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.
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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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