The power supply stiffener is just a large electrolytic cap. The new cap effectively doubles the capacitance of the power supply filter. The larger capacitance helps maintain regulation during high current demand. This can be times like when you play an open E chord as hard as you can. You may know this by another name. It is called "Power Supply Sag"
Much has been written about Sag so we won't spend much time on it. Suffice it to say, we are trying to get rid of the built-in sag of the Blues Jr.
Removing the cap is exactly the same as the tone stack caps. The real trick to this mod is to make sure the big cap is secured to the PCB so it won't tear off. I cleaned all the old silicon off real well and then rubbed the spot with alcohol so the new silicon would stick the best it can.
It is well known that the Blues Jr. is biased hot to allow the amp to distort sooner than a regular amp. This sounds cool but is hard on the tubes since they run hot all the time. It is pretty good to get a year out of a set of tubes on a Blues Jr. The bias trimmer should help to get a longer life on a set of output tubes.
We could just fiddle with the fixed resistors and get the bias close, but you really should set the bias every time you change tubes. Installing the bias trimmer allows us to do the adjustment.
This mod was the most confusing for me since the instructions did not highlight some key points. The confusion shows in the messy work. The mod worked and was mechanically sound so all was good.
Next time we tackle the output transformer...