Immediately blowing the fuse is usually due to a fault in the power supply of the amp. I would first suspect the rectifier tube (I think it's a GZ34, but check the schematic available on this site first)... and the second most likely would be a shorted electrolytic filter capacitor.
If replacing the rectifier tube still blows fuses, then I would replace all of the power supply electrolytics.
Hope this helps,
Joe
