Why is my bass amp humming?

That's not a question that can be answered over the internet. Maybe something went wrong inside it, or maybe it wasn't an amazing amp to start with... Mine does it too, though, and I think most amps hum a bit.

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Some hum on cheaper amps can be expected. If it hums when nothing is plugged into the amp, and it's louder than soft background noise when you aren't playing, most likely it's either an internal grounding issue or a filter capacitor needs to be replaced. You should take it in to have it repaired.

If it only hums when your guitar is plugged in, you have a grounding issue with your guitar or cables. Remember, single coil pickups like those found on a Fender Jazz bass are likely to pick up a lot of noise, which could cause hum.