Modifying a Fender Blues Junior

So first things first: credit where credit is due.  I learned how to do these modifications from Billm’s Blues Junior Modification pages at

His parts (which I bought from him) and his instructions were excellent and really worth reading.  It was a cool little project and took a couple of hours to do. I heartily recommend getting the parts and directions from him because they were really helpful.

The following is a photo of the back of the Blues Junior on my bench with the screws removed from the back plate.  I didn’t remove the screws on the sides on on top of the amp.  Those are for the chassis inside the speaker enclosure.

Back of Blues Junior Amp

I like to use a muffin tin to keep all of the parts that I take off of a project. It’s REALLY easy to get small parts out of the muffin holes even if you don’t have any finger nails, like me.  Some other people I know like egg cartons, but I had a bad experience with a soldering iron and Styrofoam that doesn’t need to be repeated.

The following photo is what the PCB looks like before you start voiding your warranty!

Now I have unscrewed the PCB from the chassis and I have wiggled the shafts of the pots out of their holes.  I also removed the daughter card for the foot pedal and the speaker out.

The blue resistor in the very center of the picture replaces the stock resistor and helps to cool down the amp bias.  It’s below the three horizontal caps and next to the 3 vertical gray caps. This helps to remove the mushiness (lack of definition and transient response), especially in the lower frequencies.

Blues Junior

You can see the two leads from the resistor poking through the back of the PCB. Incidentally the PCB is incredibly cheap and uses very little copper. You have to be VERY careful to not ruin the copper pads by using too much heat. The pads will peel up and roll themselves into a tiny ball.

Blues Junior

If you look in the upper left corner in the following picture you can see the big blue cap that I piggy-backed onto the gray caps that was the stock cap.  By doubling the capacitance of the power filter cap, you create a much bigger store of power which helps the amp deal with very fast transients and low frequencies.  By store the power in the cap, the amp can respond much better to the attack of picking and popping.

Blues Junior

So that’s it for the photos.  The sound was really improved.  The amp responded much more quickly to changes in dynamics and the Low B note of my baritone guitar came through easily.  Before the modification, the amp struggled to produce the low E from a regular guitar with much authority.  I also replaced the speaker with an Eminence Swamp Thang speaker which is supposed to have extended low frequency response.  Speaker specifications really suck in general and usually don’t represent what timbre humans perceive.  This speaker has roughly the same specs as the speaker I replaced, but it sounds completely different.

All in all, I am really pleased with the changes!

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