Quiet GK 400RB - Preamp Question

Discussion in 'Amps and Cabs [BG]' started by nonfatmatt, Jan 7, 2009.

  1. I was lucky enough to snag a GK 400RB for cheap, and I really like the way it sounds but it's really quiet. I use an active bass so naturally I engaged the pad which is supposed to be -10db but I can't any volume out of it. I tried messing around with the preamp volume knob, and when it gets almost to the end there's a huge jump in volume, so the head goes from quiet to thunderous.

    Thinking there might be an issue with the pad I disingaged the pad and turned the preamp input volume way down, to like 2. Even at 2 the head is substantially louder with the pad disengaged than the with the pad engaged and the preamp knob at 10 or so, so I'm thinking there's something going on with the pad. I plugged a passive P-Bass into it and it's way louder on the passive setting than my 18v active bass on the active setting.

    Edit: the boost knob is at 10 o'clock on both tests

    So I have two questions:

    1. Is it okay to run the amp with the pad disengaged and to use an active bass with the preamp volume lowered (I'm pretty sure it is, I'm not getting any distortion)?

    2. The guy I bought it from switched the footswitch jack and the input jack because the input jack was a bit wonky and he didn't use a footswitch. Can there be a problem with this, are they two different parts? Might it have effected the proper functioning of the pad?
  2. Gearhead43

    Gearhead43 Guest

    Nov 25, 2007
    Yes, you only need to use the active setting if your bass clips the input/pre in the first place. Use the passive.
  3. Edit, now the preamp cuts in and out. Has anyone worked on these? Are there common problems.
  4. davecanady

    davecanady Guest

    Aug 24, 2008
    Back to basics: Have you confirmed that there is no issues with the output of your bass? The cord? As long as the jack is connected correctly, they sould work the same.

    I have the 400 RB III and could never have the boost at 10. It's not just loud, it sounds like crap.

    If you are getting full volume with non-active pups then the pad might be the issue. I can use my non-active pups with or without the pad engaged and not have to max either the gain or the master.

    It sounds like a possible grounding issue with the pad switch. If all works without the pad, look at the pad. For now....don't use the pad!
  5. Boost is about 40% of the way up, 10 o'clock not 10. I've tried three basses and two cords, it does the same thing, now the preamp is quiet on passive and active when before I was getting volume with the pad disengaged, I think I have a wonky preamp in it.
  6. Buy some control cleaner (NOT WD-40) and clean all the pots first, then the effects jack.

    Is the amp the 400rb, 400rb II, 400rb III or 400rb IV?
  7. Yeah that was my next go to idea. It's a 400RB original for reference.
  8. If you need service info. PM me a e-mail addy.
  9. winston

    winston Supporting Member

    May 2, 2000
    East Bay, CA
    There's something up with the amp's front end. When properly working the input control & pad are smooth & linear. If the input/ex-footswitch jack is stereo, it might have been wired incorrectly. Take it to someone who knows these amps, or send it to GK.

    The amp should get LOUD and growly with the volume controls cranked--it's especially vicious with a P-bass.

    Good suggestion--the RB's FX loop jacks are prone to corrosion over time, which causes an intermittent signal. Look for a cleaner like Caig DeOxit that won't leave a residue, spray the jacks, and plug/unplug a cable several times to loosen the gunk. Clean the push switches too.

    I have a 400RB that I got for Christmas in '87 which I still love & play regularly. In fact this week I'm taking it to a tech for a once-over, I'll probably get the filter capacitors replaced so I can enjoy it for years to come.
  10. throbgod13


    Mar 26, 2005
    open the amp (unplugged) and look at the FX Loop jacks.. follow the wires to the little white molex connector to the PCB.. disconnect the molex.. spray DeOxit5% on the pins and the female connector.. use a hard bristle toothbrush and scrub the connections.. wipe off the excess.. reconnect..

    it should work just fine after that.. ;)