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Ace Frehley , the original lead guitarist for KISS, is hospitalized on life support, and the prognosis is not good ... TMZ has learned.

Sources familiar with Ace's situation tell us ... he suffered a brain bleed when he took a fall in his studio a couple weeks ago -- forcing him to cancel his upcoming tour dates -- but his health has not improved.

We're told he's been on a ventilator for some time, and hasn't gotten better -- so, his family is considering turning off support ... perhaps as soon as Thursday evening. Getty

While there are several social media posts indicating he's already passed ... we're told the 74-year-old guitar icon was still on the ventilator as of Thursday afternoon.

Following Frehley's fall and subsequent cancelations ..

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