(Updates with comments from Rigas lawyer)

By Emily Chasan

NEW YORK, June 25 (Reuters) - A U.S. judge reduced the prison sentences of Adelphia founder John Rigas and his son Timothy after an appeals court threw out one count of their convictions.

The prison sentence of John Rigas, 83, was reduced to 12 years from 15 years, and the sentence of Timothy Rigas, 52, who served as the former chief financial officer at Adelphia, was reduced to 17 years from 20 years, prosecutors said, citing an opinion from U.S. District Judge Leonard Sand.

The Rigases plan to appeal the ruling immediately, saying their remaining sentences are still too long, according to their attorney.

The pair were convicted by a jury in 2004 of concealing loans and stealing millions from the cable operator. They are serving their prison terms at a low-security federal corrections facility in Butner, North Carolina.

The Rigases were resentenced after a U.S. appeals court in New York had reversed their conviction on one lesser count of bank fraud in May 2007, citing insufficient evidence. The court had upheld the pair's convictions on 22 of 23 counts of conspiracy and securities and bank fraud.

Judge Sand noted in the opinion that "a minimal adjustment is appropriate" to their sentences, given the reversal. Sand also reduced the $2,300 special assessments the Rigases were ordered to pay by $100 each, and ordered all other terms and conditions of their original sentences remain unchanged, prosecutors said.

"The sentence was, in my view, legally erroneous for a number of reasons," Lawrence McMichael, who represents the Rigases told Reuters. "We respectfully disagree with Judge Sand's assessment of the law and the facts and we'll appeal." McMichael said he felt a more reasonable sentence would have been one to four years. The Rigases have ten days to file their appeal.