[AMRadio] Ongoing Quest: Legal Limit AM Amp

Tue Jan 24 08:02:50 EST 2012

Hi Rick: an AM linear is simply put, efficiency modulation.  IE Low eff. at 
carrier level and high at the peaks.    Exactly the same if it was an audio 
amplifier, when the drive is reduced with the same loading the output level 
falls, it is the mathematics of the system.    Now, if when the drive is 
reduced, one also reduces the loading, the eff. will rise. But, That is NOT 
a linear.

 A class C amplifier would be a littler easier to properly set up and only 
slightly more efficient when total power consumed is concerned.     Total 
over all efficiency is  is a non issue with an amateur amp.  It would only 
be a few Cents per day at best or worst depending on your viewpoint.

Grounded grid can be utilized with any type of amplifier, class A, B, C, 
push pull, Parallel or what ever.  some are just a little easier to build.

If you can buy an existing linear for $600, do it, if you think you have to 
have "legal" power, as you will have that much or more in a home built.  I 
have plate transformers here that cost more than $600.  Just get one of 
those old Heath amps with 3-500s and put much larger fans under the tubes, 
read Eimacs application notes.    You will also have to reduce the output of 
the DX100 down to about 20 or 30 watts carrier to drive those amplifiers. 
Now for the bad news, your DX100 probably can produce a little over 100 
watts of carrier, moving to what some people consider to be legal power, for 
the sake of argument lets say 400 watts of carrier, that will only be 6DB, 
one S unit!!  normal fading that I see is likely to be 20 DB, three S units, 
or 100 to 1 ratio.  In short, if you had someone listen to the DX100 and 
then went to the "legal" amp, someone with normal band conditions would not 
really be able to tell the difference.

The  best thing for you to do is use the best antenna, that to me means 
center fed, open wire feeders to a 1/2 wave at the lowest frequency.  Just 
about anything else is inferior to that, especially the so called multi band 
dummy loads sold as antennas.     In addition go through the audio sysem of 
the DX100, reducing distortion, improving frequency respose, etc.  use a 
good compressor to hold the modulation level high.    That will give you a 
far better signal than someone running " legal" power to a G5RV or using one 
of the tuners? with a balun on the output.

The next thing may cause you some consternation, but it is not meant that 
way.   If you have to ask for a schematic to build one of the simplest 
pieces of ham gear, Please do not jump in and start messing around with 
anything with high voltage.  You mention 3,500 Volts, That will not shock 
you, it will simply flat ass kill you!!!!!!!!!!!     Bernie

  ----- Original Message ----- 
From: "Rick Poole" <wa1rkt at arrl.net>
To: <amradio at mailman.qth.net>
Sent: Monday, January 23, 2012 11:07 PM
Subject: [AMRadio] Ongoing Quest: Legal Limit AM Amp

>
> 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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