Ernie Ball Super Slinky Cobalt Flatwound (.009–.042) review: the lightest flatwound Cobalt

Super Slinky Cobalt's flatwound sibling, and the lightest of the three gauges in Ernie Ball's Slinky Cobalt Flatwound line: .009 to .042, wound with a cobalt alloy ribbon instead of round wire. Ernie Ball's own product page frames the set around three claims: "Flat Feel," a cobalt ribbon "precisely polished to provide the smoothest possible feel with the least amount of finger noise"; "Round Sound," the same cobalt alloy that "preserve[s] that classic Slinky tone"; and "Plain G," where "a plain G string along with lighter gauge combinations allow for easy bending and soloing compared to traditional flatwound strings."

This is the gauge that completes the family. Ernie Ball sells three flatwound Cobalt gauges on one product page, Super Slinky (this set), Regular Slinky, and Power Slinky, and all three now have a full CYS review. Guitar World covered the NAMM 2023 launch and called it the world's "first and only" cobalt flatwound string, noting Ernie Ball was "touting their blend of old-school tone and modern playability, with the smooth feel particularly suited to slide playing."

Like every set in the flatwound family, this one skips the tradition of wrapping the G string. A traditional flat, D'Addario XL Chromes included, wraps all three treble-adjacent strings, which stiffens the G and makes wide, fast bends harder to execute cleanly. Ernie Ball's SKU 2593 leaves the G plain at .016, so the same light-gauge bending vocabulary that makes the roundwound version of this exact gauge Steve Vai's documented Cobalt anchor carries over here too, just quieter under your fingers.

SKU 2593 is Ernie Ball's Super Slinky gauge in the Slinky Cobalt Flatwound line. B0C356VJ63 is the ASIN Amazon lists for this exact SKU, cross-checked across multiple regional Amazon storefronts and independent retailers this run.

Ernie Ball sells this exact .009-.042 gauge two ways: roundwound Super Slinky Cobalt and this flatwound version. Same numbers, same alloy family, different wrap geometry, and CYS has both reviewed, so the comparison is direct instead of estimated.

The gauge decision and the construction decision are separate. Pick .009-.042 because it's the lightest, easiest-bending numbers Ernie Ball sells; then pick flatwound or roundwound based on how much finger noise and brightness you want. Cobalt vs nickel Slinky covers the alloy question if you're also weighing that.

Ernie Ball sells Slinky Cobalt Flatwound in three gauges on one product page, all sharing the plain-G design. This is the lightest of the three, and the last of the three to get a full CYS review.

Regular Slinky Cobalt Flatwound (.010–.046), full review · Power Slinky Cobalt Flatwound (.011–.048), full review

None of these three gauges have a manufacturer-published tension figure, since Ernie Ball doesn't publish flatwound tension charts. Estimating from CYS's own already-sourced roundwound Cobalt tension at each identical gauge (E standard, 25.5" scale) gives a rough but useful side-by-side that no retailer listing publishes:

The spread between lightest and heaviest is about 18 lbs across the family, roughly the same relative gap as the roundwound Slinky Cobalt line at these gauges. If your hand and your tuning are already dialed in on a roundwound Slinky gauge, the matching flatwound version should feel like a similar amount of string tension under your fretting hand, even though the wound strings themselves feel and sound different.

• Players who want the loosest, easiest-bending flatwound Ernie Ball sells. .009-.042 is as light as the Cobalt Flatwound line goes
• Rock, blues, and pop players curious about flatwound who don't want to give up bends and lead lines to get there
• Quiet studio takes and slide playing. Guitar World's own launch coverage singled out the flatwound Cobalt line's smooth feel as "particularly suited to slide playing"
• Guitars already set up for roundwound Super Slinky gauge, including Vai-style .009 setups. No setup change, since the core numbers match

• Traditional jazz players chasing a fully dead, dark tone. D'Addario Chromes is voiced darker and deader on purpose, at a heavier gauge; this set still carries some of Cobalt's brightness
• Hard-picking players in Eb standard or lower. At .009-.042 this is the loosest gauge in the family; step up to Regular Slinky Cobalt Flatwound (.010-.046) or Power Slinky Cobalt Flatwound (.011-.048) for more tuned-down headroom
• Humid climates and heavy sweat. Player reports on Cobalt's corrosion resistance are mixed: some see rust or tarnish within weeks in humid conditions or with high-acid sweat, others get long life with no issues. If you sweat heavily or gig somewhere humid, wipe down after every session, or consider a traditional stainless flat like Ernie Ball's own Stainless Steel Flatwound (buy on Amazon)
• Budget shoppers. It costs roughly double a standard nickel Super Slinky pack

Coming from roundwound Super Slinky, whether nickel or Cobalt, the swap to this set needs little more than a normal restring since the gauge numbers match exactly. The flatwound ribbon on the bottom three strings carries a bit more mass than round wire at the same gauge, so check intonation after the swap. Stretch each string the way you would any new set: press behind the 12th fret, pull up about an inch, retune, repeat three or four times before you trust the tuning.

Go easier on hard bends than you would on a roundwound set, even though this is already the easiest-bending gauge in the flatwound family. The flattened cobalt ribbon has less give than round wire, so an aggressive full-step bend can leave a faint flat spot at the bend point over years of play. It's a smaller concern here than on a heavier traditional flat, since the plain G string, where most rock and blues bends happen, isn't flatwound at all.

Super Slinky Cobalt Flatwound is the set to reach for if you already play light-gauge roundwound, Vai's .009-.042 numbers included, and want a quieter, smoother string without relearning how to bend. It's the loosest, easiest gauge in Ernie Ball's flatwound Cobalt line, trading some brightness for reduced finger noise and, per Guitar World's own launch coverage, a smoother ride under a slide. It costs more than a standard nickel Super Slinky pack, isn't a substitute for a traditional dark jazz flat like D'Addario Chromes, and gets floppy fast if you tune down and dig in hard. For an E-standard player who wants flatwound's quiet without giving up light-gauge playability, it's the easiest on-ramp Ernie Ball sells, and the gauge that completes the Slinky Cobalt Flatwound family on CYS.