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Gallien Krueger 200MB combo hum

Melik

Melik

Jan 3, 2009
628
45
Helsinki, Finland
#1
Hey everyone! I've tried to look for info on this topic on various platforms, including Talkbass, yet I haven't found a definitive answer to my exact problem, so I thought some of you could help. I recently acquired a GK 200MB (the combo amp from the 80s, predecessor of 150MB) which has this hum, even when it's in off position. So the hum isn't distractingly loud or anything as it stays the same when you turn the amp on, but for some time I thought that there's some sort of fault in the power section and the amp is drawing current even when it's off. I contacted GK and they told me that it's probably got something to do with the line out section, which is grounded, thus creating an internal ground loop. Do you guys have an idea if this is something that can be treated, or if it even needs to be cured? I'm worried that if I leave the amp connected to a wall socket, it's drawing power thus creating heat, thus creating a potential risk of fire. My local go-to amps repair person told that they don't know how to fix these old GK amps (understandably, since I've heard it's a notoriously finicky build).
 
#2
That’s an odd enough problem that I would recommend finding an older, EXPERIENCED, QUALIFIED GK service tech to diagnose what’s going on.

I emphasized experienced and qualified because I’m seeing the bad results from an increasing number of techs that are seriously under-qualified.
 
#3
The MB200 combo has the curious feature that the power switch is after the transformer. The transformer is always powered, always vibrating a bit, and is mounted in the speaker cabinet, so it's transferring its vibration to the cabinet walls.

It's not a super complex design, but getting it apart is tricky.
 
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#4
fdeck said:
The MB200 combo has the curious feature that the power switch is after the transformer. The transformer is always powered, always vibrating a bit, and is mounted in the speaker cabinet, so it's transferring its vibration to the cabinet walls.

It's not a super complex design, but getting it apart is tricky.
Click to expand...
That is correct, I just sold my 2nd ‘80s 200MB last Friday to a local TB’er who loves these. The one I got new in ‘90 did this too. Odd design decision, but not indicative of damage.
 
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#5
fdeck said:
The MB200 combo has the curious feature that the power switch is after the transformer. The transformer is always powered, always vibrating a bit, and is mounted in the speaker cabinet, so it's transferring its vibration to the cabinet walls.

It's not a super complex design, but getting it apart is tricky.
Click to expand...
Wow, that's interesting! Do you know if this can be umm, reversed, somehow? Or if there's a possibility of adding an extra component or something to make the transformer become not-always-powered. I'm not worried about modding the amp
 
#8
Melik said:
Thanks for your reply! Also, do you know if the transformer also heats when left connected to a wall socket, if turned off? In other words, is it a safety hazard?
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Note that I'm not a real engineer, so I can't say anything authoritative about a safety hazard.

However, leaving the amp plugged in would be no different than any other product being turned on indefinitely. If it's in good condition and was correctly designed in the first place, then it should be safe.

I know of no other bass amp that works this way, and also don't know if it meets currently accepted standards.

Transformers consume some power when they're powered up, even when the amp itself is idle. The energy for making noise has to come from somewhere. If nothing else, this is a waste. I think some states and countries have banned the old line frequency wall mount power supplies for this reason. Newer power supplies are required to drop down to a very low power level when idle.

A power switch could be added to the primary side of the transformer, but now you're into the territory of messing with a line voltage circuit, which would require a qualified tech. Perhaps unplugging it when not in use would be simpler.
 
#9
fdeck said:
A power switch could be added to the primary side of the transformer, but now you're into the territory of messing with a line voltage circuit, which would require a qualified tech. Perhaps unplugging it when not in use would be simpler.
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Yes, this is exactly the sort of solution I was looking for. It would effectively turn the original power switch into a standby switch. And yes, for sure I would have someone else do it! I have to ponder over it and do some more research... a million thanks, fdeck for the help!
 
#10
fdeck said:
The MB200 combo has the curious feature that the power switch is after the transformer. The transformer is always powered, always vibrating a bit, and is mounted in the speaker cabinet, so it's transferring its vibration to the cabinet walls.

It's not a super complex design, but getting it apart is tricky.
Click to expand...
Yes, it appears that there were (at least) two versions. I was unaware that they switched the secondary on the older one which is an odd decision indeed. This wouldn't comply with any of the newer safety standards for a couple of reasons, the first being that the safety standard allows single pole primary power switch to be classified as the means of disconnect when combined with a removable power cord. The second is the European Eco-directive which requires either a standby mode or power switch on the primary.

Things were a lot looser back in the day, this is why experience with the product line is important when servicing legacy products. Go back 10 years earlier and things were even looser!

I have later documentation that shows this was changed to primary switching, so obviously it depends on the version too.
 

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