Ok Roger, I will bite. :>)
We tune our amps for linearity not for efficiency. If we tuned for best
efficiency we would tune them for most power out at the point that average
SSB is prevalent. In the case of a normal 1500 watt amp the average ssb
power out is around 20% or 300 watts. If we peak the amp at that power level
we would have maximum efficiency for the average power out but very poor
linearity.
Tuning the amp up at the 1500 watt level ensures that the peak power can
reach those levels and still be linear. Efficiency at the 300 watt level
will be less than 30%. But the amp will be linear, which is what we want in
order to keep distortion products down.
Being linear means that, if the amp has a gain of say 10 db, then 100 watts
in will give 1000 watts out. 10 watts in will give 100 watts out.
It hurts nothing to tune the amp up at the 1500 watt level and operate it at
the 750 watt level. Efficiency will be a little less but we don't care as it
will still be linear.
As to IM products when the amp is operated at less drive, the overall IM
will probably be lower. However the lowest IM is not always at the lowest
power level of an amplifier. There is often a dip in 3rd order IM products
somewhere in the lower 3rd of the power curve. 5th order products are often
minimum in the upper 2/3 of the power curve.
It depends on the particular tube and how it is operated.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Roger
> Sent: Sunday, April 11, 2010 4:56 AM
> To: amps@contesting.com >> Amps Amps
> Subject: [Amps] An IM and power question
>
> We have been taught to tune up at the power we plan on using, but that
> is more for efficiency than anything else IIRC. But now when we are
> looking at power out and not efficency, there are after all those who
> run class A to get multi KW out, what about IM. Lets say you tune up
> for a very comfortable 1500 out, but for some strange reason decide you
> can get by with just 750, or half that amount. You are already tuned up
> for 1500 out, so all you do is reduce the drive to get the 750 out.
> What does that do to the IM on the output?
>
> 73
>
> Roger (K8RI)
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