[AMRadio] Legal limit AM amplifier, homebrew

Sat Nov 12 22:47:36 EST 2011

Pick up some of the Bill Orr handbooks, or pre Bill Orr.
I have 'the radio handbook, copyright 1947 by editors and engineers'.

If you really want to build something that one would have built back then, 
that is a good book, plus the 1947 ARRL handbook.

813's were around back then, and still are, very robust tubes.
A pair will give over 600 watts of carrier, 2400 watts pep for years and 
years.
400 ma at 2000 volts which is reasonable.

Both push pull swinging link and pie net were popular back then, so you can 
go either way.

For most people, its hard enough to get parts without limiting yourself to a 
time frame.

I like using old tube designs with modern updates, like solid state 
rectifiers, modern meters.
I do not like electrolytic caps and use oil filled, but using tube 
rectifiers is a waste of a lot of space, they get hot, contain toxic waste, 
etc.

Brett
N2DTS

----- 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
>
> ______________________________________________________________
> Our Main Website: http://www.amfone.net
> AMRadio mailing list
> Searchable Archives: http://www.mail-archive.com/[email protected]/
> List Rules (must read!): http://w5ami.net/amradiofaq.html
> List Home: http://mailman.qth.net/mailman/listinfo/amradio
> Post: AMRadio at mailman.qth.net
> To unsubscribe, send an email to amradio-request at mailman.qth.net with
> the word unsubscribe in the message body.
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html 