Can you add separate bass and treble controls to a Marshall-style amp channel with a single tone pot?
I recently purchased a handwired clone of a Marshall 2061 JMP and I love it. But is there any way to add a bass control or maybe a bass-boost switch? I want the bottom end without losing clarity. The amp currently has high and low inputs for both the lead and rhythm channels. The head was a kit designed by Soultone Amps.
Okay, Ask Amp Man readers, it turns out Bobby is local to me. Because I thought this might be a cool project, I had him drop off the amp. It has two channels, lead and bass, each with a single volume and tone control. We discussed what Bobby wanted, and it turned out that the single tone control on each channel weren’t cutting it for him—he needed more control over both the treble and bass. He already bridges the channels with a jumper (we used to call this “double jacking”).
I decided the best action would be to retain an individual volume control for each channel, and then install global bass and treble controls for overall tone shaping. Here’s a description of that mod, along with a couple of additional changes to enhance the amp’s performance.
First, I removed the original control set from the amp (Photo 1), leaving it connected as an assembly (Photo 2). This makes it easy for any future owner to return the amp to its stock configuration.
Next, I installed new volume pots in the front panel, placing them between the two sets of input jacks, each next to its respective channel. The other two locations can now become individual treble and bass controls.
Wiring up the volume controls is simple: The CW leg connects to the output capacitor for the individual channel outputs of the first preamp tube. The CCW leg goes to ground, and the wiper connects to the respective 470k mixing resistor.
Now for the tone controls: I didn’t want to add to the existing circuit board, so I simply built the tone stack by attaching the components directly to the controls (Photo 3). This, by the way, is the true definition of “point-to-point” wiring. I chose the tone stack values traditionally found in older Marshall amps: a 470 (or 500) pF treble capacitor, 0.022 ?F midrange and bass capacitors, and a 56k dividing (slope) resistor. The input to the tone stack comes from the junction of the two 470k channel mixing resistors. (The one for the lead channel has a bright cap in parallel). I disconnected the original 220k grounding resistor from this point, since it’s no longer needed, thanks to the new tone stack.
The tone stack’s output (the treble pot’s wiper) is connected to the input (grid) of the phase inverter. Since there’s no midrange control, I chose a value of 10k to approximate a mid-position control setting. This value could be anywhere between zero ohms (jumper) and 25k—or even higher, depending on your personal midrange preference.
That completed the new control set, but I wanted to make a couple of other changes as well. This particular amp design uses only two preamp tubes. One is dedicated to the phase inverter, so there really isn’t much gain in the preamp stage. Most of the amp’s drive characteristics are generated by pushing the output stage into distortion—which is great, since it’s a 20-watt amp with two EL84 output tubes. Since we added a traditional tone stack to the circuit, putting additional load on the signal path, I compensated by increasing the gain in the phase inverter.
I did this by decreasing the value of the cathode resistor of the phase inverter tube. The original value was 8.2k, but I lowered this to a 2.7k (Photo 4), noticeably increasing the gain. The other change I incorporated was to parallel the bass channel’s 220k mixing resistor with another 220k resistor to better match the level of the channels. (Photo 5). (I simply repurposed the disconnected grounding resistor.)
There you have it: a cool Marshall-style 2061 amp with full tone control. Now it’s a more versatile low-power gem. Enjoy!
Warning: All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.
Ultra-small headstock tuner can move like a yogi while performing spot-on tuning.
D’Addario’s new NS Micro Universal Tuner is indeed micro. I have normal-sized hands and can fit the unit in one palm, wrap my fingers around it, and make it completely disappear. This also means it’s ultra discreet when attached to a headstock. Factor in the dual-swivel clip-on mount and the ability to spin the tuning unit 360 degrees, and I was able to place the tuner on multiple spots of the headstock with great command in reaching my preferred viewing angle. The reversible LCD screen, while only about the size of an almond at best, provides a pretty stellar readout through a brightly lit, intuitive 3-color display that will handily conquer darker stages. Tuning is accurate, fast, and easy, and the built-in metronome is a nice bonus. And this little tuner provides a calibration range of 410 Hz to 480 Hz for those who like to venture from standard A440. If you tend to lose things like car keys and other small items like I often do, that could be an issue given the tuner’s size. But at 17 bucks, you can always grab a couple.
Test Gear: 2005 Larrivée Parlor, 2014 Breedlove Pursuit, 2001 Fender Precision
Quick! Who invented the fretless 4-string?
Hot on the heels of our recent columns on headless basses and their technology, here’s another “less” topic for you: the fretless electric bass. The instrument has had its ups and downs since the mid ’70s, when Jaco Pastorius made it popular. Unlike the ’80s, these days we don’t see the fretless bass too often in pop bands, but it’s still very present in jazz and progressive rock.
There’s no data on sales figures for fretless instruments among overall bass sales, but my personal guess is that fretless basses represent about three percent of the total market. Some people say it’s actually less than one percent. One uncertainty here is the number of home builds, such as in Photo 1. That’s because this mod of pulling out the frets and refinishing the fretboard is relatively easy.
That market share is pretty low considering the high number of renowned bass masters who feature a fretless instrument in their music. It appears as if the higher the skills, the more likely a musician is to pick up a fretless. For many bassists who’ve fallen in love with the fretless, it offers a more expressive way of playing. Of course, for the novice it also offers more chances to fail, but players planning to get a second (or third) bass should consider adding a fretless to their arsenal.
What makes it so special? Basically, it comes down to the enhanced possibilities of the left hand. A fretless lets you slide into notes, add singing vibrato, or vary the pressure you use to press a string against the fingerboard—and all of this with the intimate feel of the vibrating string right under your fingers.
Even the angle at which your finger presses against the fretboard makes a tonal difference. Simply put: It sounds more like a fretted bass when you use the nail side of the fingertip and more like an upright when you play with the softer middle of the pad. The fretless sounds less perfect than a fretted bass— notes are less sinusoidal and have a stronger tonal variation over the entire sustain period, and chords sound especially lively and organic.
Everything has its downside, and the fretless might not fit every playing style. Slapping comes to mind—it works, but the tone often lacks that sharp, metallic attack of the frets.
Jaco covered his fingerboard with hard epoxy, which gave his sound a less muddy, more articulate upper midrange. The Le Fay bass in Photo 2 takes this even further by using a stainless steel fretboard. Still, even with the enhanced treble range of a slapped string wobbling against a flat, metal fretboard, the fretless sound is different from that of a fretted bass, although “different” doesn’t always have to be bad.
Who invented the fretless bass? Urban legend has it that Jaco was the inventor of the fretless bass. Sometime around 1970 he removed the frets from his 1962 Fender Precision, filled the gaps with wood strips, and epoxied the fingerboard. The hard fingerboard produced a singing and lively tone that soon became his trademark. Ultimately this mod and its resulting timbres became extremely popular. More than once, Jaco added to the legend by crediting himself in interviews for the invention.
This fretless Le Fay Remington Steele bass has a stainless-steel fingerboard. Photo courtesy lefay.de
In fact, the electric bass started as a fretless as early as 1935 with Paul Tutmarc, though as a builder he focused on size and pickups, and essentially kept everything else from its predecessor, the upright. Tutmarc’s bass remained fretless until 1940, when Paul Tutmarc Jr. entered his father’s business and produced smaller fretted basses that could be played horizontally. So for the first time, the bass player didn’t have to drive in a separate car, follow the rest of the band, and get lost more than once—a real cause for celebration!
Not much happened technically until the ’50s introduction of the Fender Precision, and from that time on, all electric basses had frets. The first return to fretless came around 1965 with the introduction of a Hohner hollowbody made by the Bartell company in California. According to Bartell’s history, this preceded the Ampeg AUB-1 fretless by a year. It wasn’t until 1971 that the maker of Jaco’s modded Precision joined the party with their first production model.
So again, who invented the fretless electric bass guitar? Let’s put the Tutmarc fretless aside for a moment, simply because as a scaled-down upright bass, it’s hard to call it an invention.
The first person to use a modern fretless is the Rolling Stones’ Bill Wyman. According to his coffee-table book, Bill Wyman’s Scrapbook, he converted a cheap Japanese bass to fretless as early as 1961 and used it with his first band, the Cliftons, and later on for several tunes with the Stones. So much for the popular notion that the fretless only has its place in jazz!
Of course, the working musician will most often need a fretted bass and only switch to fretless for a limited part of a gig—or even switch within a song. And there are a few solutions for that task of switching, so that’s what we’ll investigate next time.