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Hoffman Amps Forum image Author Topic: Plug with a ground, or not, that is the question . . .  (Read 1491 times)

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Offline acheld

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Plug with a ground, or not, that is the question . . .
« on: October 03, 2025, 04:14:16 pm »
The few vintage guitar amps I've restored have always received a grounded mains plug, with a chassis ground. And never had a problem . . .

I have no practical knowledge of how the solid state components work  . . . time to learn.  :icon_biggrin:

I'm currently restoring a Marantz solid state receiver (2238b) which was originally ungrounded.  The power cord was operable, but shredded and obviously unsafe. 

I installed a new mains power cord with a modern three prong plug, and grounded the chassis.   And of course now there is a hum.  :dontknow:   I'll probably just put in a non-grounded power cord, ie go back to what it was (and had no hum).   But I have no clue why grounding the chassis would cause a hum.

Is it possible that grounding the chassis for a solid state receiver would cause a hum?  Why?

Offline rumpus

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Re: Plug with a ground, or not, that is the question . . .
« Reply #1 on: October 03, 2025, 05:23:29 pm »
Here are a few things I can think of:

Did you change anything else?

Try disconnecting the ground lead of the power cord from wherever you attached it to the chassis, does the hum go away?

Is there a "ground capacitor" from one side of the power line to the chassis? If so, try disconnecting that from the chassis (with your new power cord ground reconnected), does that affect the hum?

Are you sure your ground in the power outlet is clean? Measure voltage between that and neutral (with the amp unplugged from the outlet). (I occasionally play bass in a venue where I have a bad hum problem - I measured about 1.5 VAC between neutral and ground there.)

Offline pdf64

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Re: Plug with a ground, or not, that is the question . . .
« Reply #2 on: October 03, 2025, 06:03:20 pm »
Is the receiver connected to other equipment, eg phono turntable, CD player?
If so are they grounded?
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him! BBC News feature  https://www.bbc.co.uk/news/articles/cm26llp

Offline shooter

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Re: Plug with a ground, or not, that is the question . . .
« Reply #3 on: October 03, 2025, 06:33:52 pm »
do MetalicA headphones count for grounded?  :icon_biggrin:
Went Class C for efficiency

Offline acheld

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Re: Plug with a ground, or not, that is the question . . .
« Reply #4 on: October 03, 2025, 08:51:46 pm »
These are all really good thoughts, and are much appreciated.  I'm just going to "downgrade" to a new old-style ungrounded plug like it had when manufactured, as there is no hum in that condition.   

Quote
do MetalicA headphones count for grounded?  :icon_biggrin:
Why yes, of course they do, but I can't seem to find mine now.

Quote
Is the receiver connected to other equipment, eg phono turntable, CD player?
If so are they grounded?
Yes, both turntable and CD, and they are not grounded to the wall, though of course the turntable is grounded to the receiver.

Quote
Try disconnecting the ground lead of the power cord from wherever you attached it to the chassis, does the hum go away?

Is there a "ground capacitor" from one side of the power line to the chassis? If so, try disconnecting that from the chassis (with your new power cord ground reconnected), does that affect the hum?

Are you sure your ground in the power outlet is clean? Measure voltage between that and neutral (with the amp unplugged from the outlet). (I occasionally play bass in a venue where I have a bad hum problem - I measured about 1.5 VAC between neutral and ground there.)
Hum goes away when (the unoriginal) ground lead is disconnected from the chassis. There is a cap from the line to the chassis -- but I'll have to disassemble again to check that.
Thankfully my ground and neutral read '0'.   LOL, and no -- you should never trust wiring in a bar!  Learned that many decades ago . . .

Offline rumpus

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Re: Plug with a ground, or not, that is the question . . .
« Reply #5 on: October 03, 2025, 11:10:50 pm »
I'm just going to "downgrade" to a new old-style ungrounded plug like it had when manufactured, as there is no hum in that condition.   
If it ain't broke, don't fix it :laugh:  It's probably a good assumption that the Marantz engineers knew what they were doing.

There is a cap from the line to the chassis -- but I'll have to disassemble again to check that.
Any chance that's now between the hot wire and the chassis? That would introduce a small current flow through the chassis to the new ground connection.

Offline acheld

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Re: Plug with a ground, or not, that is the question . . .
« Reply #6 on: October 04, 2025, 10:38:30 am »
There is a safety rated cap.  I don't recall which wire(s) it was attached to . . .  but will check it out when I get home next week. 

Shooter, as usual, has got the right idea.  I'm going to stick to the schematic. 

Offline pdf64

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Re: Plug with a ground, or not, that is the question . . .
« Reply #7 on: October 04, 2025, 01:07:53 pm »
...There is a cap from the line to the chassis . . .
My understanding is that such a cap is intended to be between neutral and the chassis 0V common.
With a ground switch, the switch can be flipped to obtain the lowest noise setting, ie cap between neutral to chassis. Or if neutral was noisy, the cap could be taken out of circuit in the middle setting.

With no ground switch, the plug would need removing and flipped around to check which was the lowest noise orientation.
https://www.justgiving.com/page/5-in-5-for-charlie This is my step son and his family. He is running 5 marathons in 5 days to support the research into STXBP1, the genetic condition my grandson Charlie has. Please consider supporting him! BBC News feature  https://www.bbc.co.uk/news/articles/cm26llp

 


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