[AMRadio] power ratings

Gary Schafer garyschafer at comcast.net
Tue Jul 12 22:12:35 EDT 2005

Geoff wrote:
> Donald Chester wrote:
>>> The plate dissipation of the 6146's is the key. Your carrier steady on
>>> should not exceed the dissipation rating. This is about 50 watts if I
>>> remember right which would give you 200 watts PEP. So your power suppl;y
>>> should be capable of about 150 watts continuous and it will handle 
>>> the 200
>>> watts on peaks. I would beef up the fan so that I had a good amount 
>>> of air
>>> passing the glass of the 6146's to keep the seals cool.
>> In AM linear service, the carrier output should not exceed HALF the 
>> total plate dissipation.  A 6146 runs about 25 watts plate 
>> dissipation, so with a pair of them, you should be able to  run 25 
>> watts out.  With 100% modulation in the positive direction, that would 
>> be 100 watts pep.
>> A properly functioning AM final capable of 100% modulation should run 
>> about 33% carrier efficiency.  That means that two-thirds of the input 
>> power is dissipated in the plates of the final, and one third is 
>> delivered as rf output.
>> When the carrier is modulated, the final actually  becomes more 
>> efficient, so the plate dissipation is reduced under modulated 
>> conditions.  The DC input should not vary, so that simply means that 
>> some of the DC input that was being dissipated as heat is now being 
>> converted to rf output in the form of sideband power.
> Is that where sideband energy then is created and therefore exists as 
> long as there's a modulate AM carrier?
> I wasn't in on the big discussion about this, a few months ago.
> ---
> 73 = Best Regards,
> -Geoff/W5OMR

The side band power is created in the low level modulated stage and 
amplified by the linear amp just like the carrier.

However, the total amplifier power operates on the composite signal and 
not individually on each component of the signal. The composite signal 
looks like one signal to the amplifier. With modulation present the 
amplifier is operating at a better plate impedance match to the load at 
that power level. With modulation peaks it is operating at the load 
point where it was tuned to, its most efficient point.

So as modulation increases the load point of the (properly tuned) 
amplifier has a better match and delivers more power out at that higher 
power level (modulation peaks). The efficiency of the amplifier 
increases as load point is approached.

You can readily see how the efficiency changes with power level by 
varying the carrier drive level (no modulation) and calculating input 
power verses output power at different drive levels.

For more insight look at "efficiency modulation" in some of the handbooks.

Gary  K4FMX

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