GBE750 - Noise Issue

[ncbassman]

I got a used GBE750 last week. I demo'd it at a Guitar Center and gave it a pretty good work out. I didn't notice any loud hums/buzzes/noises then. (might of been my happiness of finally being able to afford a quality bass head, and the awesome tone that I was hearing that I didn't listen for any problems)

Since then, I have been hearing a noticeable noise from my speaker cabs when hooked up to the amp. I have heard it at home, at practice, and at a gig.

It makes the noise without anything plugged into the input. I have tried different outlets and a power conditioner. The noise is audible with the master volume completely down and/or the mute engaged. As you turn the master volume up, the noise gets louder. It makes no difference if it is on the tube or fet preamp channels either.

The noise is 'high frequency', I can dial back the high frequency knobs on the EQ and it gets a bit quieter, or I can turn them up, and the noise is louder, almost a scratching noise at that point.

The noise is more pronounced on cabinets with tweeters and horns. I have a single 18 cabinet, the noise is not nearly as bad on that cab as my Acme B2 or Mesa 2x10 cab (but it is still there).

I have tried both speaker speakon outputs as well as different speaker cables that I know work. I have also tried different power cables. The fuses on the amp are ok.

I have tried it in and out of the rack case thinking a ground to the case might be the issue. No luck.

If I hook everything up to it, the power and tone is there when I am playing the bass, just the noise is too I can plug my guitar amp into the same outlet, and it is fine, there is no noise like with my GB head.

Any ideas on anything else I could try before getting a tech to look at it? I hate buying something and having to put it in the shop almost as soon as I get it.

[KsPiNeSh]

Originally Posted by ncbassman

I got a used GBE750 last week. I demo'd it at a Guitar Center and gave it a pretty good work out. I didn't notice any loud hums/buzzes/noises then. (might of been my happiness of finally being able to afford a quality bass head, and the awesome tone that I was hearing that I didn't listen for any problems)

Since then, I have been hearing a noticeable noise from my speaker cabs when hooked up to the amp. I have heard it at home, at practice, and at a gig.

It makes the noise without anything plugged into the input. I have tried different outlets and a power conditioner. The noise is audible with the master volume completely down and/or the mute engaged. As you turn the master volume up, the noise gets louder. It makes no difference if it is on the tube or fet preamp channels either.

The noise is 'high frequency', I can dial back the high frequency knobs on the EQ and it gets a bit quieter, or I can turn them up, and the noise is louder, almost a scratching noise at that point.

The noise is more pronounced on cabinets with tweeters and horns. I have a single 18 cabinet, the noise is not nearly as bad on that cab as my Acme B2 or Mesa 2x10 cab (but it is still there).

I have tried both speaker speakon outputs as well as different speaker cables that I know work. I have also tried different power cables. The fuses on the amp are ok.

I have tried it in and out of the rack case thinking a ground to the case might be the issue. No luck.

If I hook everything up to it, the power and tone is there when I am playing the bass, just the noise is too I can plug my guitar amp into the same outlet, and it is fine, there is no noise like with my GB head.

Any ideas on anything else I could try before getting a tech to look at it? I hate buying something and having to put it in the shop almost as soon as I get it.

Try plugging a patch cord into your effects loops individually... sounds like it could be a normalling switch that freaked out.

[ncbassman]

Thanks for the suggestion. I tried all sort of combinations in the effects loops. The noise remained. I ordered an Isolation Transformer just to make certain it isn't any issue outside of the amp before I send it off to a tech.

Just really frustrating. I know the GBE750 is a superior head compared to my old Carvin BX1200, I just gotta get the issue diagnosed and get my tone dialed in.

[ncbassman]

ok. I ordered an Isolation Transformer to test out with my amp noise situation (Amazon.com: Tripp Lite IS250 250W Isolation Transformer 2 outlet 6ft Cord: Electronics)

It did nothing. My Carvin amp is also generating the same noise, albeit not as loud/pronounced. I attached an .mp3 clip of the 'noise' taken from my cell phone this morning. Halfway through the clip, I turn some EQ knobs, and you can hear the noise changing.

I know the cab and speaker cables I've been trying are OK. I have a mixer/power amp that I have hooked up with them and the noise is not there.

Anymore ideas?

[NWB]

I'm sure that if you e-mail either Agedhorse or Hasbeen from Genz-Benz that your problem will be taken care of.

Not only do you have a great amp, but it's from a company with legendary customer service.

[Medford Bassman]

Could it be your power source? i.e. house wiring. try it at another location.

[agedhorse]

A couple of questions here first...

There will always be a base noise floor with any amp. With the master volume turned completely down, and say an Uber cabinet with the tweeter control at 12:00, the noise will probably not be noticeable, even with good ears, 3 feet from the cabinet. This is a good way to establish the base noise floor of any amp.

Now, with the eq set flat, the shape filters off and the channel gain/volumes all the way down, bring the master volume up to the 12:00 position. There should be a small increase in noise, barely noticeable at 3' with the above cabinet.

Now, select the LF extend shape filter, and you may hear a very slight increase in the noise floor but probably not. The difference is barely perceptable.

Now turn off the LF extend and select the mid scoop filter. You should hear a modest increase in HF noise. This is normal becuse this specific filter uses gain to add a negative to a positive to generate this particular response curve.

Now, turn off the Mid Scoop filter and turn on the HF attack filter. YOu should again hear a modest increase in HF noise, the difference will be slightly higher than the Mid scoop (about 0.75dB) and may have a slightly different character. This is because the added HF gain is amplifying any noise prior to this stage (ie. summing bus, make-up gain, eq filter gains, etc.)

Now, select the FET channel and set the volume to 12:00 with the channel gain to 0 (fully CCW) and you should hear just a slight increase in noise above the initial baseline. Increase the input gain up to the 12:00 position and you should hear a smooth increase in the background noise floor, it should be audible at 3' from the test cabinet but will be hard to hear at 9' away.

Now select the tube channel and with the tube gain at 0, increase the channel volume to 12:00. The noise floor should be slightly higher than the same test on the FET channel under the same conditions. Now, bring the tube gain up and the noise floor should increase similar to the FET channel but there should be a bit more noise and there may be a slightly different character to it. There is more gain and noise contribution from the tube and tube channel. If you find that the tube channel is a lot noisier, it's possible that you have a tube that is failing.

Be sure to do all of this with nothing plugged into the amp.

Noise levels may increase with some active basses, as the noise floor of the active pickups is amplified by the bass amp and the noise floor can never be lower than the noise floor of the source.

You can always PM me with questions.



If you find the noise on the tube channel to be a lot higher