Double Bass GK 200MB hums when turned off

[Steve Freides]

I started noticing a hum in the house a couple of days ago. We couldn't figure out where it was coming from - I thought it was a construction generator next door or maybe an upstairs air-conditioner vibrating through the beams in the house, but it turned out to be my GK 200 MB.

When you turn the amp on, the little red light comes on and it works just fine, but when you turn it off, even though the red light goes out, it hums. The only way to kill the hum is to unplug it.

Any idea, short of a gremlin inside my amp, about what's going on here?

Thanks in advance.

-S-

 

[mbelue]

Sounds like your outlet is wired wrong, do you have a tester to check it with?
Link Removed

 

[tornadobass]

I had one several years ago that was the same. There's a transformer in the cab that stays connected to power but doesn't send it to the amp until you hit the ON switch.

...just to be clear, this is the older GK combo, not the current MB200.

 

[Jackson171]

I've had one of those amps since I bought it new over 20 years ago. It has always hummed when plugged in, but turned off. I just keep it un-plugged.

 

[Steve Freides]

Funny but I never noticed the hum before. I now have it connected to an external speaker that's more efficient - maybe that's why we're hearing it for the first time.

Bummer - for one, I don't want to be wasting electricity when I'm not using it. It's my main amp for my students, which means it gets turned on and off several times a day, and the plug is not in a convenient-to-reach spot, either. I guess I'll use a power strip and switch that on and off.

-S-

 

[dannylectro]

Curious... (I'm an electrician by day). It does sound plausible that the polarity on the actual receptacle outlet might be wrong, so that when you turn the power off with the switch, the transformer is still "live" and maybe leaking to ground. Transformers do occasionally "sing" when connected to power.
I would get one of those plug-in testers, mentioned by mbelue, and make sure the hot & neutral at the plug aren't reversed (a not uncommon situation!), and that you have a sound ground.

 

[Steve Freides]

I do have the tester gadget - will look for it here in the house, try it in this outlet, and report back. Might be the weekend before I find the tester gadget, though.

-S-

 

[Earl]

Mine has always done the same thing.

 

[B String]

Mine too. I sent it in to GK. Whatever they did made it much better, but still just slightly there.

 

[robgrow]

The old GK 200MB had the power transformer mounted in the speaker cabinet in back, near the bottom. The power switch in that amp did not shut off current to the primary winding of the transformer, so some current always flowed. There is something called hysteresis loss that causes the transformer to draw a little current even at idle, which also causes the transformer to hum or vibrate a bit. This hum is made more noticeable by the transformer being mounted on a large metal panel, which acts like a sounding board.

It's probably nothing to worry about, and the hum should only be noticeable under very quiet conditions. I've owned a few of these amps, and they all did this to some extent. You could try tightening the four panhead phillips screws that are used to mount the transformer. Beyond that, you probably should consult a qualified technician.

BTW this has nothing to do with how the outlet is wired.

 

[winston]

Good explanation, robgrow.
I've had 2 of the old 200MB combos, both always hummed when powered down/plugged in.

 

[Steve Freides]

Mine never used to hum, just started maybe a week ago.

-S-

 

[CosmicRay]

Mine never used to hum, just started maybe a week ago.

-S-

It would be more weird if it whistled instead of hummed....

 

[gerry grable]

It is "normal" for this amp. They all do it. It is annoying , but you just have to ignore it or unplug it. Power strip is best. Robgrow has it right: the primary. Steve-- it didn't just start, you just hadn't noticed it.
Good amp, dumb design. Go figure

 

[Joacocool]

I think that it has something to do with the grounding of the outlet. My 700rb hums when I plug it in my drummers house, but it doesn't when i plug it at home. I did some measures and I found that the grounding was bad at his home...

 

[gerry grable]

I think that it has something to do with the grounding of the outlet. My 700rb hums when I plug it in my drummers house, but it doesn't when i plug it at home. I did some measures and I found that the grounding was bad at his home...

I don't know about the 700rb, but all of the old GK 200MBs hum. They are built that way. It's just an annoying quirk you have to live with.
Just to be clear, I'm talking about the small, solid-state amp with a self-enclosed 12-inch speaker, about a 14 x 13 x 7 inch unit. Not the little box that I've seen pics of.

 

[tgorycki]

I saw on a forum somewhere that someone used felt washers to isolate the transformer, effectively reducing vibration to the back panel.

 

[bssist]

I do not have one of those amps. It does sound like a poor design if the primary is always hot. I would change the switch to break the incoming power instead of the secondary. If you don't know what I'm suggesting have someone who does (like dannylectro) make the change for you. It costs nothing, fixes the issue, and saves $$ on electric bill.

 

[Ric Vice]

?.........
Just to be clear, I'm talking about the small, solid-state amp with a self-enclosed 12-inch speaker, about a 14 x 13 x 7 inch unit. Not the little box that I've seen pics of.

I owned both versions of the 200MB. I do remember that hum, especially in certain rooms I used to gig in. I recently purchased a 200RCB, the head only version of the 200MB. Because they had to move the transformer up into a single case with th Pre Amp and Power Section, the black case it's housed in is pretty large, especially when compared to the little sliver MB 200, which is a totally different amplifier. The 200MB/RCB preamp section had one of the finest voicings ever for Double Bass, IMHO.

Ric

 

[tgorycki]

I do not have one of those amps. It does sound like a poor design if the primary is always hot. I would change the switch to break the incoming power instead of the secondary. If you don't know what I'm suggesting have someone who does (like dannylectro) make the change for you. It costs nothing, fixes the issue, and saves $$ on electric bill.

Excellent idea! Anyone thinking of doing this should keep in mind the stock power switch is likely rated for 12 volts or a similarly low voltage. Modifying the wiring to make the switch break the incoming wall current would require using a switch rated for at least 120 volts.