Bushy passed away Sunday morning at UCLA Medical Center in Los Angeles -- this according to his family members ... several of whom were by his side, including his wife and daughters. We're told he'd been battling some form of cancer.

The guy was the only member of IB to have appeared on all six of their albums, and is perhaps best known for his iconic drum solo on their 1968 hit, "In-A-Gadda-Da-Vida."

The track is legendary for a number of reasons -- not the least of which because it's more than 17 minutes long ... and Bushy completely shreds on the drums for a good 2-3 minutes.

RB went on to help record their second, third and fourth albums into the '70s -- and then took a break until the gang got back together in '74 for albums no. 5 and 6. They continued jamming into the '80s -- when a second breakup occurred -- but they rekindled once more.

There were an infamous amount of lineup changes for IB through the '70s and '80s -- but the one constant mostly throughout was Bushy ... who was usually around to keep the tempo and the beat going, through studio sessions or on tour.

He played with other bands too during his tenure with Iron Butterfly -- including groups like Magic and Gold. Ron hung up full-time playing in the mid-2010s due to health concerns, but word is ... he'd make guest appearances with the new iteration of the band in the years after.

He's survived by his wife, Nancy, their three daughters and six grandchildren. Ron was 79.