Built from Tweed for Extra Speed
Though rightly recognized as one of the most significant amps of the 1960s, the JTM45 can confound casual Marshall enthusiasts with modern, high-gain leanings. It effectively mirrored the Fender 5F6A tweed Bassman circuit. But small differences between the amps (as well as bigger ones, like the Marshall’s Celestion speakers) added up to a unique and powerful voice perfectly suited for the emerging British blues-rock language.
To reduce the 30 to 40 watts produced by a JTM45 (depending on spec and era) to 20 watts while using the same 5881 power tubes the JTM45 was born with, Marshall uses a cathode-biased output stage and hits the tubes with lower voltages than big Marshalls run on. The circuit also enables the ST20C, and its sibling the ST20H head, to run even quieter via a 5-watt mode. It comes with contemporary niceties like an effects loop with bypass switch, a DI output, and speaker outs for 4-ohm, 8-ohm, and 16-ohm loads. But the ST20C is still a very old-school, all-tube creation at heart, using three ECC83 (Brit-speak for 12AX7) preamp and phase-inverter tubes, just like its predecessors.
Coffin It Up
Like the Bassman-inspired JTM45, the ST20C is what Marshall calls a two-voice amp, with shared EQ and two inputs per voice. Marshall’s numbering scheme for the inputs can be a little confusing to the uninitiated (they read “2” and “1” from left to right), but the top two inputs are for the high-output voice, the two below are for the low power voice. In each voice, input 2, aka the normal input, is the warmer and rounder-sounding of the two. Input 1, or the high treble input, brighter and snarlier. Each has its own loudness control, but they share treble, middle, and bass controls, as well as a presence knob that further tweaks high-end response at the output stage. The two voices can also be jumpered to access blends of input 2’s warmth and input 1’s brightness.
“Newbies might be surprised to hear how much the ST20C deviates from a modern definition of the Marshall sound.”
In addition to the vintage-evocative control panel, Marshall went all-out on the early JTM-era cosmetics for the ST20C, and I think it looks fantastic. The black levant covering is complemented by gold string and piping, fawn fret cloth for the grille, and an art deco “coffin” logo with red enamel “Marshall” text. Construction of the 19.7" x 18.1" x 10.4" birch-ply cabinet feels robust, and with the 16-ohm Celestion G12M Creamback speaker, it still weighs just around 39 pounds.
The Studio Series amps aren’t among Marshall’s handwired offerings, so the circuit inside is PCB. Nevertheless, it looks sturdy and well-built. The use of relatively low voltages in the output stage means the 5881 tubes should last longer than they would in a higher-wattage, higher-voltage amp. But if you do need to replace them, the cathode-biased configuration means you just pop in a new pair of 5881s and go—there’s no bias adjustment necessary.
Breaking in, Breaking Through
I checked out the ST20C with a 1959 Les Paul Reissue to see how it handled the classic Bluesbreaker formula, as well as a 1966 Fender Telecaster. With each guitar, it delivered a wide and expressive range of tones that are very much in keeping with the original’s performance. Newbies might be surprised to hear how much the ST20C deviates from a modern definition of the Marshall sound. But it is surprisingly versatile for that reason. And you can make the case that the ST20C tone palette works for a greater range of playing styles and genres than the average blaring stack.
The core tone is characterized by a thick, round midrange; silky, clear highs that aren’t piercing; and low-end tones that are full, round, and a little loose when you push it. The amp also works beautifully with overdrive pedals. A Tube Screamer sounded great, and a Marshall-style Friedman Small Box overdrive effectively brought out the ST20C’s plexi side. The effects loop works perfectly for time-based and modulation effects. Meanwhile, the 5-watt power setting rips without any significant tone loss.
Many players more familiar with the plexi and JCM800 Marshalls will be surprised by, well, how Fender-y the ST20C can sound. Obviously, the tweed Bassman-derived blueprint accounts for some of that. But if you’ve ever turned up an early-’60s, brown-panel Fender Deluxe and thought, “wow, that sounds like a mini Marshall!” … you’ll recognize that hybrid sound in many of the ST20C’s sweet spots, too. Crank it up and it’s raw, throaty, and singing like an early Marshall should be. Play it at the edge of breakup and it evokes more American-style tones. The ST20C’s ability to span these sounds and assert its own personality makes it a fun, flexible and inspirational piece of gear.
The Verdict
The JTM ST20C is an awesome addition to Marshall’s popular Studio Series and it’s cool to hear the tonalities of the company’s first amplifier rendered in such compact, practical form. Some buyers might find the near-$2K price tag high for a PCB-based, 20-watt amp. But when you consider the wide range of genres and styles this compact U.K.-built combo covers, and how assertive and primed to conquer it can sound and feel, it starts to sound like money well spent.
Marshall Studio JTM ST20C Demo | First Look
Intermediate
Advanced
• Learn melodic minor scales
• Create melodic minor chord-voicing strategies
• Develop melodic minor melodic and harmonic vocabulary
First, take a look at Ex. 1, which is the chord progression for an A minor blues. I will be referring back to specific locations in this progression from time to time.
Why is the Melodic Minor Scale so Bright?
Melodic minor scales are built with a W–H–W–W–W–W–W–H step pattern—W is a whole-step (two frets) and H is a half-step (one fret). The A melodic minor scale is spelled A–B–C–D–E–F#–G#–A, or 1–2–b3–4–5–6–7–8 in scale tones. (In classical theory, the melodic minor scale has a separate formula for its ascending and descending forms. For our purposes—as is often the case in jazz—we’re using the ascending form for both directions, not the descending form, which would be the same as an A natural minor.)
It sounds bright because its structure yields an abundance of whole-steps in the upper part of the scale. Take a listen to Ex. 2 to hear how it sounds against an Am chord. You can use this scale anywhere you see an Am chord in Ex.1. Measures 1–3, measures 7–8, and measure 11.
Since this scale has a natural 7, or a leading tone, it makes the Am chord sound like tonic minor. Basically, this scale tells everyone, “Yo! We’re in the key of A minor!”
Consider the bVI chord—F7—in measure 9. This is an opportunity to use F Lydian Dominant, which is the fourth mode of the melodic minor scale system. It is built with a W–W–W–H–W–H–W step pattern, spelled F–G–A–B–C–D–Eb–F, and its scale tones are 1–2–3–#4–5–6–b7–8. When using this mode, you’re implying that your F7 chord has become an F7(#11). Since this scale is the fourth mode, the parent scale of F Lydian Dominant is C melodic minor. Listen to Ex. 3 and hear how the F Lydian Dominant mode sounds over an unaltered F7 chord.
Let’s talk about dominant function chords.
The actual V chord in the key of A minor is E7, and it’s found in measure 10. There is also a secondary dominant in measure 4. A7 is functioning as the V chord in the key of D minor and provides some forward motion, making it sound like you’re headed to a new key. The seventh mode of the melodic minor scale system is Super Locrian, and you use this mode to get a fully altered dominant chord sound. For E7, you play the E Super Locrian mode, which is built with a H–W–H–W–W–W–W–H step pattern, spelled E–F–G–Ab–Bb–C–D–E, and its scale tones are 1–b2–#2–3–b5–#5–b7–8. Using this mode over a dominant chord gives you both altered 9s and altered 5s, turning E7 into an E7alt chord.
The method is the same for A7, where you would use the A Super Locrian scale to achieve an altered dominant sound, creating more musical tension that can be resolved when you land on the tonic chord. Listen to Ex. 4 to hear each of these Super Locrian modes played over their corresponding unaltered dominant chords. Again, remember that the parent scale of E Super Locrian is F melodic minor, and the parent scale of A Super Locrian is Bb melodic minor.
So, what does this sound like when you put it all together?
Ex. 5 is one possible solo, blending traditional minor pentatonic sounds with the melodic minor scale. The lines are also not too heavy on altered sounds over the dominant chords.
Ex. 6 leans into the melodic minor scale and the altered sounds more, arriving at Ex. 7 which is more angular and rhythmic.
There are two paths to creating chord voicings for these altered sounds. One way is to take the unaltered seventh chord, find all your altered 9s, 5, and 11s, and adjust the shape. The other way is to simply use the min(maj7) chord of the parent scale. Take a look at Ex. 8 for some Drop 2 min(maj7) voicings and inversions. Substituting Amin(maj7) for Am or Am7 should be pretty straightforward, keeping in mind that a major seventh chord will sound kind of bright. For your F7(#11) sounds, use a Cmin(maj7) chord voicing, where C–Eb–G–B will sound like the 5, b7, 9, and #11 of F7.
To get the altered dominant sounds, play an Fmin(maj7) chord instead of E7, where F–Ab–C–E will sound like the b9, 3, #5, and root of your E7 chord. The same relationship holds for A7, where you would use a Bbmin(maj7) voicing to get an altered chord sound. In Ex.9, you can hear how these voices are working against a bass line.
There’s a lot to unpack here, especially if you’re not familiar with all the modes of the melodic minor scale system. Learn the modes, then start with simple, straightforward musical ideas. Just going up and down the mode will get these new sounds in your ear, and using traditional structures like thirds, triads, and arpeggios will give you something to play without feeling overwhelmed. Welcome to the bright side of the blues!
But how many of these mods are really necessary? It’s fair to assume that if you own a Blues Junior you were attracted to both its price of $749 street and at least some elements of its sound, which falls into the range of classic Fender voices, with clarity and punch. So, as PG’s “Silver and Black” columnist and resident guru of fenderguru.com, let me wade through some of the popular mods and make some recommendations about a few I think are actually beneficial, affordable, and easy to accomplish—resulting in more punch, volume, and warmth in your tone. Specifically, I’m talking about using extension cabinets, adjusting the bias by changing the resistors, replacing those electrolytic caps, and adding a resistor to tame and darken the reverb.
Before we get there, however, let’s take a look under the hood of this humble working-player’s Fender. The 15-watt, all-tube Blues Junior was introduced in 1995. It weighs only 30 pounds and measures 16" x 18" x 9", making it smaller and lighter than a Princeton, although it holds a larger 12" speaker.
Although smaller than the Princeton, the Fender Blues Junior has a larger speaker and more wattage, with a 12" speaker compared to the Princeton’s 10", and 12 watts compared to the Junior’s 15.
How Low Can We Go?
The Blues Junior was introduced before the trendy low-wattage-amp era began, and even in the digital era, players still appreciate the Junior’s printed circuit board (PCB), manual knobs, and tubes in its preamp and power amp sections. There have been many revisions over the years, including minor changes in circuitry and components, different speakers, and the addition of a fat switch that boosts the mids and pushes the preamp stage into quicker distortion.
There are three models in the current Blues Junior IV lineup. The black Tolex amp has a Celestion A type speaker, the racing green model has a Jensen C12Q, and the lacquered tweed model comes with a Jensen C12N. They all share the same circuitry. There’s also a tube in the phase inverter, while there is a transistor-driven reverb and diode rectifier. The early Blues Juniors with green circuit boards were made in the U.S., until Fender moved production to Mexico in 2001. The Mexico-made amps have a cream-colored circuit board.
PCB amps are a little more complicated to work on than handwired amps. You must be gentle and careful with wire cutters and soldering irons to not damage the small connections. It is easy to break the board’s connection traces and pins, or accidentally touch a high voltage point in the circuit. Be careful should you venture inside.
The Tale of the Tubes
So, what can we draw from inspecting the amp and reading the schematic of a Blues Junior? The control panel has volume, master, treble, middle, bass, reverb, and fat switch controls. Reading the schematic, I find that the tube layout and circuit functions are:
V1 12AX7: The first half of the tube handles preamp gain stage one before the volume and fat functions, and the second half powers preamp gain stage two, after volume and before the EQ/tone stack.
V2 12AX7: The first half powers preamp gain stage three, after the EQ/tone stack and before the master volume, and then the phase inverter. The second half of the tube’s output is not in use.
V3 12AX7: The first half juices the phase inverter’s phase one; the second half handles the phase inverter’s phase two.
V4 and V5: That’s the home of the EL84 power tubes.
Just like the Princeton Reverb, there are three gain stages in the preamp circuit. The power amp circuit design is also based on a classic Fender black- and silver-panel-era recipe. It has dual EL84 power tubes in a push-pull Class AB configuration with fixed bias and a negative feedback loop. This means we should expect a nice, clean black-panel-style tone before the amp breaks up.
Before the power tubes sits a proper, high-performing, long-tail phase inverter based on the two available amplifier functions in the single V3 12AX7 tube. All Class AB push-pull amps need a phase inverter that splits one sine wave signal into two signals with opposite phase that are fed into each power tube. The Princeton Reverb has a weaker phase inverter than the Blues Junior, and the Blues Junior has a bigger loudspeaker but a smaller speaker cabinet, which we will get back to in a moment.
The small cabinet is the most significant bottleneck with the Blues Junior. At home, it works well for me, but onstage it gets too boxy and thin. The 12" speaker doesn’t have room to breathe in that small cabinet and would benefit greatly from a bigger space. You can change the tone by swapping the loudspeaker, but you won’t get more punch and bass response.
Here’s an interior look at Jens’ own Fender Blues Junior. Space is of the essence in this mighty mite of an amp.
Extended and Amended
Bigger extension cabinets are by far the easiest way to increase the spread and punch of a Blues Junior. Good speaker combinations are closed or semi-closed 1x12, 1x15, or 2x10 cabs. The Blues Junior expects an 8-ohm speaker load but will handle anything between 4 and 16 ohms. Since 8 ohms give you the most headroom, I recommend disabling the built-in speaker entirely and sending all the power to the bigger extension cabinet for a bigger tone.
If a bigger extension cabinet isn’t enough, you could consider upgrading the output transformer to a larger one. Some will argue that the Junior’s small output transformer is a weak point, limiting fullness and bass response. I, however, think the modest power transformer is properly sized for this small amp. It is, after all, supposed to be a low-wattage amp with early breakup. A bigger OT is not the right medicine in such a small combo, in my opinion, unless you’re always driving an extension cabinet.
Fixing the Fixed Bias
Before we leave the power amp section, I must mention that the Blues Junior is famously known for having a too-hot fixed bias setting on the EL84s. Juniors benefit from a cooler bias, which also increases tube lifetime. Since there is no available bias pot, you have to adjust the bias by changing the values of the two resistors—a 22k/R31 and 33k/R37, or a R52 and R51 in more recent models. This is especially helpful if your amp wears out EL84 tubes fast. The good news is that Fender has improved the latest model IV with a cooler bias setting.
This photo shows the location of the R51 resistor in the Blues Junior circuit, important in recontouring the bias.
So, you’ll need to reduce the amp’s bias current to between 25 mA and 30 mA, from the factory-set 42 mA. (You’ll need a bias probe to measure the bias. To learn more about using a bias probe, see Jeff Bober’s story “Amp Man Returns, to Explain Tube Bias” in Premier Guitar’s June 2023 issue or online at premierguitar.com.) The simplest way to accomplish this via reducing the value of a resistor is to add another resistor in parallel. You simply solder a resistor on top of the existing one on the front side of the circuit. Twist the legs of the new resistor around the old ones a couple of times so it stays in place. That way, you don’t have to remove the original resistor. Then, warm up and apply the soldering tin to the legs. It’s that easy. I recommend adding a 220k resistor in parallel with the original 33k to see where that leads you. If you need to reduce more, use a smaller resistor or add another one in parallel until you are between 25 mA and 30 mA.
As far as tubes go, the circuit design will allow lower gain tubes—such as a 5751 or a 12AU7—in the preamp V1 and V2 positions. But you’d lose volume and break-up. I find that the amp’s sweet spot, meaning the transition between clean and distortion, is already pretty much where I want it, and I like the usable range between the volume and master volume controls, so I would advise against inserting low-gain tubes.
The tone stack and EQ balance is well-designed, too, with a nice, usable range in the bass, mid, and treble controls. The Junior can do both sparkling cleans and crunchy distortion. The mid control offers more mids than the scooped Princeton. With the mid knob set at max, I’m able to get some British-style distortion. If you think that your Blues Junior sounds a little glassy, you should experiment with other loudspeakers. Inserting a broken-in Celestion Greenback or a Jensen P12Q would easily make cranked tones smoother or the clean sound more authentic-vintage Fender. The Blues Junior III I’ve been playing lately comes with a Fender-labeled speaker made by Eminence—a good, all-round speaker with nice sparkle and a firm bottom end.
Doffing the Caps
Next up: a highly recommended maintenance mod. Electrolytic filter caps are important components in terms of noise, loudness, and clean headroom. The cheap, low-quality caps that come in blue or grey are a well-known problem with the Hot Rod Deluxe and Blues Junior. The good news is, they are quite easy to swap out. I recommend replacing these if you hear any 50Hz hum or see any signs of cap leakage. There are several cap replacement kits available online for the Blues Junior, running from about $60 to $100.
Note the 14 circled electrolytic capacitors in this photo. Filter cap replacement kits are affordable and widely available online.
Start by replacing the four largest capacitors: three with a value of 22 uF and one with a 47 uF value. Again, it’s simple. Just clip the legs off the factory-installed capacitors as close to the caps’ bodies as possible, and remove them. Then, twist the new caps’ legs around the old ones and solder them in place. Just be careful with both your clippers and soldering iron around the PCB. If that doesn’t get the results you want, replace the smaller caps, too.
Moving on to the reverb, I can understand the cost-efficiency decision that Fender made with the simplified, transistor-based circuitry. Not having a tube-and-transformer-based reverb lowers expense and weight, and there are fewer things that can fail. Luckily, there is a proper spring reverb tank in the back of the amp. In fact, the Blues Junior I, II, and III are famously known for having too much reverb. Anything above 2.5 on the reverb dial and the amp swims in overwhelming and bright ’verb.
Another recommended, and cheap, mod is to add an 82k resistor in parallel with the original 220k/330k in the reverb circuit, to tame and darken the reverb. This goes parallel to the R44 (220k) resistor on the made-in-Mexico board or the R50 (330k) resistor on the made-in-U.S. green PCB. Once again, an easy fix. As in the bias adjustment mod we covered earlier, we solder a new 82k resistor on top of the existing 220k on the front side of the circuit. How? By twisting the leg of the new resistor around the old one a couple of times so that it stays in place, and then applying the soldering tin. Voilà!
To restore the Junior’s classic Fender reverb sound, a new resistor should be wired in place atop the R44 factory resistor, circled in green.
With this mod, the reverb sounds more like the classic black- and silver-panel ’verbs, but with a wider knob-control span. I have read that the early made-in-the-States models have a darker reverb tone and might not require this move. But if you do this and find change isn’t enough, you may want to experiment with replacement reverb tanks with different decays.
So, that’s my take on this popular little warrior tube amp, and some easy mods. Even without changing the circuitry at all, with the Blues Junior, for a little money you get a lot of vintage Fender tone with a modern twist.