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Old 30th September 2012, 07:47 PM   #1
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Default 20 millivolts on chassis

Hi, I placed the positive probe of a dmm on my tube amp metal chassis and the negative probe in the earth socket of a wall plug. I observe 20 millivolts.
1) Is this acceptable? I checked my wiring and cant see any shorts . Although I am satisfied (for now) with my sound: you can hear a faint hum at full volume.Nothing audable at comfortable listening volume levels. I have heard plenty worse from commercial products. Is that faint hum the 20 millivolts?
2)The reason I did this was to check the chassis was not live after a tweak. Am I using the correct method to check I have not made the chassis live through say a wiring error?
3)I have been told by an electriction at work that he is not sure if a RCD on a domestic consumer unit will trip if DC current goes to the earth.He thinks only AC will make it trip. Can anyone tell me if the RCD will trip with DC.
Thanks Alan
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Old 30th September 2012, 08:31 PM   #2
DF96 is offline DF96  England
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Is the chassis grounded? If so, you are measuring the amount of hum magnetic field coupled into the loop formed by the chassis ground connection and your DMM probe leads.

If the chassis is not grounded then you are measuring the amount of hum electric field picked up by the chassis stray capacitance to nearby mains conductors.
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Old 30th September 2012, 08:52 PM   #3
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yes the chassis is grounded. Theres a bit more hum than I thought too. I just been trying to investigate. I got to admit I am disappointed with the amount of hum in the build as i was trying for null. I used the Morgan Jones super quiet heater circuit with the chokes (page 374). Used a star ground and used a separate chassis for the power supply. the heater and anode supplies I manually turn on. At full volume I observe 48mv when the heater circuit just runs and hear considerable hum. Then after waiting a minute and turning the anode supply ,the observed chassis voltage drops to 20mv and the audable hum drastically reduces. The hum is equal in both channels I would guess. I have the LM 317s set up as voltage regulators. I might change them as current regs as Jones does see what happens. But not tonight.Thanks for the reply
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Old 1st October 2012, 11:38 AM   #4
AndrewT is offline AndrewT  Scotland
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The distance from the wall socket to the equipment causes a measurement problem.

Somehow you have to overcome the untwisted DMM leads, leading to erroneous HUM readings.

Try doing it in smaller chunks, where you can ensure a small distance between the parts being measured.

If this 20mVac is real, then does that mean that the 20mVac is actually the voltage drop caused by earth leakage current times the PE line resistance?
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Old 1st October 2012, 12:14 PM   #5
DF96 is offline DF96  England
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It sounds like the heater circuit and HT circuit are generating hum magnetic fields which to some extent cancel each other. Measuring the voltage of the chassis is misleading, as there is no such thing as voltage - only voltage difference.
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Old 1st October 2012, 07:53 PM   #6
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I just tried measuring at different intervals from ther mains switch. There are 4 boxes 1-jones's heater circuit
2-anode transformers and rectifier valves
3-anode chokes and filter caps
4-dac,output tubes,tripath

progressively further away from the socket. As you move along the mv increases from zero milli volts to 48mv if the anode supply ie is off and the heater supply on.When the anode supply runs you see -14mv in the first chasis,11mv,16mv then 20mv at the farthest point.You can touch the chassis and can't feel anything tingle so I guess its nothing. I was more concerned about saftey. I got to admit the high voltage stuff does worry me because I dont have any formal training in electronics so I distrust every thing I do.
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Old 1st October 2012, 08:34 PM   #7
DF96 is offline DF96  England
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Yes, you are just measuring some induced voltage from magnetic fields. Should be no safety issues. It may or may not be connected to any hum you can hear.
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Old 1st October 2012, 08:44 PM   #8
nezbleu is offline nezbleu  Canada
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Are you measuring AC or DC voltages here?
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Old 2nd October 2012, 10:03 AM   #9
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Hi Alan

are you using 4 metal boxes that you have individually safety earthed....just wondering if you have created multiple paths for the DC to take .........this is in relation to the Hum you are hearing and not the voltages you are measuring .

Last edited by epicyclic; 2nd October 2012 at 10:17 AM.
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Old 2nd October 2012, 05:20 PM   #10
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yes each box has its own earth back to the mains, but they also connect up to the star ground. The star sits right between the two 6h6p valves. I just tried adding high temp 1500uf electrolytics straight on the heater pins but it has done nothing to address the hum. The pin I use as negative on the heater also goes to star ground.

I am measuring dc voltages
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