tax withholding penalty question.

In 2006, I withhold minimum tax on my two jobs, I used excess money to pay down my student loan and leftovers invested in float-rate-income closed-end fund w/ 15% yield. As a result, I will be owing a lot of tax, but I can easily close the fund to repay back the IRS. The question is will I also be penaltied for not withholding enough tax? (I purposely did that since I can use the $$$ to generate cash, 15% yield + 7.75% interest saving on student loan, instead of paying uncle sam w/ no interest.) But if a penalty is involved, then it is a different story.