If you’re just changing the preamp tube, no bias adjustment is needed. If you’re changing the EL84 output tubes, and the new tubes are bought as “matched pairs” (meaning they have the same gain), then typically no bias adjustment is necessary—and on this model, bias can’t actually be adjusted anyway. However, if the amp sounds dull or cold after the change, your best option is to have the output tubes re-biased. This requires modifying the circuit to make it variable bias instead of fixed bias, which should only be done by an experienced technician at a reputable local shop. Even if you’re just swapping a tube, be careful! You’re dealing with high voltages that can kill you. You’ll be playing a harp in heaven instead of a guitar on earth if you get zapped by the 385+ volts of plate voltage present in the output stage of this amp. Always make sure the amp is turned off and has been unplugged for quite a while before working on it—these power supplies can hold lethal voltage for hours after shutdown. Good luck! —Mendamp

When changing pre-amp and power tubes, must the new tubes be biased?