For point of clarification on the Alpha 77 series, I had forgotten that the
120V blower is actually in series with the plate supply primary such that
the blower forces more air as the load increases. The blower is provided
with reduced operating voltage of ~ 75V by the use of a 50V plate primary
winding in a "bucking" configuration. That explains a lot concerning
regulation of Ep for both the 77Dx and Sx models.
So, unless one wants to modify the 77 series with a constant rotational
blower (not necessarily a bad thing), the only solution to the safety
dilemma is to use 4-wire service to the amp.
Paul, W9AC
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: <jim@audiosystemsgroup.com>; <amps@contesting.com>
Sent: Monday, April 04, 2011 1:22 PM
Subject: Re: [Amps] what is a 4 wire 240VAC service?
> Right, Jim. I should have placed a caveat that it's the only right way
> without modifying the amp. Knowing most Alpha owners, they would already
> scoff at what I've done to my 77Dx amp!
>
> Paul, W9AC
>
>
>
> ----- Original Message -----
> From: "Jim Brown" <jim@audiosystemsgroup.com>
> To: <amps@contesting.com>
> Sent: Monday, April 04, 2011 12:44 PM
> Subject: Re: [Amps] what is a 4 wire 240VAC service?
>
>
>> On 4/4/2011 9:03 AM, Paul Christensen wrote:
>>> So, left unmodified, older amps with 120V blowers are still a safety
>>> concern
>>> if they are serviced by only 3-wires. There's only one right way: Pull
>>> 4-wire, 240/120v service into the shack with proper conductor and
>>> breaker
>>> size, and replace the 3-wire power cord with a 4-wire power cord. The
>>> latter part is easy with Alpha 70/77 series amps.
>>
>> There's another very good way to do it, as practiced by Ten Tec in the
>> Titan 425. The 120V blower runs across one half of the primary of the
>> high voltage transformer, and the center tap is not bonded to the
>> chassis. There's no good reason to bond the center tap to the chassis,
>> and in this case, it must NOT be bonded, because that would cause part
>> of the fan current to flow on the green wire.
>>
>> This configuration is shown in one of the Power Point slides that are
>> part of my presentation on Ham Interfacing, also on the website.
>>
>> 73, Jim Brown K9YC
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>
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