Help with buzz/hum

[Stickman]

I've posted before trying to nail down an annoying hum in my Bottlehead Crack with Speedball upgrade.  We've done some debugging using the chopstick and found some additional information.  The hum actually occurs without a source hooked up and varies with the volume.  We found that if you move the right most wire bundle towards the center (see arrow) the buzz gets worse.  (The bundle from the input jacks to the pot.)  Seems like it's picking up interference (maybe) from the transformer?  I've attached a photo that may help.  Any advice appreciated.  I love the amp when it works but since it doesn't it has been a paper weight on my desk.

[kgoss]

How does it behave with shorting plugs in the RCA jacks?

[Stickman]

I'm guessing that is something I purchase?  I don't have anything like that currently so I haven't tried that.  Let me know what to get and I can order them.

I appreciate the help!

[Deke609]

Seems like it's picking up interference

To try to test this, I'd do the following. Wrap the red/white/black braid tightly in 2-3 layers of aluminum foil. Makes sure the foil is well away from any components and terminals, including at the pot and input jacks.  See if that helps.  This is just a temporary test, not a solution. IF the buzz lessens, you have some options for a more permanent fix: shielded signal wire (e.g., Cat5 cable) with drain wire soldered to a ground lug, copper tape or tubing/pipe  with a wire soldered to the copper tape/tubing/pipe and a ground lug.  Or you just try increasing the number of turns of the braid and making the braid tighter -- this may require longer wires.


cheers, Derek

[Doc B.]

Move it around to different locations and see if the amount of hum changes. If the hum changes when you move the amp around the house it could be outside interference. If it stays the same it is possible that it's simply a ground connection at the RCAs or the volume pot that is not quite as conductive as it needs to be and it could use a reflow.

[Paul Birkeland]

We've done some debugging using the chopstick and found some additional information.  The hum actually occurs without a source hooked up and varies with the volume. 

How much hum do you have with the volume all the way down?

[Stickman]

Last I checked the unit has zero hum when turned all the way down. 

My friend has reflowed all the joints, I intend to test this afternoon to see if it improves.

We had previously tried a wrap of aluminum foil to shield the strands but it didn't seem to make a difference.  However it was only a single wrap of foil.  If it doesn't improve, we'll try wrapping 2-3 times as suggested.

Thanks to everyone for the help!

[Paul Birkeland]

See Crack FAQ #5

[Stickman]

Looks like the reflow may have done the trick.  My friend has certifications in soldering so she did a much better job than I did for sure.

I haven't tested it with a source but it's quiet, quiet, quiet, until you hit about two-thirds volume or above.  Per the referenced FAQ, that is probably normal.  I'll do some critical listening when I return from next week's conference.

Work is really hurting my extra play time.

Thanks again to everyone for all the help!  I'll post an update in a couple weeks.

[Stickman]

Finally got a chance to try out the Crack after the reflow and it was great!   (while it lasted.) 

So the hum that follows the volume up and down is completely gone.  However, a slight hum will pop in every so often.  I've found I can move the power tube slightly and make it go away for a while, although it usually comes back.  Initially we thought it might be due to excess dirt and/or flux in the socket, we gave it a good cleaning.  It seemed to help but it is still there.  The tube is relatively new but my thought is could it be the socket itself?  It seems kind of loose and you can wiggle it around quite a bit.  Was wondering if I may have melted something in there due to my limited soldering ability.

I can also try ordering a new tube but not sure if that would be the first path forward.

I appreciate everybody's help.  When it was working, it brought back all the goodness I remembered.  I think we're close but not quite there.

Thanks again,

Shawn

[Paul Birkeland]

If moving the tube around makes the noise disappear, the most likely culprit would be a persistent loose connection on the socket or on the other end of a wire that's connected to the socket.

The next most likely cause would be oxidation on the tube pins themselves, and the least likely issues would be a funky tube or a bad socket.

Running the amp for a few days will usually cook in the tubes enough to remove any noises that they may make, and there is also the possibility that you may have some external interference. 

Wrapping aluminum foil around the input cabling is not the safest decision to make and is very unlikely to make any difference if this is a noise that is present with the volume control turned all the way down.

-PB