Adding a bias pot to the Fender Blues Junior and Pro Junior is becoming a very common modification these days. This modification helps extend tube life and reduces hiss in these amps. Another bonus is the amp runs cooler. The factory bias setting (which is set by a non-adjustable fixed resistor) allows the EL84 output tubes to draw too much current. With the addition of a bias pot you can adjust the bias properly for each set of tubes you install in your amp. Remember every set of output tubes will draw a different amount of current which is why you need to adjust the bias every time you replace them. I’m finding this is a very effective mod for these amps.