Martin MA550T Lifespan 2.0 Phosphor Bronze Medium (.013–.056): the D-18 Molly Tuttle's factory string

Martin MA550T is the Medium .013 to .056 gauge in Martin's Authentic Acoustic Lifespan 2.0 line, built from the same 92/8 phosphor bronze wrap (92% copper, 8% tin phosphide, per Martin's own string materials guide) as the rest of Martin's Authentic Acoustic SP family, over a tin-plated high-tensile steel core. The "T" in the model name marks Martin's Lifespan 2.0 anti-corrosion treatment, a patented process Martin says protects both the core and wrap wire from the sweat and oil buildup that dulls tone fastest.

This is also, as of 2026, a factory-default string. Martin's own product page for the new D-18 Molly Tuttle signature dreadnought lists "Authentic Acoustic Lifespan 2.0 Guitar Strings Phosphor Bronze - Medium" under Recommended Strings and confirms it ships pre-strung on the instrument. If you own that guitar, or you're chasing the tone it left the factory with, MA550T is the exact set.

Martin's pitch for Lifespan 2.0 is narrow and specific: a patented treatment on both the core and wrap wire that fights corrosion "without compromising tone or feel." Martin's own materials point to an internal corrosion test pitting Lifespan 2.0 against three unnamed leading brands, and the marketing line is that the treated strings "sound better, longer than any other extended-life string on the market." That's Martin's claim about its own product, not an independently verified benchmark, so treat the superlative as a manufacturer's positioning rather than a tested fact. What's verifiable is the mechanism: sweat and skin oil corrode wrap wire over time, and any anti-corrosion treatment (Lifespan 2.0, D'Addario's XT, Elixir's Nanoweb and Optiweb) works the same basic problem from a different angle.

Country artist Thomas Rhett backs the line directly, quoted on Martin's own Lifespan Strings page:

Martin doesn't specify which gauge Rhett personally uses, so this documents his endorsement of the Lifespan line in general rather than the Medium MA550T gauge specifically.

Martin unveiled the D-18 Molly Tuttle and D-X2E Molly Tuttle signature dreadnoughts at the 2026 NAMM Show, both modeled on Tuttle's own 1943 D-18. Martin's product page for the D-18 model confirms MA550T Medium ships pre-strung on the guitar at the factory, listed directly under Recommended Strings. Tuttle's own gauge history backs up the fit: a 2017 Premier Guitar interview documented her personal touring set as a coated D'Addario Medium .013-.056, the same gauge as MA550T, just a different brand and coating technology. Medium gauge tracks her playing style, too. Flatpicking fast single-note runs across a full bluegrass band benefits from the extra low-end push and sustain Medium gauge gives a dreadnought over a Light set, at the cost of slightly firmer fretting pressure.

• Martin dreadnought and jumbo owners who want the extra low-end push and sustain Medium gauge gives a large-body acoustic over Light
• Bluegrass flatpickers running long sets where sweat and oil buildup would otherwise dull an uncoated set fast
• D-18 Molly Tuttle and D-X2E owners restringing to the exact factory spec
• Players stepping up from Light gauge for more volume when playing with a full band

• Smaller-bodied acoustics (0, 00, 000/OM, parlor): Martin's own gauge guide steers Medium toward larger bodies; step down to MA540 Light instead
• Players chasing maximum fresh-string brightness: the anti-corrosion treatment trades a sliver of top-end zing for longer life, the same tradeoff every treated string makes
• Budget-first buyers: the uncoated MA550 SP in the same gauge and alloy costs less if longevity isn't the priority

MA550T is the Medium 92/8 phosphor bronze gauge Martin now ships standard on its highest-profile 2026 signature model, wrapped in the company's own anti-corrosion treatment for extra shelf life. If you own a Martin dreadnought, or any full-size steel-string acoustic, and want factory-spec replacement strings that hold their tone longer between changes, this is the set. If you'd rather save a little and don't mind restringing more often, the uncoated MA540 family covers the same alloy one gauge lighter.

Ships stock on the Martin D-18 Molly Tuttle Approved

Martin

MA550T Lifespan 2.0 Phosphor Bronze Medium (.013–.056)

.013 – .056

Price tier: $

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