The legal limit on AM is 1500 watts PEP, just as it is for all other
modes.
There is no 375 watt carrier "limit" set by the FCC.
But it does work out to 375 watts of carrier when you modulate to 100% on
positive peaks.
You could run more carrier and modulate at a level less than 100% and
still
maintain PEP below or at 1500 watts and be legal. Or you could run less
carrier and modulate over 100% on positive peaks and still maintain PEP at
or below 1500 watts.
A 100% modulated carrier is when the audio (side bands) voltage equals the
carrier voltage. This doubles the composite signal voltage. Doubling the
voltage gives you 4 times the power (PEP). Ohms law gives this to you E^2
/
R = Power.
So if your amp is tuned up for 1500 watts PEP you would drop your carrier
power to 1/4 that value or 375 watts in this case. When you fully modulate
to 100% your peaks will hit 1500 watts.
In this case you do not want to modulate over 100% even if you were to
have
your negative peaks limited to less than 100% as you would overdrive the
amplifier and cause distortion because of flat topping, just like you
would
if you were on SSB and overdrove the amp.
With a plate modulated transmitter you can run over 100% positive peak
modulation (it is common to run 120% positive modulation) with out
distortion as long as the negative peaks do not exceed 100%. This
increases
the PEP along with the average audio power in the signal. This can be done
without changing the maximum plate load as you would have to do with a
linear amplifier.
With a linear amp if you were to tune up to the 1500 watt level and then
drop the carrier down to something less than 1/4 power you could then
modulate over 100% as long as your peaks did not exceed where the amp is
tuned at (1500 watts in this case).
If using your SSB radio in the AM mode and you exceed 100% negative
modulation it will not cause the same type of distortion (splatter) that a
plate modulated rig would. Your signal will be distorted when this happens
but no where as bad as cutting off the carrier in a plate modulated
transmitter.
With the SSB transmitter there is a phase reversal when you exceed 100%
negative rather than a carrier cutoff that you get with a plate modulated
rig which causes sever distortion and splatter
when you exceed 100% negative modulation.
All of the above operation is to have a clean signal and not just to stay
within the 1500 watt PEP limit. You always want your peaks to be below
where
the amp is tuned no matter how much power you are running.
Also remember DO NOT retune the amplifier once you drop the carrier level
down for AM. It must remain tuned at the peak level that you will be
hitting.
Efficiency of the carrier in the AM mode will be about 1/2 of what full
carrier (tune up) efficiency is. If your amp is 60% efficient at full
power
it should be around 30% at 1/4 power of the carrier.
But this is really no different than when you operate SSB. At low speech
levels on SSB the amplifier efficiency is much lower than at full peak
power. With full modulation on AM the amplifier efficiency is 60% too, the
same as it is on SSB.
You should always tune your amp for maximum output power for whatever
drive
power you are using. If the power is going over what you want (1500 watts)
then reduce the drive power.
You never want to limit your tune up by not fully loading the amplifier!
This holds for AM or SSB.
Actually slightly "over tuning" or over loading a little heavier than
maximum output so that the output drops slightly from maximum is ideal as
the amp will not flat top as easily if the speech level is driven a little
harder.
When looking at your signal with a scope your carrier should be 1/2 the
voltage that you see at full power of the amp. When you modulate the
modulation peaks will hit the level that you tuned up at. Remember power
is
E^2 / R. so when the voltage doubles that is an increase of 4 times the
power. Like wise when you reduce the carrier to 1/2 the voltage on the
scope
that is 1/4 the power.
Remember that in the above with 100% AM modulation the voltage doubles
which
is what you see.
Sorry this turned out so long.
73
Gary K4FMX
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Robert
Logan via Amps
Sent: Thursday, October 30, 2014 9:17 AM
To: Kimberly Elmore
Cc: Amps@contesting.com
Subject: Re: [Amps] AL1200 on AM - max carrier?
Thought I might mention the legal limit for AM is 375 watts carrier.
Bob. NZ5A
Sent from my iPhone
On Oct 30, 2014, at 9:04 AM, Kimberly Elmore <cw_de_n5op@sbcglobal.net>
wrote:
Any amp will be inefficient when used as a linear amplifier for AM
because as drive is reduced, efficiency is reduced. They will all
generate a lot of heat in this application. However, the AL-1200 should
hold up well to the lock-and-talk culture. In AM, PEP is 4X the carrier,
Overall, you want the carrier to be at 1/4 of the amp's rated PEP
output.
Assuming the driver generates 100 W output, tune the amp for max or
1500 W output (whichever comes first) with the amp in SSB mode. You want
SSB mode because you want the best linearity. Whether or not you use SSB
or CW makes little difference in the efficiency for your application but
makes a big difference in linearity.
Usually, when the driving rig is in AM mode, the carrier is reduced to
1/4 of the max PEP SSB output. In such cases, you should see no more
than 375 W carrier power (375 x 4 = 1500).
If you are trying to drive it with an older plate-modulated tube rig,
you ill have to be careful if you reduce the power output because the
modulation transformer is designed with a particular output impedance in
mind. If you reduce the drive to the PA, the impedance changes and you
will have to set the proper audio level using a scope or modulation
monitor.Start with audio gain at zero, bringing it up slowly. Otherwise,
you risk arcing in and destruction of the modulation transformer.
Kim N5OP
On Thursday, October 30, 2014 8:05 AM, Gary Smith <Gary@ka1j.com> wrote:
One thing for sure, run it at the CW
setting, use the lower KV setting and you
will run far cooler for a given KW out.
73,
Gary
KA1J
> I want to run my AL-1200 on AM for a short stint (2 hours operating
> time, 50% overall duty cycle). I have tested it at about 225 watts
> carrier output, and with moderately high duty cycle transmissions
(known
> in the AM crowd as 'Old Buzzarding'), it gets quite hot.
>
> What is the max recommended AM carrier power for long-winded
transmissions?
>
> Also, I'm assuming it should be tuned up for proper operation at
double
> the carrier power.
>
> I searched the list finding no real answers.
>
> Thanks,
>
> Tony, K1KP
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