These mods have become known as the “Billm” mods from my name on the Fender Discussion Pages. and the Telecaster Discussion Pages. Fender’s Blues Junior is a great little amp–it’s small, light, priced right, and all tube. It has spring reverb, flexible tone controls, and it’s loud for its size. It has a couple of shortcomings, however, that can be overcome without too much difficulty.
The biggest issue with the Blues Junior is that it sounds small and boxy. Cup your hands around your mouth and speak or sing. That’s “boxy.” Some think that this undesirable tone is due to such a large speaker in a small cabinet. But most of it is due to the components selected by the original designer for the tone stack and the lame coupling cap values. In my opinion, the original designer (long gone) sandbagged the Blues Junior so that I wouldn’t compete too much with the Blues Deluxe.
The next issue is bass performance. The low notes are fuzzy and indistinct. Often called “flub” or “farting out,” it has multiple causes. The speaker is partly to blame, so is the small, not-great-quality output transformer. Surprisingly, the limitations of both can be mostly overcome by just increasing the amount of capacitance in the power supply, commonly known as “stiffening.”
The third issue is output tube bias. Bias is a small negative voltage on the grid that sets the operating point or “idle speed” of the tube. On amplifiers with push-pull output like the Blues Junior, the tube manufacturers’ recommendation is for the operating point to be set at approximately 70 percent of the full power rating. The original designer of the Blues Junior chose a much higher operating point, and most BJrs run at 100 percent of their rated output, sometimes higher. Hotter bias is not automatically better. Hiss increases and the amp becomes mushy and inarticulate. Output tube distortion occurs earlier, but this is not always a good thing. My basic mods kit includes a simple way to make the bias adjustable.
Old BJrs Sound Different
Fender has used two different circuit boards in the Blues Junior. The original green circuit board was in production from 1995 to early 2001. The later cream-colored board has been in production since mid-2001. Although the circuits are virtually identical (except for reverb), the cream board is much brighter, with livelier treble, than the green board. Where applicable, I’ve separated the mods appropriate to each. The green board reverb is prone to hiss and hum. See my Improving Green Board Reverb page. The Green boards were made in USA; production shifted to Mexico in 2001. There is no difference in quality.
Some Experience Required
If you can wield a screwdriver, soldering iron, and multimeter, you can do these mods, but it’s a good idea to have some experience soldering and desoldering printed circuit boards. Gaining access to the back of the circuit board is not all that easy–amp techs hate these amps for a good reason. Here’s how to minimize the pain: http://billmaudio.com/wp/?page_id=17