Jan 22 - Standard & Poor's Ratings Services today assigned Dallas-based Tenet Healthcare Corp.'s proposed $850 million senior secured notes due 2021 its 'B+' issue-level rating (one notch above the 'B' corporate credit rating on the company). We also assigned the notes a recovery rating of '2', indicating our expectation for substantial (70% to 90%) recovery for lenders in the event of a payment default. The company plans to use the proceeds to refinance existing debt. The corporate credit rati