When reducing power you should calculate input power verses output to find
plate dissipation. What most people do is just back down the drive to reduce
power out on the amp. Well that drops the efficiency when you do that if you
don't retune the amp.
Let's say at full power out the efficiency is 60%. At 1/4 power out the
efficiency will be around 30%. Half power will be somewhere around 45%. So
if you reduce power down to 750 watts out from 1500 you have not reduced
plate dissipation by much at all.
At 1500 watts out with 60% efficiency, input power will be 2500 watts. That
is 1000 watts plate dissipation.
At 750 watts out with 45% efficiency input power will be 1666 watts. That is
916 watts plate dissipation. Almost what you had at full output. The tube
will get almost as hot as at full power out.
Now if you retuned for max output with reduced drive to bring the efficiency
back up to 60% that would drop the input power to 1250 watts and plate
dissipation to 500 watts. Half the heat by just retuning. Still 750 watts
out.
Retuning reduces the input power and also the load on the power supply too.
More heat saved.
But you have to be very careful if you retune. Don't forget that you did so
and later crank up the drive again with out retuning for higher output or
you will fry the grid and maybe have an arc in the plate circuit because of
the under loading.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Al Kozakiewicz
> Sent: Wednesday, April 06, 2011 8:39 PM
> To: 'Roger (sub1)'; amps@contesting.com
> Subject: Re: [Amps] Alpha vs Henry
>
> Thanks.
>
> You've reminded me that there is a blower speed selector in the cabinet
> and I should definitely make sure that it's running at the max for RTTY
> contests.
>
> 73
>
> Al
> AB2ZY
>
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Roger (sub1)
> Sent: Wednesday, April 06, 2011 6:23 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Alpha vs Henry
>
> On 4/6/2011 5:05 PM, Al Kozakiewicz wrote:
> > Can anyone tell me how much power an AL-1500 is rated to produce at a
> 100% duty cycle?
> >
> > During RTTY contests and outputting about 1000 watts, the tube side of
> the cabinet gets mighty warm.
>
> It should. Running 100% duty cycle and 1500 watts out at 60% efficiency
> means you are dissipating 1000 watts. That is a LOT of heat. The temp
> of the exhaust air depends on how much air (CFM @ however many inches of
> H20) the fan can push. The 8877 is a relatively small tube for its
> ratings, but it has a fairly efficient radiator. OTOH that means a
> blower that can produce a fair amount of back pressure is required to
> get full output from the tube.
>
> As a comparison, when I had an all electric house I heated the basement
> and ham shack with 4400 watt space heaters mounted in the walls. In the
> dead of winter it only took a few minutes to heat the 13 X 21' den to
> about 70 degrees. After that the heater would only kick on a few times
> for only a few minutes per hour. That means your amp running under these
> conditions puts out enough heat to require opening the den door to keep
> a reasonable temp. IOW you could heat a good sized room with that amp
> on RTTY.
>
> > The power supply side seems cool - but then, that's with the cover on.
> I don't know how hot is too hot and it would be useful to know at least
> how much continuous power the supply is rated for.
>
> If you were running large glass tubes that depend on radiation that
> cabinet would be too hot to touch.
>
> 73
>
> Roger (K8RI)
>
> > Tube cooling is probably another issue...
> >
> > Al
> > AB2ZY
> > _______________________________________________
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> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
>
>
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