Marshall MB450H vs tone hum/feedback

[jeffbos]

Hi all,

I've have a Marshall MB450H bass head. I've noticed that when I turn the tone control all the way up (CW) on the bass, I get a lot of hum/feedback thru my 410 cabs and thru my headphones. Turn the tone down (CCW), hum stops. Here's what I've tried with no luck....

1. Moved only the head to another room and tried with the headphones. I tried various distances up to about 10'.

2. Tried J bass with hot stacks, J bass with single coils, T-Bird, and Victory bass.

3. Tried getting 20' away using the 410's

4. Tried the amp in all 3 channels in both locations

5. Tried different gain/voice/volume settings

Here's what gets rid of the hum....

1. Turn tone knob all the way CCW

2. Touch any of the strings (with the tone knob full CW)

3. I have a Fender B-DEC combo plugged into the same wall receptacle and do not have any hum/ feedback issues with it.

Any ideas would be greatly appreciated

[TheRealKong]

Is there another power circuit somewhere? Two rooms in one house does not mean automatically, that there is another circuit. Try it.

What kind of electrik light is installed in the room? Neon lamps? They often cause big humming problems. Or is there any stagelight installed? Same reason.

Or try changing phases by turning around the power plug in the socket for 180 °. If this does not work, change the cables You use - guitar cable, cab cable, patch cables, if You use one.

I use simply the same amp, but in the combo - version, as a MB 4410. Never had any humm in any situation - but with stagelights in the same electrik circuit.

Play bass!

[tom once dead]

Are you boosting much treble?
I used to get a little bit of buzzing if I had the treble up and the tweeter on.

[parapentep70]

I had an MB4410 combo (same head) and it was deadly quiet.

But I was using it with a good active bass providing a very high output. Perhaps too high. With this I used the active inut, which reduces sensitivity to outside noise entering the amplifier.

If you touch a string and the noise becomes noticeably quieter, then you have a problem with grounding.

1) Make sure you have your amp properly grounded.
2) Move away from any "discharge lamp". Neon lights and similar gas-filled tubes are specially noisy.
3) Try a different bass, then you can consider shielding yours. A noise that stops or decreases significantly touching the strings (usually grounded via the bridge) is a good candidate to improve a lot by proper shielding.

EDITED

The amp is good but in my experience (and others), internal connections are not the best. Mine (MB4410, the big combo version) came brand new with the tweeter unconnected (!!!!). And I heard comments about many people suffering from cold solder joints. So maybe there is something in your particular unit.

WHat I cannot understand is what tone control stops the hum. Is it a passive tone control in your bass? In which one? Or is it active? In the amp? In which channel?

[jeffbos]

All,

First, Thanks for the help.

Update. It appears to be a house grounding issue. I took the amp to my neighbor's home and there was no hum. I've tried four different circuits in the house with all the fans, florescent lights and AC unit off. No luck. Next, I'm going to check my E- box to earth ground wire & rod.

Any ideas on how to tell where the issue may be in the house?

Again, The hum only occurs with tone up full. The hum will only stop in this situation when you touch the strings; or turn the tone down.

[TheRealKong]

This is not an electricians clue, but maybe the powerchord of the amp is not soldered proper? Even the grounding chord? In Germany we talk about a "cold solfdering point", this means, it is soldered, but has no contact.

If it works in your neighbours house, it maybe a house grounding problem. I don't know how to fix a complete nhouse-Power-suply. This is an electricians job.

Only thing I can say - looking at my hi-fi poweramp which is grundet like this: Try to get a extra grounding by a extra chord, fixed on a blanc metal piece of your amp and a well-grounded place in your room/house. I don't know how it is in your place, I would try the radiator, which n(in Germany!) normaly is well groundet.

Or, and this is the most secure and sensefull clue, get your amp to service. It sounds like no big thing in this case. it won't be expensive. But this will be the most secure thing. You don't want an amp, that either maybe hum in several places or not.

Play Bass!

[greencow]

Connecting the PE of any device to your radiator is a bad idea on so many levels.