[AMRadio] AMRadio Digest, Vol 96, Issue 48

[Donald Chester]

On Fri, Jan 27, 2012 at 7:34 AM, amradio-request at mailman.qth.net wrote:

>> I used to run a Ameritron AL-82 easily at full legal AM limit. That 
>> amp,
>> which uses a pair of 3-500's run at 3800 volts or so with a very 
>> hefty
>> power transformer, could just as easily run 1500 watts PEP output all
>> day long.
> <<<<<
>
> Yup, that's what their specs say.  And, Ameritron has at least one 
> other amp (the AL-1500, single 3CX1500/8877) that should loaf at 1500 
> watts PEP all day and all night.
>
> Interestingly enough, though... I engaged Ameritron in a discussion on 
> that topic over the last several days, and they completely disavow AM 
> operation at anywhere near legal limit with any of their amps... not 
> the AL-82, not the AL-1500, not anything.
>
> In our discussion, they continued to describe their amps in terms of X 
> carrier and 2X PEP, e.g. 400 watts carrier and 800 watts PEP (AL-82) 
> or 600 watts carrier or 1200 watts PEP (AL-1500).  I'm not sure they 
> even understand the concept that on AM, PEP = 4x carrier.
>

Forget that PEP bullcrap, and think in terms of real power when running 
it on AM.

First of all, with AM linear, the carrier output should not exceed 1/2 
the total plate dissipation of the tubes. So if the amp uses a pair of 
3-500s, the total plate dissipation is 1000 watts.  That means the 
maximum carrier output with those tubes is 500 watts.  The amp should 
run about 33% carrier efficiency with no modulation present, so that 
means it would be running a maximum of 1500 watts DC input (1 kw plate 
dissipation plus 500 watts rf carrier output).

So, another consideration is what 100% duty cycle power level the power 
supply would be capable of furnishing.  Typically with a SSB  linear, it 
would fall far short of 1500w continuous duty.  The manual may not give 
any data on AM operation, but see is they give any  recommendation for 
RTTY or CW.  RTTY would be the best figure to go by, since CW does not 
operate continuously at 100% duty cycle.  Fire up the amplifier to the 
tune/CW/RTTYmode, and adjust for maximum power output. Now reduce the 
carrier input  to the amplifier, until the plate current drops to half 
the maximum value.  That should be approximately the  correct power 
level for AM  operation.  Check the DC power input and rf power output. 
The input power should be 1/2 the maxumum value, while the rf carrier 
output should be 1/4 the maximum observed.  Make sure the power input 
minus carrier output is equal to or less than the rated dissipation of 
the  tubes, and that the DC input does not exceed the rated continuous 
duty cycle capability of the power supply.

Now, switch the exciter to AM mode, keeping the above adjustments.  Use 
an oscilloscope if one is available, and see if you can modulate up to 
100% without flat-topping at positive peaks, and that the DC input to 
the final remains steady when modulation is applied.  The plate current 
meter should not jump upwards or downwards when you talk.  Check the 
waveform on the scope and see if it looks normal for AM. Adjust the 
audio level for 100% modulation in the negative direction, and make sure 
there is no flat-topping on the positive.  If there is flat-topping on 
positive peaks, try reducing the carrier level and making appropriate 
re-adjustments of the other parameters, until clean positive peaks are 
achieved with 100% negative peak modulation, with no variation in plate 
current when modulation is applied.

The variables in question include rf drive level, antenna coupling and 
modulation level.  To get a good quality AM signal with a linear, adjust 
those variables for the nearest you can come to the theoretical ideal 
you can come with the tubes in the final and the contunuous duty cycle 
capability of the power supply.

With a pair of 811As, the maximum carrier output power that could be 
expected would be 65 watts.