Also called an "Edison Service" The neutral is the center tap of the
240 volt transformer. The 4th wire is a frame ground. Sou you have:
Phase "A", measures 120 volt to Neutral
Phase "B", measures 120 volt to Neutral
Phase "A" to Phase "B" measures 240 volt
Neutral is tied to ground at main panel only.
On 4/3/2011 10:57 AM, Charles Harpole wrote:
> Maybe being in Thailand, where 230vac is supplied with TWO wires, makes me
> ask.... what is 4-wire 240vac service?
>
> Charles Harpole k4vud@hotmail.com
>
>
>
>> From: w9ac@arrl.net
>> To: at6c@bellsouth.net; Amps@contesting.com
>> Date: Sun, 3 Apr 2011 10:35:01 -0400
>> Subject: Re: [Amps] Alpha vs Henry
>>
>> George,
>>
>> If 160m is of interest to you, may want to re-evaluate Henry amps, including
>> the 5K.
>>
>> Concerning mods for the 77Dx/Sx: few mods have been published and either
>> version of the 77 is fine as it is. I own a 77Dx and have avoided a
>> conversion to the Sx for several reasons:
>>
>> 1) Added cost of an 8877 during replacement;
>> 2) Slightly added difficulty in isolating trouble with a pair of tubes. A
>> minor issue;
>> 3) The added class of power really requires changing the RF output connector
>> to a DIN type;
>> 4) Fuse holders should be upgraded;
>> 5) Relatively poor Ep regulation of the Sx version unless an outboard
>> Hypersil transformer is used. Some folks have reported better regulation
>> with a direct- replacement Dahl type although it's a snug fit;
>> 6) The Dx already has way more reserve than needed for most operating modes
>> in American-licensed service.
>>
>> Added together, I just can't justify the change. However, I did make the
>> following changes to my 77Dx:
>>
>> 1) Isolated neutral from chassis ground. As it stands, these Alpha amps are
>> not UL compliant. Although not compliant with the NEC, the NEC is factually
>> a premise wiring standard -- not a product wiring standard. The mod is
>> super-easy since it involves only cutting a jumper on the Cinch-Jones power
>> plug. However, 4-wire service and 4-wire power cord is required. If you
>> have not already added 4-wire 240V service, consider it if you plan on using
>> older Alpha and Henry amps;
>>
>> 2) Elevated the filament return off the chassis. I detest the chassis being
>> used for high-current filament returns -- even more of a concern with the Sx
>> as 20A of current flows in the chassis. It's a personal bias of mine; many
>> Alpha amps have run for decades without issues;
>>
>> 3) Added a filament choke. A side benefit is that there's just enough of an
>> IR drop through the choke as a result of my 250VAC mains to ensure that the
>> filaments run at 5.0V. Before adding the choke, my filament voltage ran
>> above Eimac's specification and I was replacing tubes way too often because
>> of filament issues. High filament voltage combined with many intermittent
>> on/off cycles leads to these failures. Also, after inclusion of the choke,
>> I did not tie the filament to the cathode (e.g., AL-1500) although that
>> could be done if I were convinced of the necessity. I've seen arguments
>> both ways (primarily fault related) from respected amp designers as well as
>> former Eimac engineers. It stays unconnected for now;
>>
>> 4) Added gas discharge tubes to the filament and cathode terminals, and
>> added bypass caps to the filaments connections.
>>
>> Just my $0.02, and I'm sure others will disagree with me on some of these
>> changes. Good thing I'm not writing article for QST!
>>
>> Paul, W9AC
>>
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