How are the Power Tubes paired in a Bass 400+ bass amplifier?
Looking at the MESA/Boogie Bass 400+ bass amplifier from the rear of the chassis, you'll see there are two rows of six power tubes (6L6 or 5881 power tubes). Power tubes should be replaced in matched pairs. In the Bass 400+, a tube in the first row of power tubes works in a push-pull configuration with the tube directly behind it. It is not necessary that all six pair of power tubes match one another but rather that each tube, within a pair, match one another.