Many months ago I posted about a humming noise coming from my amp here: http://www.talkbass.com/forum/showthread.php?t=479692 Well, since then most of our gigs have been outdoors so noise hasn't been an issue, not to mention this has never happened at home! The last gig the noise was there, but it was within toleraable levels-and I was standing about 10 feet from my amp. But tonights gig, well . It was so bad that I ended up turning my amp volume *off* and just running through PA. Suituation: I'm running an Ampeg B2R head, rack mounted along with a power conditioner, into an Ampeg 410 cabinet. We then take a line out to the board and also run me through the PA (Yorkville tops and subs). I use my amp as a monitor basically. The noise is exactly like a 60 cycle hum. There's also a hiss/static noise, but that's been there as long as I can remember. This hum is *horrible*. It varied in intensity. One thing that it seemed to depend on was on how high I put the treble on the bass itself (high treble=worse hum). We tried adjusting gain versus master, the higher my volume the louder and more intense the noise. Now,the noise did come and go, it was not a constant noise. It varied depending on where I was standing, which way I was facing, etc. It also would build in intensity, and then if I moved a touch it'd change how loud. (pitch of it stayed the same). For the show I was about 4 feet from amp, when I was walking around trying to find a noiseless spot I got as far as 10 feet with no steady luck. The fun part about this is it has thus far never done the noise at home!! I've tried and tried to replicate it and no go.... After the show, the guitar player and I tried out a few things to see if we could find the source. We turned off the PA equipment (two tops, two subs, one monitor, and the mixer) one piece at a time with absolutely no effect. Once we were down to just my amp on, we physically unplugged everything in the line, until it was just my amp plugged in. Noise was still there. We switched cables out, three different ones. Mine are custom so just in case we put his planet waves one in -the noise doubled in intensity! I had just changed the basses' battery, wasn't that. I went out and grabbed my backup bass, thinking it was the bass. My main bass is an active Ibanez Soundgear, the backup is a passive Peavey. With the passive bass, as long as the treble was down I could turn the amp up to stage volume, the noise only showed at top level. However, turn up the treble and OW. We then tried plugging an acoustic guitar into the head, there was no noise at all. We check ground (we carry a ground tester), and ground was fine. Power conditioner showed at 116-120 the whole time. What would make a head/cabinet do an intermittant 60 cycle hum noise like this? How can I find which part is the culprit if it only does it at gigs? I do have the subwoofers here and was toying of running through them instead of the cabinet until I remember it doesn't do the noise at home . And if somehow *both* my basses are shielded poorly, how on earth do you check shielding?