Blues Jr - Dial In the Tone -9

Posted by Jim Black on 7/11/2015 to Mods

I've got "Ice Pic Tones In My Ears."

Love It, Love It, Love It!

Remember last time? I decided to change the tone-stack and the mid-capacitor to transform my blues jr into a twin reverb. Boy, did it pay off. I'm hearing the classic fender tones coming through---Strong mids, punchy bass and shimmering highs. This amp went from thin and raspy to the beauty I had hoped for.

Let's go back to the beginning and see how things changed.

Stock Blues Jr. Modified Blues Jr. Mod kit parts

This capture shows the stock legacy Blues Jr. It has the standard Blues Jr tone stack and the Jensen speaker.

The bass frequency is low compared to the spectrum. The majority of the output is all in the highs. Hence the raspy thin sound.

We do need to remember that this Blues Jr is a legacy amp. Purportedly, the designers deliberately wanted to make it look and sound like it was from the 50's.



The capture to the left shows a full range frequency sweep after all the billm mods were installed. Notice the increased bass shown on the left of the graph. The mids and treble have stayed the same.

This when things got very dark! Way too dark for my taste. I needed to do something about it.





The photo to the left shows all the components for the mods from billm.

  • Power Supply Stiffener
  • Coupling Caps
  • Power Transformer
  • Output Bias
  • Tone Stack

There are fender washers for the transformer and a high frequency bypass capacitor. Some Blues jr have a high frequency squeal when they first power up. My BJ had this problem. The drill bit and the wire are for the bias adjust.


Stock  Mod    Super Twin  
Treble  250p  250p  250p  250p  
Mid    22nF  15nF  22nF  47nF  
Bass   22nF  100nF 100nF  100nF  

The table at the left show values for the stock blues jr., the moded blues jr, a super reverb and the twin reverb. I wanted more mid and highs so I picked the 47nF like the twin. It took a bit of searching but I was able to find the exact replacement from Illinois capacitor. Allied electronics sells those caps in single quantities. The cap is marked with IC473MSR400K. The trick is to make sure the voltage rating and package size is correct. Bill mentions that the original caps are of poor quality. I would respectfully disagree. The originals are polyester metallized film and are used in many applications. Bill uses the Polyproplyene plastic film Orange drops that we are used to seeing in tube amps over the years.

I was reading billm's website and ran across a discussion on C10 coupling and C9 bypass cap and how you could remove C9 and jumper C10 for even more treble. I decided to do both.

These mods paid off.

Final Modified Blues Jr.

The graph at the left shows the low-lows came up and the high-highs came up due to removing C9 bypass and jumpering the C10 coupling. The mids are equal to the bass frequency amplitude. Listening to it next to another Blues Jr stock is like day and night. We set all the knobs at 6 o'clock. The moded amp is noticeably louder, which means it is at least 3 to 6dB louder. The signal doesn't start breaking up until 7 or 8 with the pre-amp gain. The bass is tight and punchy when you pound on the E-string. And, the highs are bright and chimey.

My friend, Garth, definitely liked what he heard and so do I. Worth the cost and the effort and we didn't try to make the amp into something it isn't.

Here is what I spent.

  • Basic kit, cream board: $22
  • Low Profile TO20 Output Transformer, $64
  • Eminence Patriot Swamp Thang 12" Speaker, $95
  • Titanium Drill Bit, $10
For about $200 I have an amp that will do well in small clubs and bigger clubs with a mic. Very happy with the results. If you have any questions, please don't hesitate to shoot me an email.

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