Billm Clean Boost Module

One of the ongoing complaints about the Blues Junior is that it doesn’t “step out” enough when you’re going from rhythm playing to solo. If you’re going from clean to distorted, a pedal is an excellent solution. The Fat switch gives you a boost in loudness and a bass boost, but it isn’t quite enough to cut through the rest of the band. Putting a clean boost pedal in front of the Blues Junior is often not a good solution because the BJr’s high-gain preamp stage goes into overdrive or distortion increases to unacceptable levels. The boost just isn’t clean anymore when you cut in the pedal.

The Billm Clean Boost module gives you more volume without a tone change by boosting an under-used stage late in the Blues Junior signal chain. It takes the preamp tone and makes it louder–drives the output tubes harder. It’s one square inch, mounts right on one of the existing circuit board retainer screws, and all the connections are done from the top of the circuit board–no need to remove it or get to the back side. The clean boost module adds about as much additional loudness as the Fat switch, but without the bass boost, and it does it at any volume level, all the way up. It boosts clean signals or distorted, and doesn’t change the tone. It does not suck tone or alter the Blues Junior’s tone in any way when it is off. Like the Fat switch, the Clean Boost increases the gain of one of the Blues Junior’s tube stages. But unlike the Fat switch, it doesn’t increase distortion or boost the bass significantly. When you’re playing loud, though, it does ultimately increase output-stage distortion, which is a good thing.

The Clean Boost is designed to come on when you pull up on the presence control. It can be used in conjunction with the Fat switch for a big boost in volume and heavier tone. If you play out, you may prefer an additional mod that changes the footswitch jack to a stereo jack. You can then plug in a 2-button footswitch and control the Fat switch and Clean Boost independently while you’re playing. The footswitch overrides the panel settings for both the Fat switch and the pull-up on the presence control.

The loudness boost, when combined with that of the Fat switch is dramatic. I made some tests with continuous, harmonic-rich tones, similar to what you’d get from a guitar, and wired one of my Blues Juniors so I could turn the Fat and Clean Boost on and off independently.

I tested 400Hz and 1500Hz tones, starting a higher volume level with the 1500Hz tones::

400Hz 1500Hz
Starting Level 90dB 100dB
Fat Switch 99dB 107dB
Clean Boost 97dB 106dB
Both 105dB 111dB

As you can see, the boost is somewhat more effective at lower frequencies, but the 11 to 15dB of total gain is an amazing difference. The boost remains effective even at 12 and 12 on the volume controls (wear your earplugs!). The Blues Junior has some untapped power hidden away in there, and the Clean Boost module brings it out–dramatically.


This is the Clean Boost module. It’s 1 inch square and it connects to your amp in three places. All connections are from the top of the circuit board, so there’s no need to get to the back side.

R3 is a thumbwheel trimpot that lets you adjust the amount of boost. If you’re not running the boost along with the Fat switch, you’ll probably leave it wide open. If you’re combining the Fat and Clean Boost, and the jump in volume is a little too much, you can dial it back. This is probably a one-time adjustment for most users.

The Billm Clean Boost uses premium components, including a Bourns cermet trimpot for reliable, static-free operation, and a Xicon low-ESR (equivalent series resistance) capacitor for maximum efficiency. It switches in and out silently, with no pops or clicks. The active connection uses Mogami low-noise coaxial. This is mixing board/studio-grade cable. The control and power connections are stranded, Teflon-coated wire for ruggedness and resistance to the errant soldering iron when you install them.


Here’s the Clean Boost module installed in a cream board Blues Junior. The back of the board is cushioned to prevent buzzes and vibration damage.

Although shown on a cream board, it works equally well on a green board, including those with existing Billm mods. I supply the module fully assembled and tested, with instructions and wiring options.


  1. Scott Pope says:

    Bill, if the audio taper gain and master volume controls are both installed, is there anything to keep from installing the clean boost and hardwiring it so it’s on all the time? With the new TO20 or TO22 transformer, it seems like that would be a mod you would want on all the time if you wanted your BluesJr to hold its own with something like a Blues Deluxe, or a JTM45 or such on stage?

  2. John says:

    It does exactly what is says on the tin. Great mod!

  3. Karl says:

    If you want to drive the output tubes harder, why not just change the resistive attenuator that includes the master volume control? You could swap out a few resistors; there’s plenty of gain from the preamp amplifer stages.

  4. EricL says:

    Hello Bill.
    Congratulation for all your work on the fender amps and the kit you offer. Fine new web site too.

    I was wondering what makes this boost clean compared to the fat boost since the circuit is very similar to the “fat switch”, but working not on the same tube.


    • EricL says:

      Ok, I think I got it. This is the cap in parallel with R17 (or R10 for the fat switch) that makes the difference. The bigger, the more bass (and vice versa). Am I right ?

      • bill says:

        Instead of changing the size of the cap, the Clean Boost includes a trimpot so you can set the amount of overall boost. In this stage of the amp, there is less bass boost than what you’d get in the preamp, as with the Fat switch.

  5. Rusty Milner says:

    Bill, Is it possible to make a Mod to the Fat Boost that would tame the low end when in use? Also for those who like the Fat Boost on all the time, can your Clean Boost be wired to the existing Foot switch?

    • bill says:

      You can “de-fat” the Fat switch by replacing the 22uF bypass cap that the Fat switch turns on/off with a smaller one, perhaps 4.7uF or 2.2uF. But if the amp sounds farty on the low end, you should probably do the basic mods and TwinStack to clean it up. Then decide whether the Fat switch is more useful.

      I don’t recommend rewiring the Fat switch to control the Clean Boost. It’s actually easier to replace the existing jack with the stereo jack kit.

  6. Sammy Wheeler says:

    Hey Bill!

    Just looking through the mods you offer for the blues junior! Excellent! I was just wondering with this particular one whether I have to have the prescence control mod, in order to put the clean boost module in? – I intend to use a two button footswitch to toggle between the fat/clean boost via the stereo jack kit.

    Best regards,


  7. k-o-matic says:


    Sorry, I don’t know anything about amp construction, so this might be a dumb question… But is it possible to convert this clean boost module into a mid boost module, and be able to control the amount of mid boost with the trim pot? I have a couple of D-style amps, so I am used to having the mid-boost function…

    Or is it possible to convert the Fat boost into a mid boost? Is that what the “de-fat-ing” does? (“replacing the 22uF bypass cap that the Fat switch turns on/off with a smaller one”)

    Thanks in advance for your reply.

    • bill says:

      Given the simplicity of the BJr circuit, it’s not really possible to change the Fat or Clean Boost into a mid boost. Turning up the mids control all the way gives you some extra distortion and drive, but not as much as you’d ever get out of a Dumble-style amp.

  8. scott says:

    Thanks for helping my blues jr. turn into a great amp. would it be alright to wire the ‘clean boost’ so it’s on all the time. I mean- can it be like like giving the master a 13, and 14?

  9. Paul L says:

    Hi Bill

    Till I purchase a 2 button footswitch, can I bridge tip and ring on the stereo jack to control both Fat and Clean Boost with a 1 button footswitch?

  10. Thierry says:

    Hi Bill,
    I installed your “clean Boost” and your optional “stereo footswitch” (i don’t have the presence module).
    Each button from the footswitch only triggers the “Fat” or eventualy the Fat and the clean boost at the same time, but i doubt about this because there’s only a little difference. (The footswitch is a Fender taken from your recommandation list).
    NB: I didn’t use the orange wire.

    Can you help me to solve the issue?

    Thank you
    Best regards

    • bill says:

      The Clean Boost will turn on when you ground the orange wire.

      • Armo70 says:


        so if you have the clean boost module and want run it exclusively via the footswitch (i.e with no presence switch installed) where does the orange wire connect to?


        • bill says:

          Just leave it unconnected. If you ground it, the Clean Boost will turn on. But if you have a footswitch plugged in it takes over control of the Clean Boost–it doesn’t matter if the orange wire is grounded or not.

  11. eddie_bowers says:

    I’m thinking of wiring my clean boost to come on when the Fat Footswitch is pushed (along with normal fat boost) but I want use the push pull knob to enable that option.
    What’s the best way to wire that?
    I have the presence control mos already.

    I basically want to have a Fat Boost Plus mode.

  12. teza says:

    as this clean boost mod look like a very good thing to have, I’ve been looking around for a double switch pedal and find this one on Ebay.

    What do you think.

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