[AMRadio] old BC AM FM or TV rigs


John Coleman jc at pctechref.com
Sat Apr 4 14:52:05 EDT 2009


Yep it was 1KW DC input to the RF final amp. That is plate V times plate current. And 100% modulation was not counted as input power.  Many folks believed, including myself, that 100% modulation meant not pinching the carrier off on the scope.  So we all let the positive peaks go to where they may.  As a result the carrier output could be around 880 Watts of carrier and PEP could go very high.  I could hit 7KW PEP output without pinching the carrier.  But with the new rules 1500 Watts PEP output is the limit.  The mythical 375 watt carrier limit is only true for a single sine wave modulating the carrier to 100%.  A carrier 100% modulated by a single sine wave will result in a PEP of 4 time the carrier.  If the carrier is 375 watts then the that would be 1500 watts.  In my case the carrier limit is even less because of the lopsided characteristics of my voice.  That is assuming that I send the lopsided peak positive.  But if I send them negative then I can run 600-700 watts of carrier output and still stay under the 1500 watt PEP limit.  The funny thing about all this is, that the audio power is the same in either case, only the polarity of the audio is different when the microphone wires are reversed.  But I can turn the carrier level up by increasing the plate voltage on the class C final only. (separate power supply for the modulator).  I do this until the downward going peaks on the scope no longer pinch the carrier.  Afterwards, the PEP is still the same but the carrier level is much higher.  As to whether this is good or bad depends on the listener and the type of detector being used.  Some say no difference, some say not as loud as before or doesn't sound 100% modulated.  Others say the extra carrier level helps stomp back the QRN.  All I know is audio power is the same and the power in the sidebands is the same in either case.

I wrote a very detailed article about all of this several years ago but can't find it right now.
John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim WB5OXQ inb Waco, TX
Sent: Saturday, April 04, 2009 12:18 PM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] old BC AM FM or TV rigs

This was a fairly recent Continental 5KW transmitter, all solid state except
for the single ceramic PA tube  maybe a 4cx5000.  I figured little value for
ham so it was sold to another small station.  It would be very difficult to
get a bc rig into my shack but there could be room in the garage for one.
This weekend I have to lower the crankup and repair my 40 meter dipole that
fell in recent high winds.  I listen most evenings on 7160 but often hear
little AM.  I am told when I am on I am very loud in some parts of the
country.  I understand that 375 watts of carrier is all that is allowed on
AM.  I used to think it was 1KW back in the 50s when I used to visit my ham
neighbor.

----- Original Message -----
From: "John Coleman" <jc at pctechref.com>
To: "Discussion of AM Radio in the Amateur Service"
<amradio at mailman.qth.net>
Sent: Saturday, April 04, 2009 12:02 PM
Subject: Re: [AMRadio] old BC AM FM or TV rigs


> Jim:
>        There might have been a lot of cool parts in that FM station.  For
> instance, I was given and old GE 10KW TV XMTR back about 30 years ago.  It
> had a lot of stuff I did not want, but others did, and it had a whole lot
> of stuff that was "way" cool that I wanted.  It had 270 degree 1ma meter
> movement and large precision wire wound meter multiplying resistors.  It
> had several large plate XFMRS and chokes.  It had oil capacitors.  Two of
> the chokes were 12 Hy open frame construction which Don, K4KYV,
> disassemble and rebuilt into a single 40Hy modulation reactor which I am
> very proud to have and I use still today.  There were mercury vapor
> rectifiers and filament XFMRS.  All of these things an more (I can't name
> all) were used in the construction of my PP-304tL rig.  And parts that I
> did not want went all over the South,  Of course you must have an empty
> garage to do all this in and that is where most folks say "no", as they
> might to building their own vehicle from scrap.  I
> f I had it to do now I would have to say "no" because there is just no
> time or space in my life right now.  Hopefully that will change before I
> get to old to walk.
>
> Anyway I hope someone got the old FM rig and used the parts.
> John, WA5BXO
>
>
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