60 cycle type hum when amp's master is turned up, what could this be?

kurt1981

Supporting Member
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2,546
Hi guys I have a boogie DC3 combo, as I turn up the master vol I get a louder and louder hum, which, to me seems abnormally loud. I've replaced all preamp tubes gone through them all with known good subs and it's not a tube issue. Wondering, could power tubes cause this or would there be something else going on inside?
Thanks for any help,
Kurt
 

mark norwine

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17,150
step 1: How have you verified for certain that it's 60Hz? Are you certain it's not 120Hz?

Not all hum is 60. In fact, it's almost always NOT 60.

Either way, you know the problem is 'before' the volume control, so you can most likely rule out power tubes
 

pdf64

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6,934
Have you had power tube problems recently, maybe resulting in them being replaced?
I'm thinking that the filament ground reference resistors (if that's what is used) may have blown.
Pete
 

kurt1981

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2,546
Haven't replaced power tubes, I just recently got the amp, and have noticed this issue. It's weird, you can hear the buzz if you take out any of the preamp tubes, except for the phase inverter, so I kind of think it's in the power amp section but I'm not sure. One thing I just thought of is to try and connect it to another power amp to isolate the preamp section and see if I can hear it there. Would a bad power tube, or one going bad cause this type of hum in a 4 el84 amp? The amp still has full volume, and isn't distorting or anything, it just seems like it's a weird issue with a buzz, not unlike when you have bad grounding, but it happens with no instrument connected.
 

Blue Strat

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30,065
If it's like most production amps, the Master is BEFORE the PI tube. Removing the PI tube blocks preamp signal from getting to the power tubes, so it really can't be a power tube problem.

If it's not preamp tubes, something internal to the amp must be the issue.

BTW, low E on guitar is 80 hz so if the pitch of the hum is higher, the hum is 120hZ (which is the most likely scenario).
 

Billm

Member
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574
And "buzz" is most likely 120Hz. 60Hz is pretty low, and if it's the actual sine wave getting through it's not buzzy unless it's thoroughly thrashing the speaker.

Problems could be a bad diode if it's a solid state power supply, bad ground, bad main filter capacitor.

If you have the schematic, you can build a simple audio circuit tracer like this one and chase the buzz backwards through the circuit:

http://billmaudio.com/wp/?page_id=1254
 

mark norwine

Member
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17,150
So many questions....

Does it do it on both channels?

Does it do it with the gain up? down?

Do tone controls affect it?

All that said, if you're not experienced in amp work, a DC3 is ***NOT*** where you want to learn! This amp (like most MBs) is a bitch to work on, even for experienced techs.
 

kurt1981

Supporting Member
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2,546
It's on both channels, tone controls don't effect it, and it'll do it with the channel volumes and gains all the way down, just turning up the master volume. I probably won't be able to fix it, but I'd like to try and isolate it before taking it to a tech.
Thanks,
 

mark norwine

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17,150
we just isolated it....it's after the gain but before the inverter.

The question that remains is my first one: is it 60Hz? 120Hz? or neither [a buzz]. These answers would reveal a lot.
 

kurt1981

Supporting Member
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2,546
It's around a B or C note below the low E on a guitar if that helps. It sounds like a bad ground issue, like going into a venu with bad power. Like I said, even if I turn the channel volume and gains all the way off but turn up the master vol I hear t e noise. I'll try and isolate the preamp tonight see if that comes through out of another amp,
Kurt
 

kurt1981

Supporting Member
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2,546
Okay, I'm even more lost, still hums when I use the preamp out, and defeat the internal speaker, I also tried pulling each pair of power tubes to see if there was a bad tube, no dice. Mystified now!
 

Blue Strat

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30,065
Okay, I'm even more lost, still hums when I use the preamp out, and defeat the internal speaker, I also tried pulling each pair of power tubes to see if there was a bad tube, no dice. Mystified now!

Did you miss my earlier post where I explained that it had to be coming from the preamp? :huh
 

kurt1981

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2,546
Just saw that, it's so weird though as I've tried all the preamp tubes with known good tubes and none make the hum go away.
 

zzmoore

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7,095
Just saw that, it's so weird though as I've tried all the preamp tubes with known good tubes and none make the hum go away.
He is not saying it is a preamp tube...but rather In The Preamp circuitry of the amp...but at least you have ruled out a tube problem.:aok
 

Blue Strat

Member
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30,065
He is not saying it is a preamp tube...but rather In The Preamp circuitry of the amp...but at least you have ruled out a tube problem.:aok
What ZZ said. It's time for the tech. Don't worry about finding the problem for him. If he's capable of doing the job, he can find it faster than 1000 guys who can't see the amp can guess what the problem is.
 

mark norwine

Member
Messages
17,150
call me old fashioned......but I verify with meters.

Knowing the frequency helps a lot.

- 60Hz could be a sprung jack on the loop return.
- 120 is power supply related. In this case, I'd suspect a busted ground.
- buzz is altogether different.

But since power tubes were ruled out in post #2 & #5, yet OP is pulling them in post #11, clearly nothing you nor I write is being read, at least not seriously.

So....Mike.....Been to any good restaurants lately?
 

Blue Strat

Member
Messages
30,065
call me old fashioned......but I verify with meters.

Knowing the frequency helps a lot.

- 60Hz could be a sprung jack on the loop return.
- 120 is power supply related. In this case, I'd suspect a busted ground.
- buzz is altogether different.

But since power tubes were ruled out in post #2 & #5, yet OP is pulling them in post #11, clearly nothing you nor I write is being read, at least not seriously.

So....Mike.....Been to any good restaurants lately?

DUDE! Read the thread. Does the OP seem to have any of the capabilities you're looking for? ;)

No seriously good restaurants like your place in Rumson, but in the coming week we'll be hitting some good ones. :)
 

kurt1981

Supporting Member
Messages
2,546
Great, so I start an honest thread and get called an ignorant ideot, thanks very much for the help. I was simply trying to rule out all possibilities before taking it in to a tech, and I've said from the first post it's 60 cycle hum. Again, the sarcasm and cheap jabs aren't needed.
 




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