|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: See
the circuit board on the new site.
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. I
think that the designers tried to emulate the mushy sound of a single-ended
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--replace one resistor to cool those overheated
output tubes, but it won't be adjustable. The middle ground is what I do for
almost all of the amps I mod: A single trimpot that adjusts both tubes
simultaneously. Blues Juniors use two different circuit
boards. Click on the appropriate one for you:
(green circuit board) or
mid-2001-present (cream circuit board). If you play clean/jazz, cooler
bias is a must.
ADJUSTABLE BIAS IS PART OF EVERY BLUES JUNIOR KIT NOW.
Finally--more volume from your Blues Junior, when you want it!
You can have more clean volume, more distorted volume, with this easy-to-install
kit. It can be set up to come on with your Fat switch or with a pull-up switch,
for a volume boost that lets your amp cut through when you solo or need to kick
it up a notch. Take a look.
samples of stock and modified Blues Juniors: click
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!
TwinStack mod: Super easy, great results
Solder one small jumper on the back of your mids control to change
the operation to that of a Twin Reverb. Lets you dial out all the mids
for more clean headroom and that distinctive bass-and-treble
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
Switch for Green Board BJr (obsolete)
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
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
Kits--all the parts you need to bring out the best in your Blues Junior
The components you need can be hard to find in small quantities--you
definitely can't get them at Radio Shack.
variant of the 12AX7 Family (obsolete)
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.
speaker for the Blues Junior.
Work in Progress:
Turning a Blues Junior into a
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
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.
Cathode Follower Tone Mod
My adaptation of Mark Huss's cathode follower mod. Harnesses the unused
triode in the Blues Junior as a cathode follower driving the tone stack. A
subtle mod, but interesting. Versions for cream and green board.
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.