Bill's Blues Junior Modification Pages
These mods have become known as the "Billm" mods from my name on the Fender 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.
Old BJrs Sound Different
The first thing you should know, though, is that 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.
If you can wield a screwdriver, soldering iron, and multimeter, you can do these mods. 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: Removing the circuit board.
First, the bias
Your Blues Junior is cooking its output tubes! The bias is set way too hot, meaning that it's allowing too many electrons to flow in the power tubes, which heats up the plates, shortens their life, and hurts the sound of the amp.
By adding two trimmer pots and changing two resistors, you can make the bias on your BJr fully adjustable, so that you can match tubes and adjust the bias properly for long life, cool operation, and better tone. You can also take the easier way out--two new resistors that will cool those overheated output tubes, but won't be adjustable. Blues Juniors use two different circuit boards. Click on the appropriate one for you: 1995-early 2001 (green circuit board) or mid-2001-present (cream circuit board). If you play clean/jazz, cooler bias is a must.
New! Sound samples of stock and modified Blues Juniors: click here!
People get all macho about biasing hot, claiming that they need to drive their plates to near-melting to get "the tone." Does it help or does it hurt? Here's an extensive analysis, with lots of cool oscilloscope screenshots, to show exactly how factory bias and cooler bias affect tone, headroom, and volume.
The tone stack:
Where the action is
Blues Junior owners seemingly spend a fortune on replacement speakers to improve the tone. That's an expensive cure when a couple of dollars' worth of capacitors in the tone control stack may do just as much. Or if you decide to upgrade the speaker, a modified tone stack will help you get the most out of it. Includes cool frequency response diagrams!
Blues Juniors made between 1995 and mid-2001 have a different reverb circuit from current Blues Juniors. Many of them are plagued with hum sensitivity and weak or mushy-sounding reverb. And the oldest BJrs have different circuitry in the preamp that makes them sound wimpy. Here's how to upgrade both to the latest rev.
Switch for Green Board BJr
Although many people want to brighten the tone of green-board Blues Juniors to match the crispness of current cream-board amps, there's something to be said for the old tone, especially after the tone stack mod. It has a rich, creamy, warm sound that you never hear from the new amps, and it's worth preserving. Here's how to make it switchable. Better yet, add a presence control! See the Mod Kits page.
Fender has revised and updated the Blues Junior a number of times since its introduction in 1995. Here's a guide to what changes were made with each revision.
variant of the 12AX7 Family
Everybody talks about how you can tailor the tone of an amp by swapping tubes in various gain stages or the phase inverter. But how many people have actually done it? And how many have actually documented what the swaps do? I've got scope photos of where distortion begins with 12AX7, 12AT7, 12AY7, 12AV7, and 12AU7 in the preamp and phase inverter.
Cool: My matching extension speaker for the Blues Junior.
Work in Progress: Turning a Blues Junior into a head unit.
Other People's Mods
These are not my mods, but I offer them so you can see some of the other approaches people have taken to improving this great little amp.
Günther's Vox-like Blues Junior
These modifications were created by GŁnther, a Fender Discussion Pages member from Germany. He has made his Blues Junior into a loud, bright, gig-worthy machine that sounds like a Vox AC-30 and can stand up to a drummer. His mods emphasize the BJr's clean tones; he uses a boost pedal for overdrive.
Steve Ahola's Blueguitar.org has many tips and articles about guitars and amps to make them more suitable for playing blues-style. It covered some Blues Junior mods, including some early attempts at fixing the reverb problem and a fairly radical restructuring of the preamp gain stages. Also on this page: Rooster's mods, as posted in the Blues Junior Mafia thread on the Fender Discussion Pages.
Huss's Cathode Follower Tone Mod
This is just a link to Mike Huss's pages describing how he harnessed the unused triode in the Blues Junior as a cathode follower driving the tone stack. The result is distinctly Marshall-like, especially on overdrive.
It's a shame to have to include this, but some people just like to make their problems somebody else's problem. Although all of these mods have worked well for me, you perform them entirely at your own risk. I do not warrant or guarantee that they will perform the same way for you or that you won't damage your amplifier, burn yourself, electrocute yourself, or stick an X-Acto knife through your palm. Tube amplifiers have components operating at high temperatures and lethal voltages. If you don't feel comfortable doing these mods, take the amp to someone who does.
These modifications will void your warranty. Peace and music, not lawsuits.
Questions? Comments? Email me.