Tim Vitullo
ENGL 202C
Section 024
Overdriving Sound with the Fender Blues Junior Amplifier
Image Courtesy: Wikipedia5
The Fender Musical Instrument Corporation introduced the Blues Junior amplifier in 1995. Originally
intended to recall the warm, tube-driven tones common in American blues and blues-rock music of the 1950s,
it has become the staple of diverse musicians such as Elvis Costello and The Edge of U2. Many incarnations
of the Blues Junior now exist, yet all of them contain the same internal electronic components. The Blues
Junior enlists the help of three 12AX7 preamp tubes and two EL84 power tubes to simultaneously amplify and
distort the sounds of electric guitars.5 This report details how these components interact with an electric guitar
to coax a variety of guitar tones from the Blues Junior’s 12” speaker.
Basic Terminology
As a result of the subjective nature of discussing guitar tones, a list of terminology is provided with
examples below to standardize and clarify some key phrases.
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Clean Tone: a natural electric guitar sound. (Jazz guitarists rely on clean tones almost exclusively
because of the unmatched clarity and dynamics.)
Overdrive: the product of overworking vacuum tubes, causing them to break up and send a distorted
signal to the speakers once they can no longer provide additional loudness to the speaker. (Rock
guitarists commonly use overdrive to add a “heavy,” “rough,” or “gritty” aesthetic to their music.)
Sustain: a “singing” quality in a guitar tone as a result of overdriven vacuum tubes. (While more “felt”
by the guitarist than heard by the listener, sustain allows the guitarist to craft and emphasize lyrical and
melodic lines with greater clarity and definition.)

Vacuum Tube: a device used to amplify an electrical signal by controlling the movement of electrons in
a low-pressure space.6
Figure 1. A Vacuum Tube Used in the Blues Junior.
Image Courtesy: Parts Express3

Pickup: a set of magnetic coils set below the strings in the guitar body that detect the vibrations of the
guitar strings and output them through the guitar’s output jack.
Figure 2. An Electric Guitar Pickup.
Image Courtesy: Same Day Music1

Signal Chain: the path which sound travels through a guitar and amp, beginning with the pickups on
the electric guitar and ending being output from the amplifier’s speaker cone.
The Front Panel
Like many other electronic devices, understanding how the exterior and interior elements of the
amplifier work with each other is important in understanding how it processes input from an electric guitar. The
graphic (Figure 3) found in the most recent owner’s manual depicts the front panel common to every model of
the Blues Junior. The front panel is located on the top face of the amplifier.
Figure 3. The Front Panel of the Blues Junior.
Image Courtesy: Fender Musical Instrument Corporation2
In order from left to right:
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The power switch is responsible for turning the AC power supply to the amplifier on and off.
The power indicator is a red LED light that illuminates whenever the power switch is in the ON
position, informing the user that the amp is receiving power.
The REVERB knob alters the amount of reverb (an effect that simulates the echo of a large room)
added to the guitar’s tone as it leaves the speaker by adjusting the intensity of sound waves produced
by a small speaker to vibrate a metal spring coil inside the metal reverb tank.
The MASTER knob drives the power tubes and adjusts the volume of the amplifier.
The MIDDLE knob, BASS knob, and TREBLE knob boost or cut respective frequency ranges from
the guitar tone and serve as a graphic equalizer for the amplifier in a similar fashion as one might find
on a stereo.
The FAT switch drives the preamp tubes with a step increase in power and provides extra distortion
and sustain to the guitar tone.
The VOLUME knob controls the amount of distortion and sustain with little change in loudness by
driving the preamp tubes.
The INPUT jack receives input from an electric guitar through connection with a one-quarter inch
standard speaker cable. The end of the cable not plugged into the guitar should be inserted here.
Each of these knobs can be set to numerical ratings from one to twelve, “one” corresponding to the
parameter having no influence on the guitar tone and “twelve” corresponding to the maximum influence the
parameter has on the guitar tone. This description does not touch upon the REVERB and equalizer functions
because of their lack of roles in overdriving the amplifier’s vacuum tubes.
Volume and Overdrive
In order to fully understand how tubes combine volume and overdrive, one must first discuss the
sequence of the signal chain for this particular amplifier. A simplified version of the actual circuit inside the
Blues Junior helps describe the process by which tubes amplify and distort the electric guitar’s initial signal.
The diagrams below show that the sound resonates from the strings and induces currents in the guitar’s
pickups. Then, the preamp tubes (mostly responsible for distortion) amplify the signal, and finally the power
tubes (mostly responsible for loudness) further amplify the signal before the sound waves are output through
the speaker. Each stage uses a volume knob to control the amplitude of the waveforms as they pass through
each stage. A few examples below demonstrate how the three stages (guitar, preamp tubes, and power tubes)
interact.
Figure 4. Clean Tone with Full Guitar Volume.
Image Courtesy: Old Tone Zone4
The above (Figure 4) setup represents a signal chain that outputs a clean tone through the speaker.
Both the VOLUME (referred to as “Input Volume” in these diagrams) and MASTER knobs are set at low levels.
These knobs diminish the sound waves coming from the guitar (note the smaller waveforms in these sections)
so that no breakup occurs after the tubes amplify the sound. This is represented by smooth, unaltered
waveforms.
Figure 5. Preamp Tube Distortion
Image Courtesy: Old Tone Zone4
The above picture (Figure 5) represents a scenario in which the VOLUME knob is turned to a maximum
value and the MASTER knob is left at a low level. The sound from the pickups is no longer limited by the input
volume and as a result, the preamp tubes attempt to amplify the sound above their limit and clip the sound
waves. This clipping phenomenon results in overdrive – specifically “preamp distortion.” Preamp distortion is
characterized as “muddy” and “dirty” because it does not provide individual note clarity when playing chords
(multiple notes fretted in unison). This setting serves best as a low-volume alternative for obtaining highly
overdriven guitar tones.
Figure 6. Clean Tone with a Low Guitar Volume.
Image Courtesy: Old Tone Zone4
The signal chain models a series circuit, allowing a variety of knob combinations to produce similar
tones. Figure 6 shows that lowering the guitar volume while leaving the VOLUME and MASTER knobs
unchanged from Figure 5 outputs a clean tone through the speaker (again, unaltered waveforms). By lowering
the guitar volume, the pickups are sending a reduced signal to the amplifier and the waveforms mimic the
scenario of Figure 3, where both knobs are set to low volumes. If the guitar volume were hypothetically located
in a different position, this would not be possible.
Figure 7. Power Tube Distortion
Image Courtesy: Old Tone Zone4
The final diagram (Figure 7) shows a power tube distortion scenario in which the VOLUME knob is set
at a moderate level and the MASTER knob and guitar volume are turned all the way up. This setting prevents
clipping of the waveforms to occur until the signal reaches the power tubes. The tone of the amp will be
overdriven, but would characterized be as more “punchy” or “sparkling” with improved note definition as
compared to preamp overdrive. Power tube overdrive can only occur at very loud settings because these tubes
are designed to govern the loudness of the output sound. As a result, improved clarity can only become
apparent as volume dramatically increases and power tubes dominate the mix.
Conclusion
Currently today, a plethora of new technologies exist in the guitar amplification market with a paucity of
benefits; the most popular being integrated circuits and computer software. Although circuits vary
tremendously among manufacturers and models, the concept of overdriving preamp and/or power tubes
(depending on the design) is still highly regarded. A direct result of their inferiority in most applications of
modern electronics, production of vacuum tubes is now a small demand and high cost market. Despite this, the
analog characteristics of vacuum tubes are still considered the most effective tools for coaxing organic,
seductive guitar tones out of modern amplifiers – the Fender Blues Junior is only one of the affordable and
capable models available today.
Works Cited
1
DrumZa Pics. DiMarzio Electric Guitar Pickups. Digital image. DiMarzio Electric Guitar Pickups. DrumZa,
2009. Web. 14 Mar. 2011. <http://pics.drumza.com/category/Electric-Guitar-Pickups/DiMarzio/>.
2
Fender Musical Instruments Co. "Fender Blues Junior III Owner's Manual." Fender Blues Junior III Owner's
Manual. Fender Musical Instruments Co., 2010. Web. 14 Mar. 2011.
<http://support.fender.com/manuals/guitar_amplifiers/Blues_Junior_manual.pdf>.
3
Parts Express. JJ 6L6GC Vacuum Tube. Digital image. JJ 6L6GC Vacuum Tube. Parts-Express.com, 1996.
Web. 14 Mar. 2011. <http://www.parts-express.com/pe/showdetl.cfm?Partnumber=072-530>.
4
Stauffer, Anthony. "Understanding Tube Amps." Video blog post. Understanding Tube Amps.
OldToneZone.com, 17 Apr. 2010. Web. 14 Mar. 2011. <http://www.oldtonezone.com/category/understandingtube-amps/>.
5
Wikipedia. "Fender Blues Junior." Fender Blues Junior. Wikipedia, the Free Encyclopedia, 14 Mar. 2011.
Web. 14 Mar. 2011. <http://en.wikipedia.org/wiki/Blues_Junior>.
6
Wikipedia. "Vacuum Tube." Vacuum Tube. Wikipedia, The Free Encyclopedia, 11 Mar. 2011. Web. 21 Mar.
2011. <http://en.wikipedia.org/wiki/Vacuum_tubes>.
Topic Sentences

The Fender Musical Instrument Corporation introduced the Blues Junior amplifier in 1995.

As a result of the subjective nature of discussing guitar tones, a list of terminology is provided with
examples below to standardize and clarify some key phrases.

Like many other electronic devices, understanding how the exterior and interior elements of the
amplifier work with each other is important in understanding how it processes input from an electric
guitar.

In order to fully understand how tubes combine volume and overdrive, one must first discuss the
sequence of the signal chain for this particular amplifier.

The above (Figure 4) setup represents a signal chain that outputs a clean tone through the speaker.

The above picture (Figure 5) represents a scenario in which the VOLUME knob is turned to a maximum
value and the MASTER knob is left at a low level.

The signal chain models a series circuit, allowing a variety of knob combinations to produce similar
tones.

The final diagram (Figure 7) shows a power tube distortion scenario in which the VOLUME knob is set
at a moderate level and the MASTER knob and guitar volume are turned all the way up.

Currently today, a plethora of new technologies exist in the guitar amplification market with a paucity of
benefits; the most popular being integrated circuits and computer software.