"Uh, afaik, the class C amplifier is THE reason for the
     infamous 30% AM modulation usual in broadcast
    afaik. You simply modulate the plate DC with audio
    and you get AM 30% (they could have chosen up
    to 80% AM without problem otherwise"

Whoa!

Where do you come UP with this stuff Peter?

I've held a First Class Phone License now for over 25 years,
have worked in the field in the broadcast on the transmitter
end and this, as posted above, is simply NOT true!

A few minutes with an O-scope connected to the IF strip
in any AM radio will show you FAR in excess of "30 %
AM modulation".

Your previous observation on the usefulness of class C
amps is vastly understated too. Every 2-way FM land
mobile transceiver ever built over the last twenty years
has used a succession of class C stages to derive the
legal max power of 110 W for a mobile or 330W for a
base station. The handheld radios also use "class C PAs"
as well as a driver stage or two running class C service.

To utilize the PAs (power amplifiers) in amatuer service (as
in boosting the power output from a 20 W PEP SSB rig) from
any of these 'monster' transmiter amps requires adding
"forward bias" to get then to do class AB service ...

Jim  (RF design, test and analysis from DC through Ku Band,
         microwatts through 1000's of watts)


----- Original Message -----
From: "Peter L. Peres" <plp@ACTCOM.CO.IL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, July 12, 2001 12:10 PM
Subject: Re: [EE]: Class C amps


> How is it that AM transmitters use class C?

Uh, afaik, the class C amplifier is THE reason for the infamous 30% AM
modulation usual in broadcast afaik. You simply modulate the plate DC with
audio and you get AM 30% (they could have chosen up to 80% AM without
problems otherwise). If you search the web for homebrew small AM
transmitters (favored by old radio collectors etc) you will find schemes
that exemplify this. The simplest scheme (which I've tried myself) can be
made from parts from a tiny AM radio.

You use the L.O. (complete with variable), substituting the ferrite
antenna for the AM LO tuning circuit (red core coil). You may have to
rewind the ferrite with a tapped coil - standard tap is 0% power 30%
collector 100% tuning cap - i.e. 25+55 turns on a normal ferrite antenna
that works with a 270pF tuning cap, using the original wire). Then you
wire the audio amp output secondary (which must be transformer type) in
series with the collector power of the LO, instead of the IF1 coil
(yellow) if it's a self-oscillating converter. Audio in is at the hot side
of the volume pot. That's it.

Please note that this may be slightly illegal in some countries. Do not
attempt to run high power into this, it will not work. Mine barely worked
across a room on the original 3V batteries. Man this was *years* ago.

Peter

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