[AMRadio] Legal limit AM amplifier, homebrew

[Rick Poole]

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