Is the hum through the speakers, if so, both speakers? If it is through the speakers, but only one speaker, try changing the tubes from left to right. If the hum changes speakers, you know it is a bad tube. If the hum doesn't change speakers may be a bad unit. But one last test, gently tap the tubes, and if you get a loud pop it may just be microphonic tubes.
The above testing should just take a couple of minutes. If it doesn't help, be sure to send it in for a replacement.