Not necessary to guess - Page 17 of the KPA-1500 manual discusses ALC.
"ALC is a useful safety mechanism, but do not use ALC as the principle
[sic] power control. ...Elecraft recommends using the K3 or K3s
PWR-SET-PER-BAND function to set proper drive levels, not ALC.
73, Pete N4ZR
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On 8/25/2019 9:18 AM, Bob McGraw K4TAX wrote:
Has anyone actually used, observed, and measured the effect of the ALC
feature of the amp in combination with the K3/K3S with regard to
output power variation.
I do use the method and setup as suggested in the manual for both the
KPA500 and K3S with no known ill effects. The amp drives to rated
output with 20 watts on all bands. I find it never need more drive
that this for 160M - 6M. Output power variation with temperature is
less than 10% as measured over the range of 23°C to 60°C. Fan
control is automatic and steps up and down as needed.
I do view if one hasn't used it, observed it or measured it, then "old
opinions" of ALC implementation are not valid for comment.
73
Bob, K4TAX
On 8/24/2019 11:51 PM, K9MA wrote:
Here’s a way to get a worst case estimate of the effect of SWR on
power output. Assume the amplifier has constant VOLTAGE gain and an
output impedance much smaller than 50 Ohms. A 1.3:1 SWR means the
load impedance could be as low as 39 Ohms or as high as 65 Ohms.
Let’s say the SWR is 1:1 at the frequency at which the drive is set
for 1500 W output. Then you tune to where the SWR is 1.3:1. At that
SWR, the output could be anything from 1150 to 1950 W. The range may
be less, as the amplifier output impedance may not be all that low
compared to 50 Ohms. In any case, this shows that the power output of
a solid state amplifier may be very sensitive to load SWR.
This is why the KPA1500 ATU really needs to be able to keep the SWR
low. The recently improved autotune firmware does this very well, if
you “train” it across the band. It is also why it’s important to use
the ATU, even if the antenna SWR is “only”, say, 1.5:1.
I keep hoping Elecraft will come up with a way to automatically
adjust drive power to maintain constant output. That would also
compensate for change in temperature.
73,
Scott K9MA
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