KPA-1500 Efficiency As A Function of Input Power

What is the Oh Jeeze about? I found K5UA's efficiency data surprising that
the amp dissipates nearly the same amount of heat regardless of power
level. Your comment was "I really do NOT understand why this data is so
surprising." You obviously know much more about RF amplifiers than I do. I
subscribe to this list to learn. So, my next question was if the heat
generated is approximately the same with all power levels, why do RTTY ops
use lower power. I am assuming that heat is what destroys finals or other
amp components. Perhaps there is a power supply limitation or that tube
amps have a more linear efficiency so lower power really means less heat.
Of course RTTY has a higher duty cycle than CW and SSB, that was not my
question. The RTTY duty cycle is the same whether you run 500 watts or
1500. If the amplifier heating is about the same at both power levels due
to higher efficiency at higher power than why not use the maximum power
for RTTY (assuming that you have an antenna and coax that can handle it)?

John KK9A

Sent via the Samsung Galaxy 7 edge, an AT&T 4G LTE smartphone.




Charlie K3ICH wrote:


Oh Jeeeze.......

Almost ALL amps in current use are NOT rated for continuous power output.

Only the most recent SS amp builders which use devices themselves rated for
3x or 4X their rated output would dare to specify 100% duty cycle.

Only a few tube type amps are rated at 100% duty cycle too and they also use
severe over-kill in their PA devices, such as a pair of 8877's or maybe
three,  3-500ZG's etc.

I would venture to say that 85% of the current amplifiers in use today are
NOT rated for 100% duty cycle, so THAT's why they back off on the RTTY
output.


73, Charlie k3ICH




-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at
mailman.qth.net> On
Behalf Of john at kk9a.com
Sent: Friday, August 17, 2018 9:43 AM
To: elecraft at mailman.qth.net
Subject: [Elecraft] KPA-1500 Efficiency As A Function of Input Power

So why do most RTTY users run their amps at reduced power?

John KK9A


From: Charlie K3ICH
Date: Fri Aug 17 09:24:18 EDT 2018


I really do NOT understand why this data is so surprising.
It all seems to me to be perfectly normal with the amplifier's highest
efficiency occurring at near max output.
Which curiously, I would assume,  is the way the amp was designed.

Think zero output with zero drive = zero efficiency.
Apply some drive, read some output and the efficiency goes up from there.

73, Charlie k3ICH




-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at
mailman.qth.net> On Behalf Of john at kk9a.com
Sent: Friday, August 17, 2018 8:57 AM
To: elecraft at mailman.qth.net
Cc: charles at k5ua.com
Subject: [Elecraft] KPA-1500 Efficiency As A Function of Input Power

Interesting data, Charles.  I assumed that running the KPA1500 (or KPA500)
at lower power would keep the fan from running as much. I knew that
efficiency changed with output power but I did not expect that it was this
significant. If your data is correct there is 1000++ watts of heat to
dissipate no matter what power level you use.

John KK9A

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On 8/17/2018 10:27 AM, [hidden email] wrote:

> What is the Oh Jeeze about? I found K5UA's efficiency data surprising that
> the amp dissipates nearly the same amount of heat regardless of power
> level. Your comment was "I really do NOT understand why this data is so
> surprising." You obviously know much more about RF amplifiers than I do. I
> subscribe to this list to learn. So, my next question was if the heat
> generated is approximately the same with all power levels, why do RTTY ops
> use lower power. I am assuming that heat is what destroys finals or other
> amp components. Perhaps there is a power supply limitation or that tube
> amps have a more linear efficiency so lower power really means less heat.
> Of course RTTY has a higher duty cycle than CW and SSB, that was not my
> question. The RTTY duty cycle is the same whether you run 500 watts or
> 1500. If the amplifier heating is about the same at both power levels due
> to higher efficiency at higher power than why not use the maximum power
> for RTTY (assuming that you have an antenna and coax that can handle it)?
>
> John KK9A
>
> Sent via the Samsung Galaxy 7 edge, an AT&T 4G LTE smartphone.
>
>
>
>
> Charlie K3ICH wrote:
>
>
> Oh Jeeeze.......
>
> Almost ALL amps in current use are NOT rated for continuous power output.
>
> Only the most recent SS amp builders which use devices themselves rated for
> 3x or 4X their rated output would dare to specify 100% duty cycle.
>
> Only a few tube type amps are rated at 100% duty cycle too and they also use
> severe over-kill in their PA devices, such as a pair of 8877's or maybe
> three,  3-500ZG's etc.
>
> I would venture to say that 85% of the current amplifiers in use today are
> NOT rated for 100% duty cycle, so THAT's why they back off on the RTTY
> output.
>
>
> 73, Charlie k3ICH
>
>
>
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net <elecraft-bounces at
> mailman.qth.net> On
> Behalf Of john at kk9a.com
> Sent: Friday, August 17, 2018 9:43 AM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] KPA-1500 Efficiency As A Function of Input Power
>
> So why do most RTTY users run their amps at reduced power?
>
> John KK9A
>
>
> From: Charlie K3ICH
> Date: Fri Aug 17 09:24:18 EDT 2018
>
>
> I really do NOT understand why this data is so surprising.
> It all seems to me to be perfectly normal with the amplifier's highest
> efficiency occurring at near max output.
> Which curiously, I would assume,  is the way the amp was designed.
>
> Think zero output with zero drive = zero efficiency.
> Apply some drive, read some output and the efficiency goes up from there.
>
> 73, Charlie k3ICH
>
>
>
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net <elecraft-bounces at
> mailman.qth.net> On Behalf Of john at kk9a.com
> Sent: Friday, August 17, 2018 8:57 AM
> To: elecraft at mailman.qth.net
> Cc: charles at k5ua.com
> Subject: [Elecraft] KPA-1500 Efficiency As A Function of Input Power
>
> Interesting data, Charles.  I assumed that running the KPA1500 (or KPA500)
> at lower power would keep the fan from running as much. I knew that
> efficiency changed with output power but I did not expect that it was this
> significant. If your data is correct there is 1000++ watts of heat to
> dissipate no matter what power level you use.
>
> John KK9A
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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[Richard Ferch-2]

KK9A wrote:

"...if the heat generated is approximately the same with all power levels,
why do RTTY ops use lower power..."

First, the heat generated is not actually approximately the same with all
power levels. To a first crude approximation, the heat generated is made up
of two components. One component is due to such things as biasing for
linearity, and is approximately constant whenever the amp is in transmit
(it's still non-zero, but quite a bit smaller, when the amp is in standby),
and the other component is related to the actual signal being generated,
and is approximately linear with output power. At low output powers, the
first component dominates, while at higher powers the second component
becomes significant. The power consumption efficiency goes from zero at
zero power (some mains current being used but generating zero output) to a
maximum when the signal-related component is largest compared to the
constant component, i.e. at maximum power.

Second, what is dissipated is the accumulated heat energy, not power. Heat
removal processes are far slower than output power changes, so effectively
they integrate the thermal power generation over a significant time
interval. If the instantaneous power is the same in two modes, but the
duty-cycle factor for one mode is twice the duty factor for the other mode,
then the signal-related component of the energy to be dissipated in a given
time period will be twice as high for the high-duty mode. The total energy
to be dissipated will not be as much as twice as high because of the
constant component, but it will still be higher in the higher duty cycle
mode. Depending on the thermal design of the amplifier, that may or may not
be enough of an increase to require derating.

73,
Rich VE3KI
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