Neutral wired to chassis, how to add a 3-prong cord? - diyAudio
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Old 9th March 2013, 11:51 PM   #1
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Exclamation Neutral wired to chassis, how to add a 3-prong cord?

Just picked up a nice little 60's amplifier, and noticed it had the dreaded unpolarized, ungrounded plug. I retrofitted a nice noise-filtered receptacle for a 3-prong computer/pro style plug. Thats when I notice that the power switch connects a hot power lead to ground through a ceramic cap.

Questions:

1. As long as the polarity is correct, and I wire chassis to ground, is this safe?
2. Will I wreak havoc with other grounded things if I plug this in? I seem to recall there being a good reason that Neutral was not directly connected to Ground. Is the cap doing enough?
3. Other parts of the circuit seem to pull power from a chassis connection. Would it be wise to just run the neutral wire to these and disconnect everything from chassis, and run chassis to ground or am I likely to run into issues?
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Old 10th March 2013, 12:07 AM   #2
JMFahey is offline JMFahey  Argentina
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1) PLEASE CONFIRM, DOES YOUR AMPLIFIER HAVE A POWER TRANSFORMER OR NOT?
DO NOT CONNECT IT AGAIN TO THE WALL SOCKET WITHOUT ANSWERING FIRST.
THANKS.

2) "a 60's amplifier" does not tell much.
Is it tube/solid state?
Brand/model?
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Old 10th March 2013, 12:14 AM   #3
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1. Yes, there is a power transformer. The power is tied to ground before the transformer.

2. It's a no-name tube amp. Looks a bit too nice to be hand-built, but I could be wrong. The branding may have been on the original cabinet, but that is long gone, and i see no chassis markings on the simple folded sheet steel. The single-sided circuit board is laid out like an old Binding post style, with all components arranged neatly facing one direction, with the rest of the construction being p2p. Has some nice old rca 6l6's in the power section.

Last edited by MereTenacity; 10th March 2013 at 12:16 AM.
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Old 10th March 2013, 12:21 AM   #4
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FYI, other than watching the seller quickly demo that it made noise, it has not been plugged in. When I noticed the sorry state of the original power cord, i decided a new grounded setup was in order as to not blow anything unlucky enough to be plugged in when the non polarized plug ended up the wrong way. I also plan on replacing the cap can.
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Old 10th March 2013, 12:28 AM   #5
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I don't know if it matters, but it appears to have a SS rectifier with adjustment pot, tied to chassis.
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Old 10th March 2013, 01:41 AM   #6
Johno is offline Johno  Australia
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If you have to ask these questions it means you should not be the one doing the upgrade, you are likely to kill either yourself or someone else in your household. Find a friendly licensed electrician instead, should only cost you a case of beer.
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Old 10th March 2013, 01:45 AM   #7
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Thanks for the unhelpful reply.

I understand many people who post here are idiots, but treating me like one just makes you look like an a**hole.

Did someone forget the name of this forum?

And please don't bother replying, that was a rhetorical question.
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Old 10th March 2013, 01:58 AM   #8
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If my last message came off harsh, please consider:

1. I came here looking for DIY info. This board is DIYaudio. Seems pretty simple.

2. If you know enough to accuse me of not knowing enough, you are deliberately depriving me of information that could make the amp safe(er).

3. Please consider the following from the rules page:
" While most projects on this site deal with electricity and construction which inherently involve some risk, particularly dangerous topics and procedures should include a warning in the thread that adequately explains these risks. Certain inherently dangerous topics are not allowed. At this time they include but are not limited to: discussing power supplies directly fed by mains current without a transformer, and mucking about in CRT video monitors. Posts and projects are those of individual members of diyAudio. The forum itself is not in the business of vetting projects or posts for safety, accuracy, performance, reliability, function, or fitness for use. If you attempt to make something and it blows up, or turns expensive parts into charcoal, or just doesn't work the way you were hoping, that's between you and the person posting the project or idea. The forum is merely a bulletin board which allows anyone to post ideas, criticisms, or discussions. It is up to the individual to make the final determination of how appropriate a project is for them to attempt, based on their own experience. "

I think all the posts here have been not only aware of the possible dangers with mains voltage, but specifically to deal with that problem. If everyone got a coy reply of "go see an electrician noob" when they posted, then there would not be a forum in the first place.
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Old 10th March 2013, 03:35 AM   #9
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Quote:
Originally Posted by MereTenacity View Post
Just picked up a nice little 60's amplifier, and noticed it had the dreaded unpolarized, ungrounded plug. I retrofitted a nice noise-filtered receptacle for a 3-prong computer/pro style plug. Thats when I notice that the power switch connects a hot power lead to ground through a ceramic cap.

Questions:

1. As long as the polarity is correct, and I wire chassis to ground, is this safe?
2. Will I wreak havoc with other grounded things if I plug this in? I seem to recall there being a good reason that Neutral was not directly connected to Ground. Is the cap doing enough?
3. Other parts of the circuit seem to pull power from a chassis connection. Would it be wise to just run the neutral wire to these and disconnect everything from chassis, and run chassis to ground or am I likely to run into issues?
The unpolarized plug is a problem. let me explain why. in your discription which is the neutral being wired to ground and an unpolerized plug.....that means...if you flip the plug inadvertently so that the chasis wired to neutral gets the hot side of the AC line and connect this amp to a preamp with a traditional 3 prong ground you will have a dead short, poping the breaker or flashing the gorunds on the RCA terminals. so def, replace the unploraized plug with a polarized one so that the chasis never becomes hot.

from simple to complex, change the plug to a polarized one, use a ground fualt service plug either in the wall or on a power strip, the question of changing the chasis architecture is difficult to answer with out schematics. Also, becareful of using any outlets on the chasis doing so in a hot chaiss senerio will extend the hot chasis to what ever is plugged into the socket.
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Old 10th March 2013, 03:35 AM   #10
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You put in a 3 prong receptacle, so the ground from it should connect firmly to the chassis using a star washer and nut on a bolt. The hot and neutral should go to a double pole switch so both are switched, then the hot to the fuse, and then both to the power transformer.
There may be ground loops/hum issues, but that arrangement will make it 'safe'.
If you can access a variac, you should use it to slowly power the unit up. If you can't find a variac, then google to find the procedure to use a series of ordinary (not cfl) light bulbs.
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