Fuse keeps blowing in 5150

So I bought a brand new 5150 3 EL34 50w model later last summer, used it very little, but now am looking to sell it as I just kind of lost interest in music. The problem is I blew a fuse in this amp a while back, and I think it was because the cable I was using was bad. I immediately took out the fuse and went down to my local hardware store and bought a new one. I showed them the fuse that came out of it and they got me a matching replacement. Took that one back home, installed it, and that one blew almost instantly too (using a good chord this time). I then took it to a music store to try to sell after replacing the tube again. They took it and plugged up to it, and the fuse blew again instantly. If you replace the fuse, it’ll come on for a second, then the fuse’ll blow, and it’ll shut off.

What is wrong with this amp?

I'd start with all the power tubes.

You’ve got a bad power tube. Grab a new set and make sure you rebias the amp which sounds like you might need a tech to do for you.

Yep, what ^ they said. Bad tubes. New tubes and a bias.

If you blow a fuse after you've replaced it once, stop replacing it. It's trying to tell you something and the fuse ain't the problem - the blown fuse is saving more expensive components in your amp. This is almost always a power tube problem. But a bad power tube can also take out the whole socket, so you need to check that too before just replacing the tube(s).

put a new fuse in and pull the power tubes out and turn on, if it blows again not the tubes. If it fuse doesnt blow get a set of power tubes which is likely your problem anyway.

Make sure its a slow blow, not fast blow. That'll get you every time. Made that mistake before.

Rick has it right. One bad fuse can be a bad fuse. More than that, something is wrong. Fuses are in place to protect.

Also likely the power tubes. Replace and rebias and I have a feeling that it will be up and running.

It could also be one of the transformers. If it powers on, and the fuse blows when you take it off standby its either power tubes or a bad OT. If it lights up for a second and the fuse blows just turning it on it could be the PT. I had a Ceriatone Cali mod where the OT went...no fun at all. Thing was a year old.