Hum from crack - tried resoldering

[shujinko]

Hi,

My crack has developed a quiet hum at times.  It doesn't happen when I turn it on after it's been off for a while, but if I listen to an album it's almost always there right after it's finished.  And it also comes and goes at seemingly random intervals.  It might be quiet for a few minutes straight and it might phase in and out several times a minute.  It almost sounds the way an incandescent light bulb does, but pitched a bit higher most times.  Other times it's just a plain dull hum.  The sound is audible during silence and is audible during playback if I'm listening to something quiet.  It does not change with potentiometer adjustments.

It seems to be related to the 6080 tube.  I tried swapping my current tube out with a brand new one and the same thing happens although it sounds slightly different.  If I swap out the 12AU7 tube, nothing changes.

I've looked at a few other "hum" related threads and I resoldered anything that looked even remotely sketchy, and I tried resoldering everything on the 6080 tube leads.  Every time I resolder I think the issue is gone, but then I'll listen to an album and it comes back.

Any advice?  Anything else I can do to help with diagnostics?

Thanks a lot.

[Doc B.]

If it happens with different tubes that kind of rules them out as the source. The random nature of it sounds like it could be outside interference from some appliance. This could be a refrigerator, electric heater, dimmer switch, computer, cell phone, etc. See if you can correlate the appearance of the hum with the running of some other electrical device.

[shujinko]

Ah, why didn't I think of that?  Let me try a few things and I'll get back.  Thanks.

[Doc B.]

Ah, why didn't I think of that? 

That's why I get the corner office.

[shujinko]

I moved things around, tried different plugs and a different power cable.  There were a couple of times I thought I might have solved it but it did come back.

And just now I realized I had left out a potentially crucial piece of information, so I apologize!  The intermittent hum only seems to come out of the right channel.

[Paul Birkeland]

Are your voltages and resistances all about correct? 

Hum with the volume control all the way down does tend to point to the 6080.  Hum only in the right channel eliminates the power supply and most grounding issues.  Also, does this Crack have the Speedball?

[shujinko]

Nope, no speedball (yet!).

Here are my resistances and voltages.

Resistance
1 = *
2 = *
3 = 0
4 = *
5 = *
6 = 2.5K
7 = 2.98K
8 = 0
9 = 2.96K
10 = 2.47K
12 = 0
13 = Once it started at 5.2 and dropped slowly..., another time it was up at 370K and dropped slowly, another time it was around 27 and dropped slowly...
14 = 0
20 = 0
22 = 0

B3 = 2.97K
B6 = 2.97K

RCA Jacks
Ground lugs = 0 / 0
Center pins = 87K / 92K



AC
Switch = 120
Transformer 1-2 = 120v
Transformer 4-5 = 5.9v
Transformer 6-7 = 167v
Transformer 9-10 = 167v

DC (using terminal 12 for ground)
1 = 77v
2 = 170v
3 = 0
4 = 170v
5 = 78v
6 = 0
7 = 103v
8 = 0
9 = 104v
10 = 0
11 = 0
12 = 0
13 = 171v
14 = 0
15 = 193v
20 = 0
21 = 214v

Tubes
A1 = 78v
A2 = 0
A3 = 153v
A4 = 0
A5 = 0
A6 = 77v
A7 = 0
A8 = 153v
A9 = 0

B1 = 77v
B2 = 171v
B3 = 103v
B4 = 78v
B5 = 171v
B6 = 104v
B7 = 0
B8 = 0

[John EH]

What is your music source and could it be from the source.  Disconnect your left channel input, move the right over to the left and have a listen.

Eliminate all the really easy stuff first.

[shujinko]

That's another thing I should have thought to try

Here's the lineage:  ASUS Xonar DG sound card, optical SPDIF out, into Cambridge Audio DAC magic, RCA out to Bottlehead Crack, Crack into Sennheiser HD650 headphones.

But I tried swapping the Crack RCA inputs left/right.  The hum still comes from the same channel (right).  Powering the DAC off doesn't stop it.  So I guess it's my Crack.

[shujinko]

Actually that doesn't make sense... That points to my headphones, yes?

I just switched the headphone plugs and now the sound has moved to the other channel.  So maybe I have a bad set of headphone cables?  I keep my crack at the office so I'll bring them home over the weekend and try on my other equipment.

[Mak33]

If it happens with different tubes that kind of rules them out as the source. The random nature of it sounds like it could be outside interference from some appliance. This could be a refrigerator, electric heater, dimmer switch, computer, cell phone, etc. See if you can correlate the appearance of the hum with the running of some other electrical device.

I don't get much hum in my apartment, but did notice that we have a dimmer switch leading to 5 light sockets.  When that sucker is on, I can hear (albeit faint) background noise/hum.

[Doc B.]

Yes dimmers are one of the biggest offenders. Lumitron makes dimmers that are quiet, but they cost a lot more than the average ones.

[shujinko]

Okay, I think it's definitely the Crack.  I brought my HD650s home over the weekend and tried them in a couple of different amps and they were silent.  I brought my Audio Technica ATH-AD700s into the office today and I'm getting the hum on them now.  I've used the Crack for at least 18 months in this location so I really don't think it's a dimmer or anything.  Coincidentally I just moved offices too, so I'm in a different spot than I was and the hum sounds exactly the same.

Kind of interesting that the hum comes out of the right channel no matter what orientation the RCA inputs are in.  Could it be the headphone jack?  Could it be heat related?  Like I said it seems to only happen after the amp's been on for a bit.

[Paul Birkeland]

This could be some kind of funky mechanical connection (loose RCA jack), or a loose solder joint that is responding to heat. 

Noise should be pretty consistent from channel-to-channel, so I'd stay at it!

[shujinko]

Okay, I got a soldering iron and just finished retouching every solder point, adding solder where things looked a little light.  I only had a gun previously, which was okay for building it, but not for getting into tighter spaces.  Wish me luck!  I'll check back in after the holiday.  If I'm still getting the hum I'll probably include a bunch of pictures to see if you guys notice anything off.  Happy Thanksgiving to US people