[AMRadio] Legal limit AM amplifier, homebrew

[Rob Atkinson]

Don't focus so much on power with all these calculations for the
mythical "legal limit."

Just get something on the air.  If it is a few hundred watts above or
below the myth limit no one will care one way or the other.  It's only
when you start running 1 KW or more all the time, especially to a
really hot antenna, that you'll annoy people.

If you have a really good antenna you only need 2 or 3 hundred watts
to strap anyway.  If you are like most AMers  and your antenna is
average, i.e. flat top center fed dipole up 50 feet on 75 m., you can
be less concerned.  There are errors in most measuring methods and
loss in tuners and feedlines...500 w. won't blow out front ends the
way some of these guys running 300 w. to high dipoles will.   A clean
powerful AM rig won't torque anyone off--a splattering 5 KW slopbucket
amp will.  I also advise shooting for audio power that is _at least_
equal to carrier power but half again is best.  Don't think about just
making a peak 100% up and down; think in terms of having the head room
to sustain a high average percentage--a high average envelope power is
desirable.  This is why you have broadcast rigs like the BC500J using
a pair of 833As modulating a single 833A.  Loads of audio power.
Anyway, whatever you elect to do, good luck!

73

Rob
K5UJ

On Sat, Nov 12, 2011 at 1:47 PM, Rick Poole <wa1rkt at arrl.net> wrote:
> Good afternoon from a newbie to the AM Radio reflector.
>
> I'm putting together an AM/CW station consisting of a Heath DX-100B
> and a Hammarlund HQ-129X.  At some point I would like to add a
> legal-limit amplifier to the station.
>
> I know a lot of people feed their 100-watt-class AM transmitters to
> class B or AB linear amplifiers but it seems like that is terribly
> inefficient... seems like a legal limit Class C amp with its own
> high-level modulator would be a lot more efficient, and that's what
> I'd like to consider building.
>
> I'm a bit fuzzy on the technical details of all of this (it has been
> 40+ years since I was ever into any of this AM stuff), but if I
> recall correctly...
>
> The legal limit of 1500 watts of output power in CW, at 70 percent
> efficiency (about average for a class C amp) translates to a bit over
> 2100 watts DC input to the final amp stage.  An AM final's PEP input
> power, on the other hand, is nominally four times its average
> no-modulation DC input power (if driven by a sine wave audio signal
> at 100 percent modulation), and so to stay legal the average
> no-modulation DC input would have to be limited to (1500/0.7)/4 or
> about 535 watts.  Thus the DC input power to the final amp would have
> to be switchable between 2100 watts (CW) and 535 watts (AM).  Does
> that sound right?
>
> To provide AM in this scenario at best efficiency would require a
> high-level modulator putting out nominally 268 watts (about half the
> DC input power to the amp)  In this scenario I wouldn't even be using
> the modulator in the DX-100; I'd run the transmitter in CW mode (or
> AM mode with no modulation) and feed the microphone to the audio
> input on the amp.  Am I still on track?
>
> I want this to be completely homebrew (I don't want to use some
> high-power hi-fi stereo amplifier or guitar amp for the audio stage),
> and to the extent practical I'd like it to be something that a ham
> might have built in 1947, the year of my birth (the Hammarlund
> HQ-129X is also from that year).  The ultimate goal is that the whole
> station be typical of that year... I know the DX-100 is newer than
> that but it's what I have right now and can be replaced later with
> something older.
>
> So... what do you all think I'll need for components for all of
> this?  Did they make 4-400's and 4-1000's in that year?  I don't
> think so... so what tube(s) should I be considering for the RF final
> amp and the modulator output stage?
>
> Any ideas where I can find a suitable power transformer and
> modulation transformer for this?
>
> Also, keeping it to something a ham might have built in 1947 means no
> solid state rectifiers in the power supply.  I really don't know much
> worth knowing about the tube-type rectifiers of the day for that
> power level... any suggestions?
>
> Thanks...
>
> Rick WA1RKT
> Londonderry, NH
>
>