[AMRadio] Ongoing Quest: Legal Limit AM Amp

[Paul Baldock]

Read this

http://www.w8ji.com/amplitude_modulation.htm


- Paul


At 08:07 PM 1/23/2012, 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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