Blues Jr. -- Diggin' Deeper -3

Posted by Jim Black on 2/13/2015 to Mods

The back cover is off the amplifier now that we have removed the rusty screws in the last installment.
I bought the basic and output transformer mods from the billm website in the hope that I can get a tighter low end and more headroom before it started breaking up.

  • Tone Stack
  • Power Supply Stiffener
  • Bias Trimmer
  • TO20 Compact Output Transformer
  • Swamp Thang Speaker
       billm mod kit
The Tone Stack mod is the 4 orange capacitors. The Power Supply Stiffener is the large electrolytic capacitor. The Bias Trimmer is the tiny potentiometer with the drill, the wire and the capacitor. The Transformer comes with a couple of fender washers and the ceramic capacitor for oscillation suppression. The Swamp Thang speaker is an experiment of mine. I need a speaker that can reproduce the bass and mids that these mods are going to produce.
Each of these mods comes with its own hard copy color instruction sheet. Which, I must admit, are just slightly confusing at first. There is no mention of where to start except that you must "partially remove the PCB." The instructions refer you to the website on how to remove the PCB, but, what they don't tell you is how to get the knobs off so you don't do any damage. The potentiometers are PCB mount with plastic shafts so they are easily damaged. Definitely don't want that.
removing the knobsThe left photo shows a foam pad under the flat screw driver against the chassis face. This prevents the screw driver from scratching or denting the chassis panel.
Gently push up on the knob with the screw driver while slightly wiggling the knob with your fingers. You want to turn the knob while doing this motion to get the knob to come off evenly. Do this for all the knobs.removing the jack
The right photo shows removing the input jack. This is a legacy Blues jr. so even the input jack is rusty. I did not run into any problems getting the nut off, thank goodness!
You are going to have a lot of little parts laying around so it is a good idea to get a plastic bag or container of some sort to put them all in. It gets worse if you are interrupted during your mods and are away a week or two. So, keep them all together from the start. plastic tub
The aluminum piece is the output tubes retainer bar with the springs. You don't have to take the tubes out. You need them in to set the bias. If you don't trust yourself, take them out. You can see the screw that has rusted into the chassis. I put a little WD40 on it and wiggled it out with my vice grip pliers. Man, what a long way to go just to get the chassis out of the box. chassis out
At this point, you can remove the chassis from the cabinet after you have disconnected the reverb and the speaker. This way you can put the chassis on a work bench to make things easier to access. I went to Billm's website to watch a video he did on how to remove the PCB once I had the chassis on the bench. You can see that he left the tubes in!
Next time, we'll start the with the Tone Stack Mods.



L. Harris
Date: 3/5/2015
Hi Jim, Thanks for the brilliantly detailed and interesting blog posts! I have wanted to mod my blues junior for a while but cannot find a source for the correct mod kits. Any idea how to get one? Thanks,

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