When Tom Petty sang, You got a heart so big/It could crush this town, he could have been talking about Merle Zuel — in more ways than one. When Zuel, a longtime heart-condition sufferer, got a transplant in February 2007 at St. Luke’s Hospital, his original ticker was about twice the size of the normal human heart, breaking the hospital’s record by 100 grams, according to an article that ran afterward in the Lawrence Journal-World. This year marks the third anniversary of the live-music benefit concert that Zuel, a former local music booker, founded after his surgery. Extending over two nights, MerleJam pulses Friday and Saturday at Knuckleheads Saloon. Beginning at 8 p.m. Friday, an ampersand-heavy, all-country lineup featuring Tater & the Gravy Train, Outlaw Jim & the Whiskey Benders and Justa Billy & the Bull Hollers breaks out the honk. Blues is the order for Saturday; the seven-band bill begins at 5 p.m. and includes Danny Cox, Four Fried Chickens and a Coke, Chicago’s Joe Moss and Howard & the White Boys, and climaxes with South Dakota blues-rockers Indigenous. Cost is $10 in advance for the first night, $15 for the second. Net proceeds from the concert go to the Saint Luke’s Hospital Foundation–MerleJam Transplant Fund.