[Beginner] Buzzing/Humming tone knob

Hello everyone,
I have a Harley Benton PJ that I recently purchased to learn how to maintain the bass. Being the instrument inexpensive, I figured that if I break something, it wont be too big a deal.
I just changed the strings and on a Rumble 25 it sounds good enough with either/both pickups. However, when I turn on the tone knob, the amp starts to “fry” and it also pops when I put my fingers on the strings and release. This occurs regardless of the pickup volume and it seems to be dependent on the tone control.
What can be the cause of it and what could I do about that?
Thank you in advance!

Can you describe this a bit more? Is it just a scratchy sound that comes through the amp when you turn it?

If so, this is relatively normal, or at least only uncommon, not super rare. It’s dirt/dust in the tone knob potentiometer. This is easy to clean - you can buy contact cleaner to do it, and sometimes it even can be fixed without it. Try briskly turning the knob right and left all the way 50-100 times and see if it helps.


Hi Howard, long time!
The noise I hear is like a buzzing sound that increases in volume the more I turn the tone knob up. Then, when I touch the strings, I hear a popping sound upon moving away from the strings. When the tone knob is turned all down, the amp is clean and there’s no buzzing, so I think it’s not a problem with the amp or cable.


Shortcut on mass cable?

Hello Chris,
Can you please explain this?

If you hear a noise when touching the strings, it must be because of the mass cable (which is connected under the bridge).
So my guess is that the mass cable somewhere “touches” another cable. It might be just slightly - and that would explain the buzzing too…

But I would believe anything @howard has to say: he practically invented electronics :slight_smile:

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NO. I am actually a relative noob, just two years electronics shop in High School and a bit more in college. No pro experience at all.

@terb knows much more and there are actually professional EE’s here. I am just a hobbyist.

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Sounds like a cable or jack issues. Check both jack if it’s tightened and cable. Try changing the cable to see if it goes away.

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Do you mean ‘ground wire’? when you say mass cable.

Ah, yep, ground. It’s mass in my language…

OK just so we are clear. In english, wire normally refers to a single conductor and cable is usually a group of conductors. That’s just a general rule not absolute.

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Barney, do you think my problem could be due to a wire without continuity somewhere? I just ordered a multimeter from Amazon to perform some tests.

You’re in Italy right @Rob150 ?

I don’t know any brands over there but the first thing I’d do is as @howard suggested and use a contact cleaner to clean the tone potentiometer.

A contact cleaner is safe to use on electrical items because it evaporates quickly.

You can buy them at automotive stores as mechanics use them.


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I may have got this wrong but I think @Rob150 was suggesting that the noise persists as long as the tone knob is turned up and not just when it’s being turned. I would have thought the dirty pot contacts would have only caused noise when being turned and not when left alone. I may have misunderstood.

And of course it would have nothing to do with the noise when removing his hand from the strings.


That is why I assume an issue with the ground wire that is connected to the bridge!?


That’s correct. The noise gets louder and persists when I turn up the tone knob. With it all the way down no obvious noise or humming is heard through the amp regardless of which combination of pickup volumes I have.

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I’m not super well versed in electronics, but I’ll test for continuity as soon as Santa Amazon delivers the multimeter :upside_down_face:

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In my case, I discovered that the ungrounded socket connected to my pedals and/or amp caused a hum. Using a well-insulated power adapter resolved this issue. You might want to check if your amp is grounded and see if switching to a different power source in your house helps.

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Sure thing, thank you. Some amps have the ground lift switch but mine doesn’t (It’s a Rumble 25).

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I did just that and in fact the noise has reduced drastically. Now I can only hear a hi-freq humming.

However, I have another concern.

As a beginner I haven’t trained my ear to low frequencies long enough to surely know what’s going on, but it seems to me that the change in tone isn’t that obvious when I turn the knob up (all the settings on the amp are in the middle). I’m not sure if it’s a problem with the tone control itself.

Sure enough, I’ve already bought new pickups, and maybe the rewiring and some further shielding might also help.