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help needed

First I would connect the bass directly to the amp. Does it pulse? If so then it is either the amp or the bass. If not then it is probably the A/B switch.

If it did pulse above connect a different bass to the amp. Does it pulse? If so then it is the amp, if not then it is the bass.

If it is the bass or the A/B switch then the best place for the moderator to have moved this thread would have been Pickups and Electronics rather then Amps.

Of course the possibility does remain that the pulsing will not occur with any of the three devices in your chain when they are used with other devices. In other words the pulsing may only occur when two of the above or all three of the above are connected together. In that case the simple troubleshooting tree above will not lead you to the problem and we would have to give you a more complex one. To start I would try the steps outlined above but before I did that I would put fresh batteries in the bass and the A/B switch to see if that fixes the problem.

Good luck, I don't hang out in the Amps section much and I only noticed this thread because there is still a link to it in Basses so I might not remember to check in to see how you are doing unless the moderators move this to Pickups and Electronics. I'm sure the good folks in the Amps section will be able to help you out though.
 
Can you borrow another bass (or guitar) and amp to mix and match to isolate the problem piece?

If not, a music store probably let you plug in to some of their stuff.

If the bass is active, make sure it has a fresh battery, although that usually shows up as static or weak sound or cutting out rather than a pulsing sound.