[AMRadio] AM linear operation

[D. Chester]

> That's a terrible combination. The Warrior has four 811A's. That will
> give you about 130 watts of carrier if you stay linear. You can get over
> 100 watts out of the Apache without the amplifier. Exactly what are you
> gaining using the Warrior?
>
> Darrell, WA5VGO

Correct. That combination would not give any perceptible difference in 
signal strength over the bare Apache.

A  rule of thumb is that for 100% modulation capability, the carrier output 
is limited to half your total plate dissipation.  The dissipation of an 811A 
is 65 watts, so 4 of them give you a  total of 260 watts. Half that is 130 
watts.

You don't want to use up all the amplifier's output capability to run the 
carrier; you have to keep the carrier output low enough to allow enough head 
room to accommodate the positive modulation peaks. Running the carrier at 
that reduced level results in about 30% carrier efficiency, while 2/3 of the 
DC input is dissipated as heat in the tubes. It is this 100% duty cycle heat 
dissipation that limits the output capability of the AM linear.  The maximum 
pep output and the peak efficiency are exactly the same as with SSB.

The efficiency of a linear amplifier is a function of the amplitude of the 
signal, up to the saturation point, which is about 60% maximum.  At zero 
signal, the efficiency is zero.  You have some DC input from the static 
plate current of the tubes, but zero output.  The amplitude of the carrier 
should run midway between zero and maximum, to achieve 100% modulation. At 
that point, the efficiency is midway between zero and 60%, or about 30%.

With an AM linear, the plates actually cool down a little when modulation is 
applied.  When operated properly, the DC input does not vary with 
modulation.  The plate meter should stand perfectly still, but in  reality a 
slight wiggle is not uncommon.  With modulation, you still have the same 
steady carrier output, plus the upper and lower sidebands.  That extra 
energy has to come from somewhere.  It comes from the increased efficiency 
at the output stage that occurs when modulation is applied.

Don k4kyv

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