[TenTec] AM with Centurion

["Rob Atkinson, K5UJ" <k5uj@hotmail.com>]

The Centurion is rated at 1.2 kw pep for SSB and 1 kw for cw.   On AM, I'd 
run it at 250 watts just to be on the safe side (particularly in the 
beginning) and kind of watch the plates and check the heat coming out if 
you're doing a long transmission.  I think Ten Tec specs it at around 10 
minutes continous duty for digital at low power output but those modes 
aren't putting out all that sideband energy.  I have no idea how much drive 
you will need for 250 watts.

My experience has been modern transceivers are lousy on AM.  I know of one 
that puts out 25 watts which is understandable (100 w. sideband rig) but 
modulates the carrier to only 60%, perhaps because the PA can't handle more.
AM is pretty tough to do well these days.  By well I mean a solid signal 
with nice sounding full modulation.   The AM guys got the short end of the 
stick when the FCC changed the legal limit and measurement method.  About 
the only way to do it right is homebrew or finding a boat anchor that allows 
for 100% plate modulation of the final amp with a big modulation 
transformer, then you have to deliver a decent clean audio frequency signal 
to the rig.

Everything about AM screams continuous duty.  Running a 375 watt (or maybe 
more outside U.S.) carrier for very long puts you in a different league 
demanding beefed up matching networks and feedline if you have much SWR on 
it.  Power supplies have to be collossus along with cooling.  Modern day 
sideband amps are just too dinky for this.  Now you know why the old full 
gallon transmitters of the 1950s were 6 feet tall racks and had to be on 
concrete floors.  Also, hanging a carrier and modulating it may get you into 
more trouble with the neighbors.

I've tuned across the AM windows and many of the signals I have heard have 
not sounded very good probably because of the aforementioned difficulties.  
Some of them make a clean 3 khz wide SSB signal sound beautiful.   I am not 
an AMer but I can say that if you are willing to make the effort it can be 
extremely rewarding.  You will learn a lot.  Most of the AM guys are very 
very knowledgeable about circuitry--they have to be these days.

Summary:  Modulating a low level stage to drive an amp -- okay for 
infrequent AM operation but not the best if you are going to operate it a 
lot.  You might investigate Class E AM solid state transmitters.  I have 
heard them and they sound like broadcast stations.  Do a google search for 
Class E AM for more information.  One more point:  If you go the boat anchor 
route, I advise not even hauling one home until you have set up a fairly 
complete basic repair bench unless you already have one.
My opinions--yours may (probably will) differ.

Rob Atkinson
K5UJ

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