Hi Tom,
I know you are technical guru and I really value and thank you for
explanation about linear amplifiers.
Well, it is true that my Alpha 8100 was hotter with 750W out as
initially we started RTTY contest in 2008. I was intigued and took user
guide to look something about proper tuning etc. Then I found this in
alpha user guide and drove it to 1500W and all was OK for the rest of
the contest.
Here is what Alpha 8100 manual says.
"If you tune the amplifier for maximum power output and then decide to
operate the 8100
at a power output much different from 1.5 kW, it must be re-tuned for
efficient and RF-clean
operation. Under such “lightly loaded” conditions, the green grid LED
will probably be fully lit.
Note that if you tune the amplifier at 1,500 watts and then simply
reduce power to 1,000 watts,
the tubes are actually going to run hotter than at 1,500 watts. It is
better to tune the amplifier
with close to 50 watts of power, but reduce the loading control to get
1,000 watts."
I guess we had to read the manual first :)
Again, thanks for correcting me about some technical details regarding
clean signals.
73 Danny E73M
On Tue, 6 Sep 2011 07:58:11 -0400, Tom W8JI wrote:
> Not to get into the complaining fun, but some comments are not
> technically accurate and may mislead people trying to have a clean
> signal.
>
> From a purely technical standpoint:
>
>> I ran Alpha 8100 at 1500w for 48 hours in RTTY mode for several
>> years
>> now without problems. Even Alpha users manual states that amplifier
>> has
>> to be driven at design power otherwise it will heat up, which we
>> experienced when tried to operate at 750W.
>
> Not true.
>
> While dissipation doe not decrease linearly at the rate we expect
> with reduced power, it does not run hotter at less power. That is
> just
> a silly idea.
>
> Typically **without retuning**:
> 2500w input/ 1500 w tube output / 1000 watts heat
> 1250w input/ 375 watts tube output / 875 watts heat
>
> Other heat like tank heat or power supply is linear, half power =
> half heat except in the output device.
>
> With retuning then half power = half heat.
>
> Tube heat does not go down as fast as we sometimes expect, but it
> always goes down. If it does not, then someone has a serious design
> problem or they do not understand their amplifier.
>
>> Everybody with some amplifier building knowledge is familiar what
>> happens when you run amplifier at 1500w designed with output filter
>> for
>> 5+ kW. It is not properly tuned and splatter is one of the products.
>
> Not true.
>
> First, the output filter has NOTHING to do with splatter. It only
> affects harmonics that are out of band.
>
> Second, splatter is more often caused by people who turn a 100 watt
> radio up to 120 watts or more to make a marginal amplifier run more
> power or who do not know how to tune an amplifier.
>
> Running a larger amplifier at lower power very rarely makes IM3 or
> high worse. Usually it is the opposite.
>
> I know of many Hams who get inside FT1000's and other radios and turn
> things up to get a few dB more power. This is where the problems
> really are, and in people who do not know how to load an amplifier up
> properly.
>
> Most of the equipment and amplifier damage here, and I am sure bad
> signals and damage everywhere, comes from people trying to squeeze
> that extra drop.
>
> One of the stupidest most selfish things people do is to turn the
> power limit controls up inside radios. They are also usually the
> biggest complainers about other people.
>
> 73 Tom
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