[AMRadio] Ongoing Quest: Legal Limit AM Amp

Tue Jan 24 09:47:04 EST 2012

nowhere does it say a 375W carrier
----- Original Message ----- 
From: "Warren Elly" <w1gud4 at gmail.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Tuesday, January 24, 2012 8:45 AM
Subject: Re: [AMRadio] Ongoing Quest: Legal Limit AM Amp


> Me thinks the gentleman needs something less than a dx-100...
> You're not going to see much difference between a barefoot 100 and 375 
> watt carrier allowed by FCC
>
> 73, Warren
> W1GUD
>
> On Jan 23, 2012, at 11:07 PM, Rick Poole wrote:
>
>>
>> Looking for a good design for a legal limit amplifier for my DX-100
>> (looks like I'll be homebrewing it).
>>
>> I've pretty much narrowed down the choice to a linear amp capable of
>> 375 watts carrier output or 1500 watts PEP output at 100 percent
>> modulation, vs a Class C amp with high level modulator driven from
>> the DX-100's audio output.
>> .
>> I was thinking about a pair of 3-500's in Class B push-pull.
>>
>> Everybody keeps telling me that I'll get maybe 30 percent efficiency
>> at best during dead carrier times.  At 100 percent modulation and
>> class B I should get 60 percent at worst.  I really, Really don't
>> understand that at all... seems like if I can get 60 percent at 1500
>> watts PEP I should be able to get 60 percent at 375 watts dead
>> carrier.  But, I'm accepting it for the moment, and anyway, I digress...
>>
>> 375 watts unmodulated carrier output at 30 percent efficiency means
>> 1250 watts input, or 875 watts dissipation or 437.5 watts per tube.
>>
>> 1500 watts PEP at 60 percent efficiency means 2500 watts peak input
>> or 1000 watts dissipation, 500 watts per tube peak.
>>
>> So it seems to me like a pair of 3-500's should be able to cut it,
>> especially if I give them plenty of air.
>>
>> I want to stick with glass tubes because I'm planning to put them
>> behind a window so I can "feel the glow" during operation.  Anyway,
>> it'll impress the living crap out of the XYL :-).  Hence, the
>> ceramics have been ruled out for now.
>>
>> So, a few questions...
>>
>> 1.  Will a pair of 3-500's in Class B push pull really handle the
>> load with no problem, assuming adequate air?
>>
>> 2.  This may be a rookie question, but google hasn't given me a clear
>> answer... is there such a thing as class B push pull grounded grid?
>>
>> 3.  There are some commercial class AB pair-of-3-500 amps (e.g.
>> SB-220, TL-922) that are limited in what they can do by the power
>> supply.  So, I plan to make sure the power supply can easily handle
>> the load.  Any suggestions on the best places to get power
>> transformers that are up to the job?  So far I haven't found one
>> under about $600.
>>
>> 4.  I'm actually not that certain that I'm competent to design
>> something like this from scratch.  Any plans available on the net
>> that I can use for a starting point?
>>
>> 5.  Current tentative plans call for the power supply to be separate
>> from the RF deck, and they may be separated by as much as 6 feet
>> (there are compelling reasons for that that go way beyond the scope
>> of this post).  Is that practical?  Where is a good source for high
>> voltage (3500 VDC) cable and connectors?
>>
>> 6.  As noted above, I plan to put the tubes behind a window so I can
>> see them.  Is there a glass that can be used for the window that will
>> maintain shielding?
>>
>> Thanks...
>>
>> Rick WA1RKT
>> Londonderry, NH
>> Catch me on 14.300 MHz Intercon / MMSN most days
>>
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