SAN DIEGO — The San Diego Symphony’s 1921 Robert-Morton Theatre Organ will be in the spotlight at the orchestra’s Friday and Saturday concerts, which will feature Weicheng Zhao performing Saint-Saëns’ Symphony No. 3 in C minor, Op. 78, Organ Symphony.

This will be the first concert to prominently feature the organ since the symphony’s downtown home, Jacobs Music Center, reopened in September following its $125 million renovation.

Organ curator Robert Knight has upgraded the organ and added new pipes for the two concerts. He provided some key facts and details about the historic instrument, which was built in 1921 and installed in San Diego’s Balboa Theatre in 1923. The instrument was moved in 1929 to the then-brand-new Fox Theatre, now known as Jacobs Music Center.

—The organ cost $50,000 when it was built 104 years ago, which would be more than $1 million today. It was rebuilt 16 years ago by Knight, who is the symphony’s organ curator.

—The rebuild took about eight months and a team of six people, led by Knight, to complete.

—The original organ mechanisms from the 1920s were made of hair sheep leather imported from England. They have been replaced with new hair sheep leather. “We actually got the leather from the same supplier in England,” Knight said.

—The organ’s console is made from Cuban mahogany. The keys are made of walrus ivory.

—Each of the organ’s four keyboards has 61 keys. There are 32 foot pedals and at least 200 stops, each of which controls a specific sound on the organ.

—The organ originally had 3,000 pipes. It now has 3,410 pipes and even more are being added between now and this weekend’s concerts. The organ originally had 32 ranks of pipes, with approximately 61 pipes per rank. One or two more ranks were added in 1929, and two or three more ranks in the 1960s. An additional eight ranks were added when the organ was overhauled in 2010. Between November and this week, 21 more ranks are being added.

—Most of the ranks are in the two organ chambers to each side of the proscenium and are mounted about one floor level above the stage.

—The echo organ, which contains six ranks of pipes, is at the rear of the hall’s top level. There is one rank of pipes in the angel walk contained in the ceiling level arches to the right and left of the stage.

—The organ bench is made of poplar and was made in the 1960s after the original bench disappeared.

—The organ console is equipped with seven lights that illuminate the four keyboards during performances.

—A drawer on the left side of the console contains a touch screen connected to a desktop computer that was installed in the back of the console in 2008 and provides an automatic playback function. The computer will be replaced with a newer model later this year. “Having the console run on Windows Seven really scares the hall’s supervisor,” Knight said. “But since we also have the touch screen, it doesn’t scare me.”

—The organ has a one-of-a-kind Moller brass post horn, which was added in 2008 and simulates the sound of a brass band.

—The organ’s percussion sounds include a snare drum, a bass drum, tom toms, a triangle, a celeste, two xylophones, orchestra bells, castanets and simulated horse hooves, which are all triggered by stops and keys on the console.

—The organ also includes a Chrysoglott, a unique theater organ voice that Knight describes as sounding “like a celeste on steroids.”

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