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Old 14th November 2012, 08:24 PM   #1
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Default Voltage between chassis ground and earth ground

Newbie here so go easy on me. This is probably somewhere in the basic discussion section but I honestly cannot find it.

In short, the main question is should there be a voltage difference between the chassis ground of an tube amp and the earth ground? If so, what should it be? At length, I am converting an old 6V6 pushpull PA amp (Pacemaker PM 20) to a guitar amp. Warm up the amp and turn up volume and it is very quiet. Touch the metal chassis and it hums. Check the voltage between the chassis and earth ground and it is about 50 V AC, which seems a bit high to me. Checked another one of my amps and the chassis to ground voltage was 7 V AC.

My inclination is to just ground the chassis, but am afraid that the create a excessive draw on the power transformer and burn it up.

Again, newbie here so go easy. Thanks for your help.
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Old 14th November 2012, 08:34 PM   #2
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Old amplifiers are often just flat dangerous, and this is no exception. I'd very strongly recommend that you fit a three wire power cord and bolt the green (safety Earth) wire to chassis.

If it were just your old carcass or mine at stake it may not be much of an issue, but children or pets could get hurt, and nobody wants that. So, yes, your inclination is completely correct.

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Old 14th November 2012, 09:11 PM   #3
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According to modern regulations chassis has to be connected to the safety ground. I even heard that everyone who repairs an old guitar amp have to add 3-wire cable and ground it, otherwise can be sued.
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Old 14th November 2012, 09:12 PM   #4
DF96 is offline DF96  England
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Mains filter caps can do this. Some old amps included a cap which is intended to electrocute the guitarist. Be careful. If possible get someone else to have a look at it.
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Old 15th November 2012, 02:23 AM   #5
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No mains connected caps, according to schematic I have. But the primary of the transformer has some capacitance to the grounded core, which provides one current path, and there could be some leakage in the insulation as well. I wouldn't trust 50-year-old paper and wax to keep ME isolated form the power line - GROUND it!

Current though the green safety ground wire is normally close to zero. When it DOES carry current, you can thank your lucky stars that the current is not flowing through YOU!
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Old 15th November 2012, 07:44 PM   #6
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Thank you all for the responses. I will be grounding the chassis with a three wire cable this weekend. Right now I have it grounded with a separate wire.
There is also one remaining rather scary looking "paper & wax" filter capacitor that I will replace. Looks like the others have been replaced in the recent past, so I will leave them for now and see if this reduces the "floating" chassis voltage.
However, my fundamental question remains ...Are these old amps just designed to have a higher "floating" chassis ground voltage than that to earth ground? . . .or is this 50 V AC leaky voltage due to old worn out capacitor. I did measure the current from the chassis to ground and it is about 0.05 amps.

This is the schematic for this thing that I found on the internet.

Thanks again for your help.
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File Type: jpg pacmaker pm 20 schematic.jpg (250.8 KB, 230 views)
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Old 15th November 2012, 08:22 PM   #7
DF96 is offline DF96  England
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0.05A leakage current is very high, and if correct suggests a major problem. There is no capacitor to cause this so it must be something else - transformer insulation? However, check carefully that a previous fiddler has not added a cap which is not shown on the diagram.
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Old 15th November 2012, 08:29 PM   #8
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Quote:
Originally Posted by DF96 View Post
0.05A leakage current is very high, and if correct suggests a major problem. There is no capacitor to cause this so it must be something else - transformer insulation? However, check carefully that a previous fiddler has not added a cap which is not shown on the diagram.
There will be leakage capacitance between the transformer windings and so a voltage passed across. So the chassis must be connected earth to be safe.
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Old 15th November 2012, 08:47 PM   #9
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by nigelwright7557 View Post
There will be leakage capacitance between the transformer windings and so a voltage passed across. So the chassis must be connected earth to be safe.
Sure, but if the measured current was 50mA, that's huge for "leakage". The level of current to be expected due to the coupling between transformer windings would be a tiny fraction of that figure.
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Old 15th November 2012, 09:25 PM   #10
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Hammond organ had a standard for earth connecting old chassis. 10k resistor connected between organ amp or preamp and "known earth ground". That known earth ground is the new 3rd wire of your power plug, if the wall plug is wired correctly. Measure voltage across the resistor with a AC VOM with more than 5 kohms / volt. (The cheapest RS meter is this high, typically). More than 4 VAC, the organ has to have the power transformer replaced. Before the earth ground is installed. To prevent insulation breakdown of the transformer (ie fire).
Not assured that the power transformer is the cause in the case of your amp, but cotton insulated wire could be the problem.
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