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Old 15th November 2010, 04:22 PM   #11
20to20 is offline 20to20  United States
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Quote:
Originally Posted by M Gregg View Post
Is the Gnd an Earth electrode or combined N & Earth incomming at the breaker board?


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M. Gregg
In a safety ground system, 2-wire plus bare ground, the white neutral and the bare ground are both ''earthed'' at the panel. They are not tied together at the green ground screw of the outlet.
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Old 15th November 2010, 04:32 PM   #12
M Gregg is offline M Gregg  United Kingdom
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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!

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M. Gregg
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Old 15th November 2010, 04:44 PM   #13
20to20 is offline 20to20  United States
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Quote:
Originally Posted by M Gregg View Post
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.

Last edited by 20to20; 15th November 2010 at 04:46 PM.
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Old 15th November 2010, 04:47 PM   #14
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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.
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Old 15th November 2010, 05:03 PM   #15
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Originally Posted by Rosinante View Post
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.
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Old 15th November 2010, 05:15 PM   #16
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Originally Posted by 20to20 View Post
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.

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?
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Old 15th November 2010, 05:22 PM   #17
M Gregg is offline M Gregg  United Kingdom
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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

Last edited by M Gregg; 15th November 2010 at 05:27 PM.
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Old 15th November 2010, 05:27 PM   #18
20to20 is offline 20to20  United States
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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?
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Old 15th November 2010, 05:34 PM   #19
DF96 is offline DF96  England
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My guess is that fixing the electrical safety has introduced a hum loop. Correcting this requires care, because safety and sound are leaning in opposite directions. We must bear in mind significant differences in domestic wiring practice on different continents.
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Old 15th November 2010, 06:20 PM   #20
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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.
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