Now, with the amp off and unplugged, find the bias trim pot – it usually looks like a tiny screw head on the circuit board. Turn it a tiny, tiny amount. Clockwise = more current (hotter), counterclockwise = less. Think of it as a water tap for electrons.
Fire the amp back up (yes, it’s tedious), remeasure, and repeat. It’s like Goldilocks, except you’re playing with enough energy to char your eyebrows off. Hot tubes give you a beefier, creamier tone, but they die faster. Cold tubes are clean but sound thin. Find your sweet spot.
