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.
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.
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.
All good fortune,
Chris
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.
All good fortune,
Chris
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.
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.
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!
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!
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.
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.
Attachments
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.
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.
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.
Not assured that the power transformer is the cause in the case of your amp, but cotton insulated wire could be the problem.
I would recommend a new power transformer. sometimes those old power transformers don't have the greatest isolation. and remember, when you plug your guitar amp in, you are holding onto the amp's ground.
Its better to error on the side of caution. there has been many musicians that died from electrocution this way.
and to add I would recommend taking it to someone like a tv repair man to test the leakage current just to be on the safe side.
Its better to error on the side of caution. there has been many musicians that died from electrocution this way.
and to add I would recommend taking it to someone like a tv repair man to test the leakage current just to be on the safe side.
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So off the top of my head thats like .4ma and this one is 50ma a factor of 200 high . Given that the main transformer is in need of replacement unless there is a resistor not on the schematic from the ht to chassis leaking the 50 ma which is like 2.5 watts at 50 volts ( 50 volts at 50ma ) there should be some amount of heat or some thing should be warm I would think. Reverse the polarity on the ac plug and measure voltage as you did be fore and please report back . so it 50 volt one way what is it the other ?
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Thank you all for your guidance. I will spend a sometime this weekend testing this thing as well as changing a capacitor or two.
Regarding "2.5 watts at 50 volts (50 volts at 50ma )"; note that I did not measure these at the same time. I measured the static voltage (essentially no current) at 50 V and 50 ma current through the multimeter () (essentially a ground wire with little/no voltage difference). I think maybe I am chasing ghosts with these measurements and that the voltage accross a 10k resistor will be much more instructive.
I still think something is wrong. Hopefully it is not the power transformer, but with my luck it is always the most expensive component.
For what it is worth, the amp does sound great except for a crackly issues at high volume. I will go through my modifications and problems in more detail after I get this safety issue figured out.
I appreciate all your help.
Regarding "2.5 watts at 50 volts (50 volts at 50ma )"; note that I did not measure these at the same time. I measured the static voltage (essentially no current) at 50 V and 50 ma current through the multimeter () (essentially a ground wire with little/no voltage difference). I think maybe I am chasing ghosts with these measurements and that the voltage accross a 10k resistor will be much more instructive.
I still think something is wrong. Hopefully it is not the power transformer, but with my luck it is always the most expensive component.
For what it is worth, the amp does sound great except for a crackly issues at high volume. I will go through my modifications and problems in more detail after I get this safety issue figured out.
I appreciate all your help.
If you are measuring 50mA leakage then there is a serious fault and a serious safety issue. Be very careful.
Gotta say everyone who is suggesting you replace that power transformer is giving you the best possible advice!
As has been pointed out 50mA of leakage current is extremely high, the allowable limit on a metallic enclosure is something on the order of 0.1mA and with a single fault 0.5mA - remember this is going to be a guitar amplifier and you will be well coupled to it when playing a guitar, all of that leakage current will travel through you to any grounded object (like a mic.) or vice versa..
Grounding is insufficient in this case as you cannot count on a reliable ground under all circumstances so a good power transformer will provide you protection under those circumstances.
Using a GFI or two to provide power and ground fault protection while gigging is not a bad idea - will provide protection against dangerous leakage currents flowing through various devices being handled during a performance.
As has been pointed out 50mA of leakage current is extremely high, the allowable limit on a metallic enclosure is something on the order of 0.1mA and with a single fault 0.5mA - remember this is going to be a guitar amplifier and you will be well coupled to it when playing a guitar, all of that leakage current will travel through you to any grounded object (like a mic.) or vice versa..
Grounding is insufficient in this case as you cannot count on a reliable ground under all circumstances so a good power transformer will provide you protection under those circumstances.
Using a GFI or two to provide power and ground fault protection while gigging is not a bad idea - will provide protection against dangerous leakage currents flowing through various devices being handled during a performance.
And since this is a guitar amp I'm going to move this to Instruments & Amplifiers per forum policy.Guys,
Recent advice on leakage current limits from a reputable source.
Leakage Current Limits for Electronic Equipment Used in Non-Patient-Care Areas
THe 50mA (50,000 micro amps) you measured means that the amp in its current state would be condemned as UNSAFE by any testing authority. Do NOT use until fixed.
Cheers,
Ian
Recent advice on leakage current limits from a reputable source.
Leakage Current Limits for Electronic Equipment Used in Non-Patient-Care Areas
THe 50mA (50,000 micro amps) you measured means that the amp in its current state would be condemned as UNSAFE by any testing authority. Do NOT use until fixed.
Cheers,
Ian
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