400RB hum at idle

Greg Clinkingbeard · #1 10-16-2010, 05:53 PM

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?

sunbeast · #2 10-16-2010, 06:37 PM

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!

Greg Clinkingbeard · #3 10-16-2010, 07:12 PM

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?

bluesrx · #4 10-16-2010, 07:18 PM

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 !!! )

bluesrx · #5 10-16-2010, 07:21 PM

I also use hi temp clear RTV silicone to tack down the caps.

bluesrx · #6 10-16-2010, 07:26 PM

If you send me your email... I'll send you the service manual

sunbeast · #7 10-16-2010, 07:46 PM

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?

sunbeast · #8 10-16-2010, 07:53 PM

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!

KramerBassFan · #9 10-16-2010, 09:41 PM

Try grabbing the speaker cable of a Ampeg V4 with no ground on its cord....

But, careful about tapping big caps on live amps...

Greg Clinkingbeard · #10 10-16-2010, 10:15 PM

Quote:

Originally Posted by sunbeast

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?

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.

Greg Clinkingbeard · #11 10-16-2010, 10:22 PM

Quote:

Originally Posted by bluesrx

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 !!! )

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?

sunbeast · #12 10-16-2010, 10:43 PM

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!

Greg Clinkingbeard · #13 10-17-2010, 01:07 PM

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.

Greg Clinkingbeard · #14 10-17-2010, 05:36 PM

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.

twobit · #15 10-18-2010, 05:44 AM

Hows the tone on the 400rb? Is it like the 800?

Greg Clinkingbeard · #16 10-18-2010, 06:43 AM

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.

sunbeast · #17 10-18-2010, 03:56 PM

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!

robgrow · #18 10-18-2010, 06:28 PM

Quote:

Originally Posted by twobit

Hows the tone on the 400rb? Is it like the 800?

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.

Greg Clinkingbeard · #19 10-20-2010, 04:34 PM

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.