[Elecraft] KPA-1500 Power Output Variation

[N4ZR]

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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