Cheater plug safety
Probably more so if the amp’s old AC wiring and power transformer are original. What are the chances of a user receiving an electrical shock if an internal hot to chassis fault were to occur? It all depends on the circumstances of where the amp is located and associated equipment it is connected to. It also depends if the amp and all other associated equipment is plugged into the same wall receptacle outlet. Examples where a hot to chassis fault has occurred: First for the user to receive an electrical shock he must place his body, or some part of, in series with the hot chassis and an earth grounded object. In this case the user completes a ground fault circuit path from the Hot chassis through the part of his body that is in contact with the earth grounded object, back to the source the electrical panel grounded service neutral conductor. Current will pass through the user’s body and he will feel the shock. Grounded objects? Is the floor bare concrete? As in a basement? In this case if the user was in his bare feet and touched any metal part of the amp the user would receive an electrical shock. On the other hand if the amp was on an insulated floor like wood he would not. His body would be insulated/isolated from a grounded object. Like a bird perched on an overhead high voltage power line. Back in the day the Dynaco ST70 was first designed what other associated equipment that might be connected to the amp by ICs used a safety equipment ground? Was there even a 3 wire grounding type wall receptacle back then? What other grounded objects, within arms length or other parts of the body, could the user come into contact if he touched the hot chassis of the amp? Say the user has a preamp that uses the wall receptacle safety equipment ground. The outer case/enclosure of the preamp is metal. Maybe the knobs are metal. There is now a grounded object in close proximity of the user for the possibility of the user to come into contact at the same time while touching the metal chassis of the amp. But wait. The signal ground of the ICs will ground the equipment right? Maybe, maybe not.... First I would never rely on the small signal ground wire of an IC to carry the high current of a dead short ground fault back to the source. For a typical 20 amp branch circuit the initial ground fault current can be over 100 amps before the breaker can react and hopefully trips open breaking the ground fault circuit. What if the user buys a new pair of ICs. What happens when he disconnects the second IC from the first RCA jack? Is there a chance his fingers could come into contact with the outer metal shell of the IC and the metal case/enclosure/chassis at the same time? So what if the preamp has a ground lift switch that lifts the signal ground from the safety equipment grounded chassis? The false feeling the IC will protect the user from an electrical shock is gone. Bottom line, just be careful if you are using old vintage 2 wire cord and plug equipment. Especially if the plug is non polarized. When changing ICs unplug the vintage equipment power cord from the wall receptacle outlet first. . |
audiolover, Hopefully the responses posted on this thread answered your questions in your original posted message. Just a guess you will continue to use the ground cheater on the sub amp because it eliminates the ground loop hum. You are not the first to experience a ground loop hum problem with a subwoofer amp, that uses a 3 wire cord and plug, and you won’t be the last. Jensen transformer does make an isolator you could install between the preamp and the sub woofer amp that should/will break the ground loop circuit, thus eliminating the ground loop hum. If you are worried about it degrading the SQ of the sub I doubt with the Jensen isolator you would hear any difference. http://www.jensen-transformers.com/product/sub-1rr/ I would recommend the Jensen transformer isolator over the ground cheater. But if you must use the ground cheater a GFI receptacle in place of your existing wall duplex receptacle would be a safer option. You could add a level of personal protection from any risk of a life threating electrical shock by changing out the existing wall duplex receptacle outlet to a GFI. A GFI receptacle does not need an equipment ground to function/operate as designed. Just buy a good GFI duplex receptacle outlet, not one of those really cheap ones. I have seen the cheap ones fail and not trip open when called upon. Leviton makes a pretty good GFI duplex receptacle. If you do decide to change out the existing duplex receptacle to a GFI replace the old one with a GFI of the same ampere rating. So if the existing is a 15 amp, more than likely the branch circuit is a 15 amp, you must use a 15 amp rated GFI duplex receptacle. You cannot install a 20 amp receptacle on a 15 amp branch circuit. A 20 amp receptacle can only be installed on a 20 amp branch circuit. You can install two or more 15 amp receptacles, (a duplex is two), On a 20 amp circuit though. . |
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 mmm |
Perhaps you should read the code that you referenced: Cleeds, Please go back and reread my post. I said two or more 15 amp receptacles can be installed on a 20 amp branch circuit. A duplex is two receptacles. Trust me it does meet NEC code. It is done all the time in commercial office buildings. Jim |
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.
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. . |
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. . |
but it appears possible that the signal uses a floating ground and that the chassis and power supply ground are not bonded together.Eric, What power supply ground? Equipment today that have a 2 wire power cord and polarized plug, that use double insulated AC power wiring, does not have an earth ground. In the world of this audio equipment the metal chassis and metal enclosure that houses the chassis is it’s ground for the power supply of the secondary and signal ground is all it knows. Am I wrong?
Agree. That could be a possible problem, IF..... Jim . |
