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Cheater plug safety

So I recently decided to swap out my subwoofer amp for another one I had to see if it worked better with the driver. It did, but I have run into a slight problem. My old sub amplifier had a captive two prong cord. The new one has a 3 prong cord.

The subwoofer amp resides on a different AC circuit than the rest of the system. (Can’t be avoided)

Although the new amp is an upgrade for the sub, there seems to be a ground issue that is hazing up my transparency and sound stage. The system looses some air when the sub amps ground isn't lifted. There is no hum problem through any speaker per se, but just an overall sonic degradation when the ground is left in place. Using a cheater plug just on the sub amp brings back The transparency and it sounds absolutely beautiful.

I read that this can be dangerous, though. (Then why are these created plugs made?) anyway, I also read that as long as the amplifier with lifted ground is connected to the rest of the system via interconnect , and the other components are grounded, then the amp will use the ground from the other circuit that the other equipment is connected to and you are "safe", and only some very unlikely happenings, such as an internal ac wire becoming loose and touching the chassis and standing in a puddle of water should cause a concern. I even had a parasound 750 amp in my college dorm that even suggested using a cheater plug if needed for hum, sooooooo.........

At any rate I am addicted to the sound at the moment and going back without the cheater plug just sounds dull and fuzzy. The system is resolving and shows just about anything you do to it. Mind you, it’s not a night and day difference that would say that there is something wrong with any of the equipment, just a subtle but very tangible improvement that I definitely like and desire. The equipment is working 100% properly.....

It sounds so much better than the old amp and the transparency and air I’m getting right now are rediculous, but I don’t want to get shocked, but the chances seem to be very slim. Can anyone advise?
audiolover718
jea48
4,310 posts
 

cleeds said:

The NEC is updated every three years and the source you previously cited is 14 years old. Sorry, but using a 15A device on a 20A circuit is not compliant with current code. I don’t doubt, however, that "it is done all the time."

Yes, I know the NEC is updated every three years.

//

From my first response to you when you questioned what I posted to the OP.

Cleeds,

Sorry, you are wrong. It is NEC Code compliant.

2014 NEC 210.21(B)(3) table. Receptacle Ratings for Various Size Circuits.


Page 70-59


That’s right out of my copy of the 2014 NEC edition.

//

As for the link I provided you that is 14 years old, it is exactly the same information as found in the current 2014 NEC. I provided you the link because I thought it would help a layman understand the subject better. I though the author did a good job.


All 15 amp rated duplex receptacles have a pass through rating of 20 amp. In most cases spec grade or better duplex receptacles use 20 amp contacts. The only difference between a manufacture’s 15 and 20 amp duplex receptacle, (for the same style/series number), is the face plate. Example, the 15 amp receptacle will not accept a 20 amp rated plug. It won’t fit.... Just use a flash light and look inside the face plate of a 15 amp duplex receptacle, (neutral side), of a spec grade or better outlet. Good chance you will see the "T" 20 amp contact.

//

As for your Bob Vila link. The only guy that knew what he was talking about is Billhart.

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jetter
2,961 posts
 
Ralph, you mentioned above regarding the Dynaco ST70....
"No- not saying you can't do it, its just that its more complex than that. The amp employs some chassis grounds that would have to be modified. This is the sort of project that should be done by someone that understands Class 1 grounding and also understands the significance of ground loops."

I have not thought much about shock hazards before, except common sense when operating the units, and the person who works on my equipment (when necessary) has never brought up modifying it to prevent same.  Does the chassis ground you mention above lessen the chance of a shock occurring.  I guess I am asking is there anything really to worry about leaving it well enough alone?
jetter
2,961 posts
 
Hi again Ralph,
Sorry to hear about your auto accident.  A little less than two years ago, on my way home from work, I took 1/2 second to turn down the radio in my car.  Looked up and there were headlights one foot in front of my car, in my lane, coming right at me.  A 19 year old girl said she hit "black ice", here in Vermont.  Hang in there, time takes time.
George  
atmasphere's avatar
atmasphere
11,615 posts
 
Thanks!

I guess I am asking is there anything really to worry about leaving it well enough alone?
That amp has been around since the late 1950s. I would not worry about it.

The concern is usually about what happens if the power switch takes a whack and gets shorted to the chassis. The fuse should blow. But the kind of switch on a Stereo 70 is really really hard to damage in that sort of a way!
jea48
4,310 posts
 

jetter said:

In order to make my Dynaco st70 safe to use, I would need to have a three prong plug attached? Easy to have done, but just asking if that is the solution?

Not sure if this is the schematic wiring diagram for your amp.

http://diyaudioprojects.com/Schematics/Dynaco-ST70-Tube-Amp-Schematic.htm

If it is you will notice the 120Vac power line side of the power transformer does not use a noise filter capacitor from either AC power line connected to the chassis. Given the age, if original not restored, to me that is a good thing from an electrical AC leakage to chassis standpoint. Why? Is the male plug on the end of the power cord polarized? Meaning it will only plug into the wall power receptacle outlet one way. If not you would have a 50/50 chance of having the noise capacitor fed from the HOT conductor from the wall outlet. If the capacitor was bad, leaking more AC to the metal chassis than it normally should, the chassis would be HOT with respect to earth ground. Depending on the condition of the old capacitor could/would have an impact on how much current could pass from the chassis through your body to a grounded object, your body might come into contact with.

It only takes a few milliamps for the body to feel the sensation of the current passing through the body. 4ma or 5ma you know you are definitely being shocked. 

So with no noise capacitor on the AC Line, that eliminates THAT possibility of a HOT chassis.

 The 3 wire power cord and grounding type plug would be the safest way to go. But like Ralph said, doing so could create noise problems and possible ground loop hum. You would be safe, but your audio system could sound like crap.

As I posted in an earlier  post, GFCI protection will also protect from a lethal electrical shock. Note I said lethal. A GFCI trips open, (is supposed to), when it senses a hot to ground current flow around 5ma to 6ma.

//

Speaking of noise... Have you ever measured the AC voltage potential from the metal chassis of the amp to the wall receptacle safety equipment ground contact? If not, you should check it.

First the ICs from the preamp need to be disconnected from the amp. The chassis of the amp can not be connected to any earth ground.

* Turn on the amp.

* Measure the voltage from the amp chassis to the equipment ground contact of the wall receptacle. Note the voltage reading.

* Turn off the amp. Reverse the AC power plug  in the wall receptacle.

* Next. You might want to wait about 5 minutes for the electrolytic caps in the power supply to bleed  off before turning the amp back on. Turn on the amp and check the voltage from the chassis of the amp to the wall receptacle equipment ground contact again.

* The lower of the two voltage reading is the correct polarity orientation for the plug. Mark the side of the plug that aligns on the same side as the neutral contact/neutral conductor of the wall receptacle.

What you are actually doing is connecting the AC power hot and neutral lines to the primary winding of the amp's power transformer for the proper polarity orientation. The lower of the two voltage reading can lower the noise floor of the amp and the overall sound from your audio system.

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