If your guitar amplifier emits a low, steady hum, it could be caused by lighting in the room. Fluorescent lighting can cause a hum in your amp, which can be a problem if you typically play your guitar in a room with this style of lighting. First, switch off the lights to determine if they're causing the problem. If so, consider adding alternative sources of lighting to the room, at least while you're playing.
The closer you place your guitar to your amplifier, the more you'll notice a steady hum emanating from the speaker. Virtually any amplifier will create a feedback sound when you put your guitar near it; numerous musicians, such as Jimi Hendrix, incorporated this feedback sound into their music. To avoid this hum, play your guitar as far away from your amp as possible. If necessary, obtain a longer guitar cable.
To diagnose a loose connection causing a humming sound, wiggle your guitar cable at either end. It connects to both your guitar and the amplifier through jacks, which can become loose over time. If the humming worsens, or begins to crackle as you wiggle the cable, this is the likely source of the problem. Tighten both jacks until they're snug using a socket.
There's always the possibility that the humming in your amplifier is due to a wiring problem in the amp itself or perhaps even in your home. Try using the amplifier in different rooms of your house to determine if the problem persists. If it does, take it to a guitar shop and explain the problem. The amp may need to be repaired or have a faulty tube replaced.