• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hum Ho

Rosinante

Member
2010-11-02 8:58 pm
Newbie here. Be gentle.

I have done a Search and looked through some threads, but it seems that these amps commonly making a humming sound and there are several potential causes. I would like to benefit from the findings and opinions of you guys.

My Yaqin MC100B now makes an unacceptable hum. Both channels. The hum volume increases as the volume pot is turned. Low hum. Not a buzz. It hums on every channel into which a device is plugged, but does not hum when the input selector is turned to a position where no device is connected. When the input selector is turned to a position where a device is connected, the hum occurs and it makes no difference whether or how much input signal is entering. In other words, if a device is turned to "mute," the hum is unchanged.

This amp did not hum a week ago. Two changes I have made that could be affecting this behavior are:

1) This weekend, I relocated my entertainment system about two feet to the left. All devices were removed then reinstalled for this operation. In the meantime, I repaired an electrical outlet behind this setup. The outlet was not being used because, as it turns out, it had an open "neutral" circuit. Fixed that. The outlet tests just fine. 117 volts. The neutral conductor is connected to the ground lug on the outlets. That is.....there is no separate ground conductor, but the chassis of the outlet (and the ground pole) are connected to the neutral circuit. This means ground current would have a path back to the main box. The outlet tester smiles on this arrangement, declaring the outlet to be properly grounded. The outlet that powered this amp last week (with no hum) was wired this way.

2) I adjusted the output tube bias last week. On three tubes, the bias was close to 35 mV. I turned them all to about 55 mV.

The hum, as I say, is unacceptable. Very noticeable, particularly at listening volumes. Any advice you experts can offer would be appreciated. I have a friend whose boat needs a new anchor. That's becoming a reasonable alternative for this unit.:devily:
 

20to20

Member
2010-06-23 9:25 pm
If that amp uses a 3-wire plug, the chassis safety ground, and thus the chassis, is now connected to the neutral since you have connected the neutral to the safety ground screw on the outlet. Your shielding may no longer be floating and all the other components may have a connection to the neutral when they may need to be floating too.
 

Rosinante

Member
2010-11-02 8:58 pm
I can use an extension cord to connect this amp to a true, 3-conductor outlet. I may have done that already. The amp did not hum last week, when it was connected to an outlet in which the ground and neutral circuit are connected. I appreciate the advice, and am doubtful this is the source of the hum.
 

20to20

Member
2010-06-23 9:25 pm
I can use an extension cord to connect this amp to a true, 3-conductor outlet. I may have done that already. The amp did not hum last week, when it was connected to an outlet in which the ground and neutral circuit are connected. I appreciate the advice, and am doubtful this is the source of the hum.

Any outlet that has the safety ground tied to the neutral is creating a very short ground loop instead of a long one that would be formed at the breaker panel. Since the hum is variable with volume control it suggests the hum is coming from the outside and not getting shielded or isolated.
 

20to20

Member
2010-06-23 9:25 pm
Also, house wiring can be paralleled from outlet to outlet and if one is miswired they will all be miswired. The power may be joined at a ceiling light box, there are all sorts of circuit paths that electricians have used to wire a room so be carefull about a quicky safety ground fix you have created.
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
By connecting your safety ground to the neutral you are essentially grounding one side of the tranny primary directly to the chassis.

20to20,

Did I read correctly has the OP connected the return mains to input Gnd?:eek:. Is there no seperate return connection?

I assume there is a neutral return in the mains cable? OP is not using input Gnd as return?

Regards
M. Gregg
 

20to20

Member
2010-06-23 9:25 pm
20to20,

Did I read correctly has the OP connected the return mains to input Gnd?:eek:. Is there no seperate return connection?

I assume there is a neutral return in the mains cable? OP is not using input Gnd as return?

Regards
M. Gregg

The neutral conductor is connected to the ground lug on the outlets. That is.....there is no separate ground conductor, but the chassis of the outlet (and the ground pole) are connected to the neutral circuit. This means ground current would have a path back to the main box.

Unless I misread this.
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
20to20,

Please correct me if I am wrong!

This means:
If the return fails all the equipment will rise to mains potential. if the other equipment is on a different socket which has Gnd then the input interconnects will carry the return power. If the OP pulls an input cable they will have mains in one hand and Gnd in the other!

Regards
M. Gregg
 

20to20

Member
2010-06-23 9:25 pm
20to20,

Please correct me if I am wrong!

This means:
If the return fails all the equipment will rise to mains potential. if the other equipment is on a different socket which has Gnd then the input interconnects will carry the return power. If the OP pulls an input cable they will have mains in one hand and Gnd in the other!

Regards
M. Gregg

Good readback. The neutral can loosen up under the lug in the panel, it could have a loose wirenut in a junction box, it could be a loose outlet screw on an adjacent outlet if the elecrtrician used the outlet to outlet wiring method by using both silver screws on the outlet instead of using pigtails connections in the outlet box. Many places a neutral can get compromised over time.
 
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Rosinante

Member
2010-11-02 8:58 pm
I appreciate the input, but do not fully understand. When this amp was running with no hum, it was plugged into an outlet that had no separate ground conductor. Two-wire house wiring. So....the electrician had made a short (two-inch) wire to connect the neutral-side of the outlet to the green ground lug, also on the outlet. So....there was no separate return circuit to bring ground back to the service box. Ground could get back to the circuit box, but would have to use the neutral conductor for that path. No hum.

When I opened the non-working outlets (two of them so.....four plug receptacles), there was an open neutral circuit. Those two outlets had been configured like the other one, with a short wire going from the empty neutral screw on the side of the outlets, to the green ground screw. I simply fixed the open neutral problem. Now, whether I plug the amp into the new, repaired outlets, or the outlet seven feet further down the wall which had been used last week, the amp hums.

BTW, there is one more bit of evidence. After I changed output tube bias (from 35 to 55 mV), when I powered the amp up (volume off), there was a period of about a half-minute or more when a very high-pitched tone was heard coming from the unit. Then it went away, and the amp made no hum. Now that it is making the hum, there is no high-pitched whine during warmup.
 

20to20

Member
2010-06-23 9:25 pm
I appreciate the input, but do not fully understand. When this amp was running with no hum, it was plugged into an outlet that had no separate ground conductor. Two-wire house wiring. So....the electrician had made a short (two-inch) wire to connect the neutral-side of the outlet to the green ground lug, also on the outlet. So....there was no separate return circuit to bring ground back to the service box. Ground could get back to the circuit box, but would have to use the neutral conductor for that path. No hum.

When I opened the non-working outlets (two of them so.....four plug receptacles), there was an open neutral circuit. Those two outlets had been configured like the other one, with a short wire going from the empty neutral screw on the side of the outlets, to the green ground screw. I simply fixed the open neutral problem. Now, whether I plug the amp into the new, repaired outlets, or the outlet seven feet further down the wall which had been used last week, the amp hums.

BTW, there is one more bit of evidence. After I changed output tube bias (from 35 to 55 mV), when I powered the amp up (volume off), there was a period of about a half-minute or more when a very high-pitched tone was heard coming from the unit. Then it went away, and the amp made no hum. Now that it is making the hum, there is no high-pitched whine during warmup.

Certainly don't rule out other possibilities. Sometimes it takes 2 failure points to create a problem. But you have a potential safety problem now with your chassis being directly connected to the neutral. If you look at the diagram of your amp I doubt you will see a grounded primary. It may have a 1000v cap to ground but not a direct to chassis ground.
 

Rosinante

Member
2010-11-02 8:58 pm
Certainly don't rule out other possibilities. Sometimes it takes 2 failure points to create a problem. But you have a potential safety problem now with your chassis being directly connected to the neutral. If you look at the diagram of your amp I doubt you will see a grounded primary. It may have a 1000v cap to ground but not a direct to chassis ground.

I still do not understand much of what is posted, including this, but I'm trying. I'm what you might call a fairly bright ignoramus.:cool:

Okay, I will play around with house circuits. I will plug this unit into a properly-grounded outlet. I will also do what was suggested earlier and unplug all the inputs, plug them in one at a time. Perhaps power the input devices from another properly-wired, grounded outlet.

What about other hum sources? Am I hearing that a ground problem is the likely source of hum? Or were we just chasing the cat that was running (focusing on a potentially dangerous ground problem)? What about the high-pitched whine that went away as the hum appeared? What about the change in output tube bias? Would changing output tube bias back to 35 mV possibly eliminate the hum? Do I need to replace the flux capacitor?
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
Rosinante,

Please do not "play around" with the mains sockets!
You may find that if you remove all input connections the hum may stop. Don't disconect or reconnect with the mains on!

If the hum stops you have a difference in potential between your input source and the amp! That means that the Gnd on the amp is at a value above Gnd on the input source or vice versa! I will leave you in more capable hands!

Regards
M. Gregg
 
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20to20

Member
2010-06-23 9:25 pm
I've had a hum problem show from a bad RCA cable not making hard clean connections. I threw away a 25ft premium cable because it was a source of noise.

Does that amp use a tube rectifier? When you turned up the bias current, did the high pitched whine come from the amp itself or the speakers?
 

20to20

Member
2010-06-23 9:25 pm
Am I hearing that a ground problem is the likely source of hum? Or were we just chasing the cat that was running (focusing on a potentially dangerous ground problem)?

The ground problem was my original focus for the hum.

As far as the safety problem goes, it exists but it's a trade off by making the chassis short take out the breaker. It would be hard to guess which saftey issue is more common, chassis shorts or a lost neutral. Even a chassis short may not take out the breaker if it's a high impedance circuit failure. It may just blow the fuse in the amp first. Or if it doesn't take out the fuse it will still be a shock if you are shoeless.