Why would a pre amp blow fuses?

Short circuit. In a tube or a capacitor. Maybe even a loose wire.

why would it decide to work for a month then blow, then work for another month then blow again? after changing tubes out for brand new ones? if there is a short it should all ways short out and blow. could my transformer be going out? it makes a slight hum.

You have of course checked it's nothing in the mains?

Fuses blow most of the time due to an over current condition. You may have a cap or maybe a diode thats breaking down. Does the fuse blow upon powering up or after long times of being powered up.
UNDER NO circumstances replace the fuse with a higher value. If you do you will find out whats causing the problem most likely after causing considerable damage not to mention a fire hazard.