I've been searching the net for some help and landed here after reading some technical info at GuitarNuts. Thanks for offering the forum.
Very long story made as short as possible. I got a new amp. When it's powered up, there's no ground loop hum until I make the final connectiong into the bass. Then I got hum which disappears when I touch the strings. (Actually, it only hums when connected to three of my four basses...of course, that's the one bass that I play the least.) Anyway...
I didn't have a ground loop problem with my old amp. Just this new one. The three-way (+, -, zero) "Ground Polarity Switch" on the back panel is supposed to be in the zero/neutral position but there's less hum when it's in the + position.
I don't know how three basses can suddenly become ungrounded...and I don't know how to correct the problem. I'm grateful for any help. Thanks.
The fact that it's a hum that goes away with touching the strings makes me think it's more of a shielding problem, with the basses picking up hum, rather than a ground loop issue. Fiddling with your grounding switch (which to be honest, I'm not familiar with), may just be making some compenating negative hum that cancells it a bit.
Your strings are probably grounded, so touching them grounds you, and your body then acts as a shield behind the guitar. Do you want to try a test?
Place the bass on a bench, with a sheet of kitchen foil behind it, plug it in, turn it up, but don't touch it. Does it hum a bit? - probably yes.
Then connect an aligator clip (or some other wire) from the kitchen foil to the outer barrel of the jack plug, or to the bridge. That will ground the 'screen', and maybe reduce hum. If it does, then if you go to the original guitar nuts site (see link at top of this page), you can read about shielding ('Quieting the Beast').
As to why your new amp is different, it could be it has a different input impedance. Higher values could be more sensitive to some types of hum.
All the above is guesswork, but worth testing for.
I did the test exactly as you described. During the first part of the test, I turned up both gain and volume. The hum actually got louder and louder and scary loud. So, I turned it all down clipped the foil to the bridge and ... dead silent ... no hum whatsoever. It sounded so good for a change...just like it should. Wow!! What a load of stress relief. Thank you, thank you, thank you.
Both amps have the three prong plug. Same room, different outlet...but I tried going back to the old outlet (as well as several other outlets in the house) with no difference in hum.
The new amp def has more power. I went from a 50 watt at 4 ohms to a 450. The 450 watts is at 2 ohms but I only have an 8 ohm speaker, so I think the watts is about 250.
I just ran the shielding test on all four basses. On the two major culprits (japanese fenders), the lack of hum was amazing when connecting to the foil...just awesome for the P, and better but not awesome for the Jazz. On the third and fourth (mexican fenders, one with an anodized pg), the hum wasn't too bad to start with...at least not as bad as the others...the foil connection made a diff too ... just not as dramatic.
Not sure what all this means...but I'll check out Quieting the Beast in a few minutes.
It's funny how the basics can easily escape me at times. I was so mindful of the new amp/cab that I was sure it had to be the culprit. If shielding is the first rule on this forum then it's a good one, for sure.
Yes, the 50 to 450 will really humm...and I'm looking forward to a 'quieter' increase in power...I'm sure you know what I mean.