Fender Blues Junior Tone Stack Mod

Lots of people love the Blues Junior. The amp is a huge hit because it's small, light in weight, has real tube tone, and has a master volume control, which lets you choose your tone and loudness independently. The BJr is also one of the most affordable tube amps.

Many players, however, especially fans of vintage Fender "blackface" amps, criticize it for having boxy tone or lacking "soul." Players say that these old blackface amps sound the way Fenders ought to sound. Aftermarket speaker suppliers do a brisk business selling replacement speakers to Blues Junior owners to correct the perceived deficiencies in tone. The speakers most often recommended for Blues Juniors have higher sensitivity (they're louder) and tend to emphasize the bass and midrange more than the stock Eminence-made speaker. One exception is the Celestion Greenback, which is a very sensitive speaker that emphasizes highs and early breakup--the classic British Vox/Marshall sound. Replacement speakers cost anywhere from $60 to $200. Too much, in my book.

People tend to attribute the boxy sound to the speaker, but it's a combination of things: The speaker, to be sure, but also the size of the cabinet and the amplifier's circuitry. Larger cabinets reproduce more bass because there's less cancellation of low frequencies from the back of the cabinet. There's not much you can do about that unless you want to build a custom cabinet.

But there's a lot we can do to alter the tone that the amp produces. The Blues Junior has treble, mid, and bass controls, with a lot of flexibility in the tone, but you can't boost the bass and mids enough to overcome the boxy sound. Fender has been using the same basic arrangement of resistors, capacitors, and potentiometers since the dawn of time, even on amps that lack a midrange control (it's a fixed resistor instead of a pot). I focused on the Princeton Reverb because it gets a lot of sound out of a small speaker and a low-powered amp.

The Princeton's tone controls are located right after the first preamp stage, while the BJr's are after the second stage, but the layout is identical, except that the Princeton lacks a midrange pot.

Here's the circuit used by the Blues Junior: And here's the circuit used by the Princeton Reverb:

Many of the values are the same. But the circuits are more notable for what's different than for their similarities. The Princeton uses a .1uF (100nF) bass coupling capacitor. while the BJr uses a .022uF. The midrange caps are the same, at .022uF (22nF), and the treble caps might as well be the same, with the BJr's 250pF and the Princeton's 270pF. Other classic Fenders use values similar to the Princeton's, but there isn't a whole lot of consistency--the 5F6 Bassman uses the same values as the Blues Junior. But the Bassman was driving four 10-inch speakers while the Princeton was trying to wring tone out of one lone 10-incher. The point is that you can alter the tone stack values to suit your speaker, cabinet, and playing style.

I used Duncan Munro's Tone Stack Calculator program to analyze the stock Blues Junior tone stack. It lets you change any of the components interactively and draws a graph of the resulting frequency response curve.

 

The magenta line is the stock BJr tone stack with all controls set at midpoint. The white line is the Princeton's tone stack. The green line is my modification to the BJr stack.

Increasing the BJr's bass cap to .1uF has a profound effect on the amp's tone. It deepens and richens the sound, even though a tone stack simulator/calculator indicates that most of the increase happens at 100 Hz and lower, below most guitar frequencies. But the couple of dB gain in the 100-1000Hz range makes all the difference because it changes the relative balance to the other tones.

Increasing the treble cap from 250pF to 270pF? No audible difference. I tried it. The tone stack simulator shows no difference, either.

But what about the midrange cap? Is .022uF the right value? You might expect that a bigger cap would get you more midrange--and you'd be wrong. The cap actually sucks the midrange frequencies out of the circuit, and creates a dip in response. Popular music is built around strong lows, crisp highs, and dialed-back mids. A midrangey tone sounds too much like it's coming through a telephone. But the Princeton stack has a bit too much scoop for my taste. A little time on the simulator shows that a .015uF cap has promise: a little boost in the lower mids and high bass, but not enough to mask the good, rock and roll sound of scooped mids.

But the proof of any mod is in the listening, not the theoretical result. So I soldered one in and gave a listen. The result? Along with the .1uF bass cap, a transformed amp. The boxiness is gone. and a little more tone is getting through the stack, making the whole amp a bit louder. Breakup and touch sensitivity start half a notch lower on the volume control, and the amp takes on a different vibe: more authority, more soul.

And now for a dose of reality: sweep tones and audio samples. I used two computers for the sweep test, one to generate the tones, the other to record them. I used Cool Edit for both--it has an excellent tone generator and powerful analytics. I chose a 20 to 6 KHz sweep over 3 seconds, so that no tone would last long enough to cause resonances in my test room, but all guitar frequencies would be covered. I recorded the modified amp and a stock Blues Junior through a Shure SM-58, direct into an IBM T21 laptop. The mic was located 2 feet from the speaker, 45 degrees off center.

Stock Blues Junior The vertical axis is loudness as measured by the mic; the horizontal axis is time, as the signal generator sweeps from 20Hz to 6 KHz. The waveform of the input tone was flat, perfectly level at all frequencies. Obviously, guitar amps are anything but flat. I was surprised to see the big emphasis building up to 3500 Hz and the sudden pinch-off at about 5KHz. Bass response is lacking. Stock Blues Junior This is the frequency view of the same waveform. The vertical axis is frequency; the horizontal axis is time, over the 20Hz to 6 KHz sweep. Loudness is indicated by color--the darker the louder. The two lines over the main line are harmonic distortion. The second harmonic is what gives tube amps their characteristic sound. I was surprised to see so much third harmonic, though.
Modified Blues Junior The frequency response is much flatter, thanks to the boosted bass and mids. It still exhibits a pinch-off at 5kKHz. The lower amplitude overall means that I messed up the level equalization. I thought I got it right with 1KHz test tones, but apparently not. Modified Blues Junior You can see that the loudness (dark blue) is spread more evenly in this view. The second and third harmonics are still strong, though. The pinch-off is apparent as the light blue lines interrupting the darker blue. No matter--guitars don't go higher than that--but the 3rd harmonic goes up to around 7 KHz.
Guitar thru Stock Amp This MP3 file has some chords from my Formicaster. Both amps were set with the tone controls at midpoint, the volume at 3, the master volume at 5, and the reverb at 3, FAT switch in. The stock sound is trebly and boxy, but crisp. The second, surf-ish arpeggios and chords sound good to my ear. The Formicaster has 3 single-coils, wired in series, with individual volume controls. The first tone is full neck with a little middle and bridge; the second tone is full neck and bridge. Guitar thru Modified Amp The modified amp is set exactly the same as the stock amp. It has much more authority in the bass, even though the sample isn't as loud. I think the highs sound dull compared to the stock amp, but they can be brought up with some treble boost, more master volume, and some bass and mid cut. It gets dirty faster than the stock amp--chords are thicker and fuller when the other amp is still clean.

And Finally, a Photo
On the older Blues Juniors, The .1uF capacitor replaces C11 and the .015uF cap replaces C13. The Orange Drop caps are much larger than the stock caps, but they fit without difficulty. You can also see my reverb mod for older, rev B thru D Blues Juniors on the left. The picture on the right shows the tone stack mod to a post-2001 Rev A board, where they replace C6 and C7. You can also see the adjustable bias trimpots.

  

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