GK 700rb Head Issues - my head has brain damage apparently

Discussion in 'Amps and Cabs [BG]' started by ZhuAnRen, Dec 6, 2013.

  1. Greetings,

    A few months back, I picked up a used GK700rb mk1 head, that came with a SWR Workingman's 2x10" cab.

    One rather huge issue I've been experiencing with the amp, is that it keeps kicking engaging safety mode circuits at the most inopportune times. Obviously this sucks for gigging, but I absolutely love the sound of the thing. At first, I looked and looked for others who had the same issues with the head, and figured out the SWR cab it came with was rated way too low in both wattage and resistance to be used alone (I would have needed another 8ohm cab for it to be happy), I really don't know what the previous owner was thinking...

    I ended up getting an Eden 2x15" cab, rated at 450w @ 4Ohms, which is the exact rating I needed for a single cab. Once I plugged in the Eden cab, things seemed to work right for a while, Recently, the amp has been engaging the safety mode circuits randomly again. I've tried separating the head from the cab (in case it is a loose capacitor getting vibrations from the cab or my drummer or something), I've plugged different cabs into it, used different cables, etc. Currently, it is connected with a 1/4" speaker cable...no luck.

    My EQ, and most knobs are set flat, between 10-Noon, no Bi-amp functions turned on, No contour/presence, -10db pad on (for an active bass). I have a small pedal chain, but even when running directly in from bass to amp, I still get safety circuits engaging at bad times. Meanwhile, my band members are getting restless and unhappy that there is suddenly no bass mid-song. Can't say I blame them. I guess its good they care.

    I am wondering if anyone else has experienced this type of thing? I've tried using the search function, but the results seem vague and don't appear to apply to my situation. Is there any course of action BESIDES getting a new head that I cannot presently afford?

    Thanks in advance.
  2. The question I have is does the fan run in the amp? It should blow warm air out the front, variable speed depending on how hot the output section gets. If the fan is dead it will go into thermal shutdown at inconsistent times. It is a 24V DC, 160MA, 80X80MM, HIGH SPEED fan.
  3. Thank you for your response. I suppose it might be the fan. I don't think I've heard it ever engage. Naturally, this was a bit concerning at first. Toasted heads are never a good thing. I took it to a qualified dealer and had them check it out after I initially picked it up. They assured me the "stationary" state of the fan was normal, and just never started up since it didn't need to. According to them, the amp never got hot enough. (I don't think I believe them)...Occasionally when the safety circuits engage the head is warm to the touch, other times, its the same temperature as everything else in the room. Its a bit strange that way.

    That gives me an excellent starting point. Thank you.
  4. I have seen the fan just "kick" at power on. They will not run until needed. If your amp was warm on top then you should be feeling warm air blowing out the front. The fans are pretty quiet and you really have to stress the amp before they turn fast enough to be noisy.
    I was "breaking in" a new cab with my Fusion 550 and the son wanted to do the same with a new car sub he bought. At the last minute of him ordering he unknown to me, switched to a 4 ohm driver. After 20 minutes the fan was pretty darn loud, 2.67 ohm load on an amp not meant to go under 4 ohm. Everything survived fine but the screaming fan really surprised me!
  5. NWB


    Apr 30, 2008
    Kirkland, WA
    Not really. I've muted my amp mid-song and nobody in the band said a word.:scowl::rollno::(
  6. Steve Dallman

    Steve Dallman Supporting Member

    I can't believe the "qualified dealer" would not have heated the amp up to the point where it needed the fan, to see if the fan is operating properly. My GK1001RB fan ran often at gigs, not continually but often. I never had a fault during years of gigging with it.
  7. I can having worked in more than one shop. Not a stretch of the imagination at all, they are independant businesses and employ people.
  8. DWBass

    DWBass The Funkfather

    Can the fan be set to run at all times? That would bother me. One of the reasons I got rid of the short live Yamaha BBT500 head. It never quit on me but I didn't like how hot it got and it seemed the fan would not come on. I could have fixed it but I had already moved on to something else.
  9. You could modify the circuit. But generally modifying a design is not the best idea and any respectable shop should refuse. The next owner of the unit may find the modification problematic and blame the original design when in fact it has been altered. :)
  10. The more I think about it, the issue almost HAS to be the fan. I never see it spin when I power the head on, or ever for that matter. I'm also pretty sure no warm air gets blown out of the front of the amp. At first I figured the problem was entirely the SWR cab being a terrible match for the head rating-wise, which, I am sure certainly really didn't help matters.

    That Fusion 550 is a real monster of a head. I tried out one a while back. They've got great tube warmth and clean tone. Frankly, I'm rather impressed GK can make a head that is capable of handling a 2.67 ohm load and not explode. That's pretty impressive. Not that it should ever be tried intentionally...I bet that fan was just howling. Talk about amp stress! I really wasn't expecting my 700rb to be solid at all. And every time the safety circuits kick in, I immediately panic and hope it will power on again after leaving it off for 20 minutes. Luckily, it always has. Granted the 550 is a tube/ss hybrid, and the 700rb isn't, I guess GK builds heads like tanks. I have to admit, I was pleasantly surprised my head was so quiet, but that is likely the problem anyway ;)

    Looks like I'll be having a new fan installed.
  11. lol, I quite possibly may have done that once or twice intentionally to express anger towards a guitar diva or two....unfortunately the drummer always ends up being the angry one IME....Last time it happened a thundering voice came booming forth from behind the drum set....it said "WHERE THE HELL WERE YOU?! I COULDN'T HEAR YOU AT ALL!"

    The guitarists were too busy grooming their hair.

    True story.
  12. It is worth having a good shop do the replacement. If the fan failed in a short condition there are a couple transistors and some diodes, resistors as well as the thermal sensor on the heatsink that could be damaged or even at fault with a good fan.

    The Fusion 550 just barely nudged out a 700RB II. I am quite pleased with it :).
  13. gregmon79

    gregmon79 I did it for the muff... Supporting Member

    Dec 20, 2012
    Chicago IL
    I have a 1001RB II and I absolutely LOVE that amp. Its the same version (actually a later/newer version but just a little more powerful than your 700) as your and it def kicks. I looked away from GK for years thinking they were, sorry but its what I thought, garbage. I have no idea where I got that impression, I mustve played through a cab I didnt dig a long time ago. But Im very glad I decided to try it again when amp shopping last year at this time. Its been a blessing ever since. I will probably use GK for the rest of my life.

    To throw in an opinion about your 700, it def sounds like the fan. My fan is on, not sure if its a lot or a little but I know it comes on and blows hot/warm air out the front. Mine is in a rack space too so I can feel the warmth come out the back too. If I didnt feel that, I would be worried. I hope the fan fixes your head. It would be a pity to see you have to ditch it, sounds like you love your GK too.
  14. Troph


    Apr 14, 2011
    Kirkland, WA
    I have a 700RB mark II and I honestly haven't ever really noticed the fan coming on, even at 4-ohm load. But then again, I rarely need to play at club volumes without a DI, so maybe I just don't push it hard enough. I am not too familiar with what changes were made between the Mark I and Mark II series heads in terms of protection circuits and/or cooling.
  15. I agree with you there. I'd much rather have it installed by someone who has the proper tools and experience, rather than doing it myself. I am sure I could, but would rather not take the chance of wrecking it. Now..to find a good shop...
  16. Contact jason@gallien.com for a shop recommendation in your area if you don't get some reliable leads elsewhere.