Why would a pre amp blow fuses?

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.

The piece is an anthem pre 1. When a fresh fuse is installed and then plugged in but not turned on the tubes in the preamp glow fine, but when turned on the tube in the power supply starts to glow then when about to click on due to the soft start, POP! the tube in the power supply flickers and every thing is out. this all happened last month and I took it to a local shop they told me that my transformer was toast and the whole thing was junk. So not quite believing them I purchased another tube for power supply, replaced it and every thing was fine for a month. it wound turn on and off fine play loud fine
no sign of problem at all. all of the sudden after a month it all starts again, POP! I'm pretty sure I could get another tube and it would play but, what could I do to insure its definite survival???????Did I mention I sounds great great great!!!when it works :(

this all happened last month and I took it to a local shop they told me that my transformer was toast and the whole thing was junk.

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The transformer is a suspect. In THEORY if some of the windings started to short that would lower the resistance and raise the current level. Its possible that the humming maybe the windings vibrating - wearing away the insulation (enamel). Thats not always the case with a hum, its more of the exception.