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::
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.