[Amps] An IM and power question

[Gary Schafer]

I forgot to mention that there is an article by Bill Orr "rating tubes for
linear amplifier service" that was published in Ham radio magazine March
1971, page 51 that deals with IM products at different power levels.
I think that same article became one of the Eimac bulletins.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Gary Schafer
> Sent: Sunday, April 11, 2010 4:12 PM
> To: 'Roger'; amps at contesting.com
> Subject: Re: [Amps] An IM and power question
> 
> 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 at contesting.com [mailto:amps-bounces at contesting.com]
> > On Behalf Of Roger
> > Sent: Sunday, April 11, 2010 4:56 AM
> > To: amps at 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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