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400RB hum at idle

Discussion in 'Amps and Cabs [BG]' started by Greg Clinkingbeard, Oct 16, 2010.


  1. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    I just picked up a 400RB made in '90. Everything seems to work well, but it's noisy, giving a loud hum. This is not affected by the pre or master volume positions.
    Is this the nature of the beast or something new filter caps might correct?
     
  2. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Los Angeles, CA
    I just recieved an 800RB with the same problem- the hum on mine was loud enough through my 6x10 to drown out an acoustic guitar!

    I went to a tech, and the filter capacitors were indeed the problem. Mine had taken a bit of a hit apparently during shipping, and a couple of the filter caps had even come loose from their solder connections (you can test this by tapping on them with a pencil with the amp turned on and plugged into a cab- they shouldn't make noise unless they have loose connections). Don't touch the conductive portions of the caps unless you have properly drained them of charge though. I learned this the hard way years ago!

    I currently have new caps on the way, and am excited to actually get to play the thing! If you decide to have the caps replaced (or do it yourself, as I am), also make sure to epoxy them to the circuit board to ensure they don't get loosened over time- for some reason the ones in my 800RB were not held in place by anything except their solder connections, which is pretty ridiculous for something so large and important! And again, to reitterate- MAKE SURE TO PROPERLY DRAIN THE CAPS BEFORE TOUCHING THEM!
     
  3. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    Thanks. I got a nasty shock from filter caps on a B+ tube supply-350 volts! Never doing that again.
    I count (4) 470uf at 63V but the values on the other 4 are not visible. Would be nice to know the values before tearing into the amp.

    Also, would there be any problem with higher value caps if I can make them fit?
     
  4. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    The caps you can't see are the same value. I would stay at the same value uf you can go higher on the voltage but not lower. Make sure you mark where the positive and negative are (make a drawing ) and put the new ones in the same way. ( Important !!! )
     
  5. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    I also use hi temp clear RTV silicone to tack down the caps.
     
  6. bluesrx

    bluesrx Supporting Member

    Mar 31, 2006
    Lakeland, Fl.
    If you send me your email... I'll send you the service manual
     
  7. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Los Angeles, CA
    It's my understanding that a higher value uf is also better if you can make them fit (will cut out more hum), ditto with the voltage (able to withstand a higher spike if necessary). I am no tech though, this is just what I was told by the guy that checked out my amp. These aren't amps that generally seem to have an issue with hum or exploding caps though, so the original values seem to work fine!

    As far as values- the 800RB uses 6 1000uf 100v caps for the 300 watt amp and 4 smaller caps for the 100 watt amp and the preamp rails. The 400RB doesn't have the biamp capability does it?
     
  8. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Los Angeles, CA
    And I got my first/ hopefully last big shock from a cap on my old Musicman HD-130 tube amp. Also never making THAT mistake again!
     
  9. KramerBassFan

    KramerBassFan

    Jan 3, 2009
    Try grabbing the speaker cable of a Ampeg V4 with no ground on its cord.... ;)

    But, careful about tapping big caps on live amps...
     
  10. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    No biamp.
    Thanks for your help. I love the sound of this thing and it looks really nice. Once I get this sorted out, I hope to run it for another 20 years.:D
     
  11. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    I wondered if those four were flipped over. Although it's been a few years, I used to build and mod vintage HI-FI amps so it should be pretty simple.

    What's the preferred method of discharging the caps? Any problem taking them to ground?
     
  12. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Los Angeles, CA
    The correct way (and what the tech did for me) is to take them to ground via the chassis through a resistor then check the voltage on them with a multi-meter. I believe bridging them direct to ground is alright as long as you plan to replace them anyway- it can apparently harm the caps to do it that way...I could be wrong about that though- which is why I had somebody who knows more than me give me the go-ahead!
     
  13. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    Bought a bag of (50) 1000uf 63v caps to replace the 470uf 63v for under $10. These are only slightly longer and the same diameter so they will fit fine and double the capacity.
    Will post the results as soon as it's done.
     
  14. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    Nevermind. I fumbled around inside and cleaned the connector from the pre to the power board. Dunno for sure if that was the problem but it's dead quiet.
     
  15. twobit

    twobit Supporting Member

    Nov 3, 2005
    Allen, Texas
    Hows the tone on the 400rb? Is it like the 800?
     
  16. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    No experience with the 800 for me, but the 400 gets the tone I hear in my head and have been trying to get. I haven't gigged with it yet, but at home playing a fretless with passive J barts I can get that burpy mid bump out of the bridge that wasn't quite there with any other amp I've used. With my fretted J it can go really old school or get a nice slap sound. It also sounds good with my old german upright which makes me very happy.
    FWIW, I think the 800 and 400 share the same front end.
     
  17. sunbeast

    sunbeast Supporting Member

    Jul 19, 2006
    Los Angeles, CA
    Seeing Avishai Cohen play (upright and electric) through a pair of 800RBs was part of what got me searching for one- I'm no upright player, but his sounded awesome through the GKs!
     
  18. I have a GK 400RB and a 800RB both from the early 90's, and they sound very similar. Two of the best sounding amps I've ever used with older Fenders and other BG's with passive electronics.
     
  19. Greg Clinkingbeard

    Greg Clinkingbeard Commercial User

    Apr 4, 2005
    Kansas City area
    Black Dog Bass Works
    Just to update, I went ahead and recapped it. With the 1000uf caps it may be just a tad quieter but that could also be due to them being new. It takes a bit longer to shut down due to the added juice available but I don't hear any difference in the sound.
    It took right at an hour to do and another hour or so to bring it up slowly on the variac.
    There's a vague memory bouncing around in my head that caps should be 'formed' but I could be imagining that.
     

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