The Fender Blues Junior despite being small is a very loud amplifier and benefits hugely from an adjustable bias mod.
Unfortunately we've seen far too many where the user has changed the output valves and because the amp is known as 'fixed bias' think they can just change out without any readjustment because there is no adjustment..
Ever Fender Blues Junior we've seen through the door has a biasing set at between 30% and 60% higher than the Nominal setpoint that valves are usually biased to based upon plate voltage and power using ohms law.
It's straightforward to add a biasing trim control.
We would advise that this is carried out by an experienced person such as ourselves due to the high voltages present on the PCB. There is also a very high chance of damaging the output valves and possibly transformers if the mod isn't carried out correct or you short out a solder pad.