, why is the 100K R40 spec'd at 2 watts? Doesn't seem like needs to be that robust?

https://ampeg.com/support/files/Sche...matics%201.pdf

[nevetslab]

I am refurbishing an old Ampeg Jupiter, and wish to make the bias voltage adjustable. Would replacing the 39K R38 with a 50K pot do the trick?

Also, why is the 100K R40 spec'd at 2 watts? Doesn't seem like needs to be that robust?

https://ampeg.com/support/files/Sche...matics%201.pdf

That seems like a sensible solution. Top of the pot goes to the Anode of the rectifier diode, wiper to C22 and the junction of R33 & R34, and bottom of the pot to GND. Maybe an additional cap at the Anode of the diode and top of the pot, leaving C22 on the wiper.

[Enzo]

I figure it has to dissipate a large percentage of a watt. So 2 watts seems reasonable. SOmething using 3/4 watt doesn't want a 1 watt resistor.

I never make bias adjustment that can be turned to zero or turned to something super low. SO a 22k resistor and 25k pot in series make more sense than just a 50k pot. If you turn my pot all the way down you still have 22k there as a default.

[Randall]

nevetslab, I think in your version if the wiper is turned too far or all the way down, or if the wiper fails, you lose all bias voltage to the tubes. Wouldn't it be better to use the pot as a two leg variable resistor in place of R38, or as Enzo suggests, in series with 27K R38? That way if the wiper fails it creates an open to R38, and you get all the bias voltage to the tubes.

Correct me if I'm wrong of course.

[clarkleach]

nevetslab, I think in your version if the wiper is turned too far or all the way down, or if the wiper fails, you lose all bias voltage to the tubes. Wouldn't it be better to use the pot as a two leg variable resistor in place of R38, or as Enzo suggests, in series with 27K R38? That way if the wiper fails it creates an open to R38, and you get all the bias voltage to the tubes.

Correct me if I'm wrong of course.

Another safeguard against wiper failure is to connect a 100K resistor from the top of the pot (raw V-) to the wiper. That way, if the wiper fails to make contact, the bias defaults to a safe level.