Preamp repeatedly blows fuses

I've got an Antique Sound USA PA-5 preamp from 1997 that keeps blowing fuses. I originally found the problem last night when I walked away from my chair and came back to find the preamp dead.

I immediately pulled the fuse and it was an "F3A250V" quick blow. I replaced it this afternoon and as soon as I turned on the preamp, it blew the new fuse. No fireworks, no pops, just a dead preamp.

I don't know where to go next, other than to call my local tube amp repair shop.

Any thoughts?
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I had the same experience in my Rotel 2-channel amplifier. It blew some rail fuses and I had it in to ListenUp for repair. The service tech called me and said he went through and tightened down all the connections, replaced the fuses, etc.; said after a repair like this he doesn't see these amps again. Well it blew another rail fuse in about 5 weeks. I changed two things: replaced the quick-blows with slow-blows, per a Rotel document I found on the internet that listed recommended replacement fuses; and I stopped using the 12v trigger to turn the amp on and off. Its been running without problems since, which is about 8 months.
If the unit was suppose to have quick blow fuses in it then slow blow may not protect it; I would try to find out which it had originally. If it was quick blow and the unit still blows them then I would get it checked. Just because it works with slow blow does not mean that it is safe with them, I know people who remove the fuses entirely, something that I would not do.
I assume the fuse you replaced was on the outside of the unit.
That would be the line fuse. Open the unit and see if there is an internal fuse maybe mounted on a circuit board. If yes check if it has blown and if it has replace. If it blows again you can contact Divergent Technologies for help. They are ASL reps and helped me troubleshoot my Hurricanes
Th fuse I replaced was the line fuse. after a thorough inspection, I couldn't locate any other fuses on the unit. Keep in mind, this is an ASUSA product, not an Antique Sound Labs product. If memory serves, ASUSA was bought out by ASL. It's a pretty complex design...everything is through-hole and point to point with not much spare room inside the chassis. I did the old "sniff" test and smelled nothing "burny" inside. I also tried my best to find a bare wire shorted to the chassis, or a tube socket...but I just couldn't. I'm going to call Music Technology here in VA, they are about 15 minutes south of me and I'm sure they can help.

Keep the suggestions coming, though :)

Just asking, but did you disconnect all inputs and outputs? This way you will be sure that another item connected to your preamp is not causing the fuse to blow.
Maybe you have a shorted interconnect.
I did disconnect everything, and isolated the preamp on a different outlet in the apartment.

Luckily I dropped it off at Music Technology this afternoon and Bill set me straight. Hopefully in a week or so I'll have it back and better than ever!