Noise with extension cab

[benson]

I've just bought a 1x12 Mark V cabinet to go with my Mark V combo. I've connected the internal combo speaker to a 4ohm output and connected the extension cab to the other 4ohm output but although both speakers are working I'm getting a constant low end hum. I've got no effects connected at the time. Am I doing something stupid here?

 

[MrMarkIII]

Are you using speaker cable, not shielded instrument (guitar) cable?

 

[benson]

It was a cable the store threw in for free and not my usual Planet Waves cables. I'll change it out and try again. I'll be both embarrassed and pleased if it is!

 

[benson]

Tried a known cable and the problem persists. However, the hum is also there without the extension cab attached but I either didn't notice it or it started when I moved the amp to fit the cab underneath it. I'll take another look tomorrow.

 

[Mork]

My V hums, Lonestar is dead quiet but the V definitely has a very loud hum....(and it's not the fan!)