GK 700RB in Protect Mode - Causes?

Discussion in 'Amps and Cabs [BG]' started by agreatheight, Jul 20, 2013.

  1. So played a gig last night and the ever faithful GK 700RB started switching in to protect mode during sound check. Would go into protect for 2 - 5 seconds and then go back to normal for a bit. Then back in protect. Fun! At first, I thought it was a heat related issue as it was hot-as-balls last night at the venue (no AC and 95 outside). So I took off the top and noticed the fan wasn't on. Problem identified, right? I am thinking "the fan is busted and the amp is overheating." Well, not so fast. I got my hand in their real close and it wasn't hot AT ALL, and it wasn't even warm anywhere on the chassis either. Um, OK, that's confusing. So I decided to leave the lid off of the amp and I pointed a very powerful fan across the innards and thought "that should fix the problem if it is an overheat issue!" And... no change. Still going into protect mode, just as before. What? So super confused. I then turned down my amp from where I was running it (eq flat, pre amp vol at 1/2, boost at 1/2, woofer vol at 1/2, tweeter vol off, eq flat) and then it was fine. I have run the amp at those settings, give or take, many times before with no problem so I don't think I was running it too hard. But I am not sure what turning the volume down would 'fix' in the situation to allow the amp to run without dropping into protect. Maybe a power issue, like the house circuit isn't providing enough power when I am pushing the amp a bit? Anyway, I figured that some stage volume was better than none, so I ran with it set up with low volume and had the sound guy push my through my monitor. I should mention we also lost a power amp in the mains rig, kept going into protect mode as well. Fun times last night. No matter - we limped through the gig and got paid so all good, lol.

    So my questions are these:
    What makes an amp go into protect mode?
    Which of these things is likely at work in my situation?
    If you think it was an overheat issue, why was the protect switching only for a few seconds and didn't the lid off / fan fix work?

    Thanks for reading this, long I know. Appreciate!
  2. 1958Bassman


    Oct 20, 2007
    The first thing I would do is find out if you amp was on the same circuit as the mains amp and if it was, measure the voltage/check the circuit for ground lift/reversed hot & neutral, etc. I would also measure the resistance between the neutral and ground on that circuit and another circuit. Since these should have no voltage present, this should be safe, but measure for voltage before checking resistance in each test. Try playing through the amp with the original settings on a different circuit- if it doesn't do anything wrong, I would guess that the circuit may have been overloaded, but not to the point of popping the breaker. It could also be a problem at the panel, like corrosion, a loose clip on a breaker or a breaker that has been stressed many times over the years.

    If you didn't feel heat, it probably wasn't an overheat issue, although the device that "senses" temperature may be loose or out of range. The amp may have a circuit that senses DC at the output, too. In addition, if your voice coil(s) became extremely hot, you may have noticed that the amp didn't sound as loud as the night progressed, and this would cause you to turn up the volume controls. Did you need to turn the controls up before the protect mode issue started?
  3. I was a turned up a bit from where I have been lately, but I thought that was due to my playing my low output passive backup bass versus my high output active main bass. But if it is the voice coil running hot, shouldn't I be able to feel that? Shouldn't the amp be noticeably warm? And is that something that should occur at the start of a gig? I was playing less than five minutes.

    I think I need to get this on someones bench, lol. But I may just buy a new head first. I got this along with an RBH410 cab for $300 total (score) and have gotten my use out of it (probably 100 gigs and lots of practice too). I am not sure it's worth throwing $100 at versus getting a brand new one for $600. I'll grab my spare amp for tonight but I'll play around with it to see if the 10 degree difference in temperature (cooler today, yeah!) makes any difference.
  4. georgestrings

    georgestrings Inactive

    Nov 5, 2005
    I'd see if it still goes into protect mode without a speaker cable hooked up to it... What are you using for a cab???

    - georgestrings