[AMRadio] Ongoing Quest: Legal Limit AM Amp

[Warren Elly]

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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