Sudden onset of unusably high noise floor - CMV563/N57 supply

[espen askelad]

Mid session, the noise floor on my CMV563 escalated from typical East German noisy to truly unusable. The tone of the mic appears unchanged, so far as I can tell under the noise (can't be certain and the noise level is extreme and overwhelms a significant portion of the signal.)
I have only owned the mic since last year, and am unaware of service history for mic or power supply. There were no problems leading up to the noise increase which happened fairly quickly. Power supply is an N57 w/ an external voltage converter.
Suggestions on where to start the search? I've ordered a new tube, though past experiences with failing tubes have been accompanied by ringing and/or increased distortion.