[AMRadio] Legal limit AM amplifier, homebrew

Sat Nov 12 18:40:06 EST 2011

Hi Rick: if you really want full CW power ( 1500W) then your tube selection 
becomes more narrow, unless you are willing to run four in push pull 
parallel. A pair of 833As is an easy choice or the 304Ts, 450 Ts will do it 
but are really hard to find.   you will need 2500 to 3 KV for CW. These are 
triodes and you have more than enough power to drive them, at least on the 
lower bands.   Getting them higher than 20 meters will require a little 
creativity but can be done.  The 304Ts can be run efficiently at voltages 
down to about 1500, but they do perform better at higher voltages.  My 833s 
run at 2800 and take about 50 watts of drive.    Best performance will be 
achieved by running push pull with criss cross neutralization and plug in 
coils if you want to change bands, you will have to wind the coils.   If you 
never intend to exceed the so called AM limit a linear is probably the 
easiest, no big mod transformer required and no need to change the voltage. 
Just run them at the suggested range and get around 300- 400 watts of 
carrier. a bit harder to adjust( quite a bit actually).   Efficiency is 
always in the eyes of the beholder. The actual efficiency of a large linear 
is only a little bit lower when one accounts for the filaments etc of a 
modulator.  certainly not enough to increase your electric bill enough to be 
concerned about. You probably would only have it on for a few hours per 
week. If you were concerned to that extent then you need to design it so 
that the filaments are off when simply tuning around, and who does that? 
Bernie

----- Original Message ----- 
From: "Rick Poole" <wa1rkt at arrl.net>
To: <amradio at mailman.qth.net>
Sent: Saturday, November 12, 2011 2:47 PM
Subject: [AMRadio] Legal limit AM amplifier, homebrew

> 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
>
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