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GK MB500 problems

GroovinOnFunk

GroovinOnFunk

Supporting Member
Apr 30, 2008
2,150
2,327
40
Nashville, TN
#1
Last night at my gig, my MB(2)500 kept restarting on me. I had my Roscoe with Aguilar pre going into it. I had the -15db pad pressed and everything. The amp was going into a 4 ohm avatar 410 cab.

So I'd be playing and all of a sudden the amp would shut off and turn back on. I assume I'm driving the amp too hard, but I couldn't raise the "gain" more than past 9 o'clock or it would do this.

Any thoughts/suggestions?

Oh, so you might be saying "easy, just leave the gain down" but I wasn't getting enough volume out of it.
 
#4
Club power sounds like a good explanation. Also, I think the MB series is prone to cutouts with certain 4 ohm cabs if their impedance curves dip below 4 ohms at a particular frequency. It's conceivable that you may have been pushing frequencies that pull the cabs impedance down tripping the self protection on the amp.

If you've used the cab on a number of gigs in the past with no issue it could also be a faulty cable or jack. Shorting will cause the amp to cut out and restart. Just taking shots in the dark here....

Good luck.
 
#5
Thanks! I know all you guys can do is guess. This is only my second time using this cab. I just acquired it (used). It seemed somehow related to the signal coming into the amp. By that I mean, I eventually turned the gain on the amp down to almost zero and it did stop cutting out, but then if I'd turn on one of my boosts or my envelope filter which has a slight volume boost, it cut out again (was great when it happened during my one solo of the night as we broke down to just bass and drums... Then it became just drums cuz my amp cut out. After about 5-10 seconds which felt like an eternity, it came back on)
 
#6
How many times have you used the head? If it were related to input levels I would have expected you to have encountered this with your old cab? If not, it might be the cab or club supply power. The switching power supplies in general don't really like dipping supply voltages and when an amp is run at it's minimum impedance it is most demanding on the internal switching power supply. Is it possible your head was on the same circuit as the main P.A. amps? You could have been seeing voltage dips from the mains.
 
#11
It did happen once before at a rehearsal but it only happened once and I had the gain up to like 11 or 12 o'clock when it happened. I know the obp-3 preamp has a really hot output but I'm thinkin it's not that. This sucks cuz I dig the tone of the avatar. Would something like an Eden wt800 have this problem? I can get one of those from work for a good price
 
#14
The fan does work... It was really hot on stage, but it's only ever done this with this cabinet (though I've never run this amp at 4 ohms before getting this cab).

I'm not sure how I could reproduce that problem... I've plugged it in at home and never had that issue. But then again, if I turn up to "gig volume" in my apartment, I'll probably get evicted.

Again, I did have the -15db pad (or whatever it is... might be -12) to counteract the level of the preamp, and it's not like I was cranking all my EQs up to 11. I gave the mids and trebl just a slight slight boost and that was it. (especially once the amp started crapping out on me)
 
#16
Spacecase said:
You're driving into 4 ohms...and you're driving it with a preamp. I'm guessing you're going into thermal protection shutdown. Is your fan working? Is air circulating freely?
Click to expand...

+1. have you tried it without the preamp ? Also, you said it was an MB 2 500? Those versions of that head had some problems early on, and there were several revisions of that head before GK called it the MB500.
 
#17
GroovinOnFunk said:
Again, I did have the -15db pad (or whatever it is... might be -12) to counteract the level of the preamp, and it's not like I was cranking all my EQs up to 11. I gave the mids and trebl just a slight slight boost and that was it. (especially once the amp started crapping out on me)
Click to expand...
If you're not saturating the intermediate stages of the amp (i.e. lots of distortion), the bottom line is the levels being demanded of the output stage. So, if you're using the -15 dB pad and giving it an additional +15 dB of signal from your preamp to make up for it, it means little to how hard your output stage is working.

I'd call customer service at GK and see if there's a thermal protection circuit doing its job. If so, you might need to consider a more efficient cabinet or a more capable amp.
 
#19
GroovinOnFunk said:
Do you think an Eden WT800 which runs at 1100 watts at 4 ohms will be ok with the avatar which can handle 1000 watts?
Click to expand...
I can't offer an opinion on the Eden. Do some homework first. See if anyone had issues with it. And call G-K customer support and see what they say.

If you need more power, there are a ton of options. My opinion: You frequently gig and already have a preamp. I'd find a rack-mount Class-D amplifier to go with it. I'd consider a used but well-cared-for Stewart 2.1 and wire it in bridged mode. It's a robust amplifier capable of 1800 W into 4 ohms. You'd need to make a bridge cable for it, though.

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