#1
Hi!

I have a problem with my amp. Amp model is Ampeg BA-112. And the guitar is Fender 60 Classic Jazz Bass (Mexico). So the problem is when I turn up neck pickup it starts to hum. What could happen to be a problem?

Thanks!
#2
How bad is the "hum". Because I suspect your amp is fine but the root cause is the Jazz bass. When the pickups on some jazz models aren't set to similar levels, you tend to get a hum, especially in the neck pickup.

The best recourse is to swap out the factory pickups for hum-cancelling pickups. Aguilar, Seymour Duncan, and Fralin amongst others make hum cancelling pickups for Jazz basses.
#3
The problem is probably just the nature of the pickups. That's just what happens with single coil pickups. If it's bothersome, you could try looking for some noiseless pickups.
Gear:

ESP EC-50
ESP FB-204

MXR '78 Custom Badass Distortion
Dunlop Crybaby
MXR EVH Phase 90
MXR Analog Chorus

"Music is the strongest form of magic." - Marilyn Manson