When people think about the Wanamaker organ - if they think about the Wanamaker organ - they likely associate it with the light show that illuminates Macy's Grand Court in Center City during the Christmas season.

That, cautions L. Curt Mangel III, is a mistake.

Europeans "worship this instrument," said the organ's curator and chief caretaker. "This is a mecca for organ lovers and organ builders, [yet] most people in Philadelphia don't even know it's here."

Mandel hopes that Philadelphia Open House will help change that.

A tour of the 109-year-old organ and its massive inner workings is one of more than a dozen behind-the-scenes offerings of Philadelphia Open House, which will run from Monday through Sunday, May 19. Proceeds from ticket sales will benefit Philadelphia Hospitality, a nonprofit that, as it touts itself, "open[s] the doors of private Philadelphia" with tours of exclusive clubs, unique homes and gardens, and well-known but little-explored public spaces.

"This is consistent with our mission of promoting the city as a world-class destination," said executive director Bill Mifflin. "There is so much to learn about the city's history, culture, and its future."

At Pennsylvania Hospital at 10 a.m. Monday, curator and lead archivist Stacey Peeples will take visitors back nearly three centuries in medical history to the country's first surgical amphitheater, where pre-op patients were numbed with rum or a hammer to the head.

Also featured: American artist Benjamin West's early-19th-century painting, "Christ Healing the Sick in the Temple." (Look for the "lunatic boy" on the right side. It is a somewhat creepy tribute to the hospital's early work with the mentally ill - though perhaps not nearly as creepy as the 7-pound salivary-gland tumor on display in the library.)

"We are real living history, doing the same mission we were designed to do 262 years ago," Peeples said. "This is where the history of medicine really takes off."

If 1751 is too contemporary for you, drop into 1332 B.C. At the Penn Museum at 11 a.m. Sunday, May 19, Stephen Phillips, an anthropologist who specializes in Egyptology, will describe how plants and flowers found in King Tut's tomb offer hints of what killed the boy pharaoh.

"We all know about the gold and the incredible treasures that were found there," Phillips said. "But there were also juniper berries and cardamom pods. No one thinks about that when they think of King Tut."

Another surprise awaits at Yards Brewery in Northern Liberties, where many sustainable practices are in place: The brewery is totally wind-powered; the used grain goes to a local farmer who pays with bison meat for the brewery's bar; the bar top is a reclaimed bowling alley.

Oh, and have a beer. Along with a brewery tour, Open House will offer a tasting.

"People who come here love beer, and they are excited to learn how we make it," said Zachary Artz, coordinator of the event, which begins at noon Saturday, May 18. "We like to link the processes they're seeing with the beer they're drinking."

Then, of course, there's the Wanamaker organ. Valued at $70 million, it has 29,000 hidden pipes ranging from 32 feet long to smaller than a pencil. One "blower room" is so loud that it sounds like a small plane about to take off.

And that's a real person playing it twice a day, six days a week. You'll find him - most likely Grand Court organist Peter Conte - at the console tucked in the rear of the women's clothing department. (Don't let the 65 percent-off signs distract you.)

When Mangel became curator 12 years ago, about 25 percent of the organ was functional. Now, he says, it's at 100 percent. He also has improved the store acoustics.

"Our ancestors never heard the organ like this," Mangel said.

Keeping the organ in shape is a lot of work. Mangel and another Macy's employee work exclusively on it. Two other full-timers - including J. Anthony Nichols, the organ's chief tuner/voice - hold positions funded by the nonprofit Friends of the Wanamaker Organ.

In advance of the Open House tour at 10:30 a.m. Wednesday, Nichols played a few notes for a visitor, then looked over the balcony at the shoppers pausing at the sound.

"It's amazing to watch people walk through the court, and if they hear something playing will stop and look up for 30 seconds and think, 'Wow, that's cool,' " he said. "Even if they don't like classical music or care about organ music or know about the organ, this instrument is of such fine quality that it always gets attention."