Problems with blown fuses on Conrad CAV 50 ?

I am experiencing a recurring issue of the F1 fuse (4 amp, slow blow) blowing about once every three weeks.
I use the amp about 1 to 2 hours per day on average. During operation it works beautifully, but I am keeping Circuit City in business by buying their fuse inventory on a regular basis. I noticed that although the replacement fuse from Circuit City is of the correct rating, its design sports a droplet design (if there is such a qualifier) as opposed to the coiled wire design of the original fuse. Could that be the issue ? Where could I find such a fuse ?
Thank you in advance for any advice.