John Rigas, who turned a $300 investment a half-century ago into cable behemoth Adelphia Communications, was sentenced to 15 years in prison yesterday for his role in the looting and debt-hiding scandal that pummeled the company into bankruptcy.

Rigas’s son Timothy, 49, who like his father was convicted last year of bank fraud, securities fraud, and conspiracy, was sentenced to 20 years in prison. U.S. District Judge Leonard Sand could have sentenced both men to life.

The sentences are among the harshest handed down in an American court since the fall of Enron in 2001 touched off a rash of corporate scandals that rocked the markets and have cost investors billions of dollars. Raising the possibility that the frail, 80-year-old Rigas could die behind bars, Judge Sand said the sentence might be cut short if Rigas serves at least two years and prison doctors believe he has less than three months to live.

“This is a tragedy lacking in heroes,” the judge said.

Prosecutors had accused the Rigases of using complicated cash-management systems to spread money around to various family-owned entities and as a cover for stealing about $100 million for themselves.

They were accused of spending the money on a lengthy list of personal luxuries. Prosecutors said John Rigas had ordered two Christmas trees flown to New York for his daughter at a cost of $6,000, ordered as many as 17 company cars, and had the company buy 3,600 acres of timberland to preserve the view outside his Pennsylvania home.

Worse still for investors, the company collapsed into bankruptcy in 2002 after it disclosed a staggering $2.3 billion in off-balance-sheet debt that prosecutors said was deliberately hid by the Rigases.

“Our intentions were good. The results were not,” Timothy Rigas told the judge.

Adelphia, founded by John Rigas in tiny Coudersport, Pa., and the lifeblood of that town for 50 years, now operates under bankruptcy protection in Greenwood Village, Colo.

Judge Sand declined to force the two men to pay restitution, noting the family has already agreed to forfeit more than $1.5 billion to settle regulatory charges.

At the most dramatic moment of a hearing that stretched nearly three hours, John Rigas slowly rose from his chair just before being sentenced, shuffled to a lectern and addressed the judge, speaking slowly and softly.

“In my heart and in my conscience, I’ll go to my grave really and truly believing that I did nothing but try to improve the conditions of my employees,” he said.

He said repeatedly he had led a blessed life, and even thanked members of the military “that fought for America and gave their lives because they believed in America and what it stood for.”

“If I did anything wrong, I apologize,” he said.

Just after he was sentenced, the elder Rigas, hunched forward in his seat, held his right hand over his mouth and dabbed at his eyes and nose with a white tissue.

The judge, while expressing concern for Rigas’s age and poor health, made repeated reference to the investors who had placed their trust in the Rigas family, many losing their retirement security.