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

["Rob Atkinson, K5UJ" <k5uj@hotmail.com>]

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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