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 Post subject: antenna and ground voltage in tube radios
PostPosted: May Sun 26, 2019 7:35 pm 
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Joined: Feb Fri 27, 2015 3:19 pm
Posts: 54
I was checking the voltage between the antenna and earth ground as well as between the radio ground and earth ground of some of my tube radios which are connected to ac through a surge suppressor. I noticed that reversing the plug of the suppressor into the outlet or the radio plug into the suppressor caused the above voltages to change. In one of the radios the difference caused by reversing the plug was dramatic, from full line voltage to zero. Interestingly, since I can't find a proper earth ground (the cold water pipe does not provide any grounding), I ground the radios to the heat radiator but only when I use them. Voltage was checked with the radios on, one at a time.
I guess it all has to do with whether or not the chassis is hot but why the antenna - to - earth ground voltage in any given radio was equal to its radio ground - to - earth ground voltage?.
Any enlightening please?


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 Post subject: Re: antenna and ground voltage in tube radios
PostPosted: May Sun 26, 2019 11:11 pm 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 9527
Location: Ohio 45177
Well if the antenna is a wire and is connected to the primary winding of the RF input coil, that primary is connected to chassis at one end. You could isolate the antenna wire from the radio with something like a .001uF capacitor or smaller value(down to the point it affects reception) which would pass the radio signals but appear as a high impedance at line AC or DC voltages. Of course rated for several hundred volts, at least. A 500V. or higher ceramic cap or even maybe film cap would not be hard to get.

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 Post subject: Re: antenna and ground voltage in tube radios
PostPosted: May Mon 27, 2019 12:04 am 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 2013
Location: Austin, Texas
As Wazz noted, a 1000pF cap in the antenna circuit will have a high enough impedance that you will not measure any significant 60Hz voltage.
Some radios have that built in and others don't. Nearly all radios have a capacitive coupling from the antenna circuit to the power line to serve
as a ground reference. Often it is a 0.1uF capacitor that can couple about 5mA of AC current into the antenna circuit. That isn't enough to kill you
but you will feel it and it will definitely show up on a high impedance voltmeter. On radios with a non-polarized plug, the 0.1uF capacitor can be
on the hot side of the power line or the neutral side depending on which way the plug is inserted in the outlet. The way most AA5 radios are wired
from the factory, the antenna circuit will have AC coupled to it either with the radio on or with the radio off. The AC coupling to the antenna is
something you have to live with on some radios unless you want to add a polarized plug and rewire the switch or add the 1000pF cap in the
antenna lead.

Jay


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 Post subject: Re: antenna and ground voltage in tube radios
PostPosted: May Mon 27, 2019 6:21 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11803
Location: Powell River BC Canada
Do not ground anything to a heat register. If a fault occurs needing a safety path,
you cannot know where that current will cause a fire.

Like the MGM Grand fire. Look into the report.

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 Post subject: Re: antenna and ground voltage in tube radios
PostPosted: May Tue 28, 2019 2:09 pm 
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Joined: Feb Fri 27, 2015 3:19 pm
Posts: 54
Thanks to all for your help.


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