Re: [Amps] Centurion cooling improvement ideas for AM service

[Gary Schafer <garyschafer@comcast.net>]

Hi Rob,

The old QST article kind of uses voltage and power as one and the same 
in the discussion. He also fails to mention that the power "INPUT" to 
the amp also increases as well as the power out with modulation. If the 
power INPUT did not increase with modulation then amp dissipation would 
probably go down with modulation. But power input does NOT stay 
constant. Even though the plate meter may say it does.

When he says "the carrier is at half level the output is one-fourth the 
modulation peak output", he really should be saying that the carrier 
VOLTAGE output is at half level and that the power output is one-fourth 
peak envelope power out with modulation.

You have to be careful reading some of the old AM stuff as the mindset 
was that what you saw on a scope when looking at an AM signal was 
exactly what was going out on the air. I.e. it looked like the carrier 
went down to zero and up to twice the carrier on mod peaks.

While this is true for the composite AM signal (the sum of the voltages 
in the carrier and the sidebands) it is not what the power actually does.

Looking at the power spectrum on a spectrum analyzer you can see that 
the carrier output stays constant whether it is modulated or not. The 
additional power from modulation appears in the side bands which are 
independent from the carrier.

Put an old thermal couple RF amp meter in the RF output of the AM amps 
antenna line. You will see that the RF current increases with modulation 
from what it was with just carrier. The average power out increases. You 
will not see that on a bird watt meter. It will remain relatively 
constant. The same thing is happening to the input power of the amp that 
you can't see on the plate meter.

The amp is only tuned for maximum efficiency at the very peaks of the 
peak envelope power. (same is true for SSB) At any power level under 
that the efficiency is lower. The bulk of the power (average) which 
causes all the heat is nearer to carrier power levels and at lower 
efficiencies.

Look at the power comparisons again that I posted before and you can see 
the increase in dissipation with modulation even though the efficiency 
increases. It is not a linear increase though.

73
Gary  K4FMX


Rob Atkinson, K5UJ wrote:
> What's your source Gary for this?
>
> Here's mine:
>
> "The a.m. signal, however does not start from zero with no modulation.  
> It works up and down about the carrier level, which is just one-half the 
> modulation up-peak level.  When there is no modulation the tube is still 
> called upon to deliver the unmodulated carrier, and since the carrier is 
> at half level the output is one-fourth the modulation-peak output and 
> the plate efficiency is one-half the modulation-peak efficiency.  
> Actually, the efficiency increases with modulation, but since there are 
> times when a speaker has to stop to catch his breath, the amplifier has 
> to be designed to handle the unmodulated carrier safely.  The tube gets 
> hotter with no modulation than with it -- quite in contrast with the 
> s.s.b. case."-- "Linear Amplifiers for A.M." p. 41, Feb. 1956 QST.
>
>
> Rob/K5UJ
>
>
> <<<Not really so in either case.
>
> A 1200 watt pep out amp with 60% efficiency will have an input power of 
> 2000 watts. That will burn 800 watts in heat with full carrier.
>
> That same amp with an AM signal applied and transmitting 300 watts of 
> carrier with no modulation will have an efficiency of 33% for the 
> carrier. That means the input power will be 909 watts with 300 watts out 
> and no modulation. That will burn 609 watts in heat. With modulation 
> added the efficiency goes up but so does the power. The efficiency at 
> the carrier level still remains at 33% and will still waste 909 watts on 
> its own. Add the side band power of the AM signal to that and the amount 
> of heat only increases.
>
> The amp is tuned and operated the same for AM as it is for SSB. If tuned 
> for 1200 watts pep out, (SSB or AM) The efficiency on SSB at 300 watts 
> out is exactly the same as it is for AM at 300 watts out. The efficiency 
> at the 1200 watt level is also the same for the AM peaks as it is for 
> the SSB peaks.
>
> The only difference is that the AM signal has a higher duty cycle than 
> the SSB signal. That raises the average dissipation higher for the AM 
> signal.
>
> 73
> Gary  K4FMX>>>
>
>
>
> Joe Isabella wrote:
>
> > But that's only if you transmit a full power CW carrier.  A keyed amp 
> > with no drive at all
> > dissapates less than an amp putting out 300W carrier (DC input power), 
> > and less than an amp
> > putting out 1200W PEP, regardless of mode.
> >
> > Joe,
> > N3JI
> >
> > --- "Rob Atkinson, K5UJ" <k5uj@hotmail.com> wrote:
> >
> > > > > SNIP<<<
> > > AM isn't so simple, and there are some paradoxes for those familiar 
> > > with SSB operation only.
> > > One is that an amp in AM service actually runs cooler when the 
> > > driving carrier is modulated and the
> > > amp is boosting sidebands and the carrier; it runs a bit hotter when 
> > > you are not saying anything and it's keyed and txing carrier only.  
> > > since it's tuned for 4x carrier power, efficiency is higher and heat 
> > > dissipation less when getting full voice.  So, don't shut up!  Dead 
> > > air = meltdown hi hi.
> > >
> > > Rob/K5UJ
>
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