I recently had the pleasure of modifying a Blues Jr for a customer. If you’ve never had the oppertunity to use this amplifier, it’s a pretty solid design and a decent value. Fender must think so as well. Introduced in 1995, the Blues Jr has been a consistant seller for over 20 years! However, no amplifier is perfect, and no amplifier will possibly satisfy every user. Fortunately, there are some modifications that can be done to improve it’s usability.
One strange thing about the Blues Jr, is that it has three 12AX7s, but half of one of those tubes remains unused. In the golden age of rock and roll, guitarists would have technicians drill into their amps just to add another tube, yet Fender has done half the work for us! Many of the modifications are based upon how best to use this free gain stage.
One option is to use the tube stage as a buffer before the tone stack. This tube buffer provides just a touch more drive, and improves the response of the tone stack. The cleans develop a bit of a sparkle to it. When overdriven, this mod takes the edge off the distortion, giving it a creamy sound.
Another option is to use the tube stage as a boost stage. It’s like adding a boost pedal to the input! Imagine the possible tonal range with a built in boost pedal!
Lastly, my favorite option, is to use the tube stage as a bias driven tremolo. This type of tremolo was rare on Fender amplifiers. It was used on the Princton, the Brownface Vibroverb and a few other non-Fender amps. It is unique in it’s smooth, round modulation. Since it changes the output tube bias, it also has a subtle distortion effect that varies with the depth control. In the Blues Jr, whenever the amp is cranked, the effect will seem to disappear, only to fade back in as the notes ring out. This provides a surprisingly versitile touch sensitivity on the effect! Since the effect never truely shuts off during overload, it will reappear with the same beat that it dissappeared with, never losing time. This is an effect that cannot be acurately reproduced with any other type of built in tremolo or pedal tremolo.
In addition to utilizing the “lost gain stage”, there are many other modification options:
- Tone stack tweaking
- Customize the range of the stack and overall tonality of the amplifier
- Bass or Treble boost or cut
- Give the amplifier a vintage sound or add a little bit of bite to the tone
- Reverb mods
- Older Blues Jr (pre 2003) were known to have a noisy reverb circuit
- Output stage adjustments
- The output tubes are run relatively cool for long life. Running the tubes hotter gives a fuller tone at the expense of tube life.
- Output transformer replacement
- A higher quality output transformer greatly enhances the bass response of the amplifier
- Speaker replacement
- Opens up the tone of the amp
- Capacitor/Resistor replacement
- Change out all the capacitors and resistors in the signal path to high quality components
Please contact me if you are interested it getting the most out of your Blues Jr.
JDP Amplifiers 
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