[Elecraft] K2: KPA100 fan issue + current question

[Bob McGraw K4TAX]

Not at all unusual for the currents to be different on different bands.

Since most power meters are voltage sensing devices, the value of the 
dummy load will affect the power indication.  Thus as the R value 
increases the voltage across the R increases for a given power value and 
thus the meter will indicate more power than actually exists.   And the 
better power meters are typically +/- 5% full scale, i.e. Bird 43.   
Many ham meters are +/- 10%.   The LP-100 is +/- 3% with NIST factory 
calibration.

As to dummy loads, I have 3 and they all say 50 ohms on the label. In 
fact they measure between 48 ohms and 54 ohms when cold.  The value goes 
up as the element and radiator heats.   While that's not much, in fact 
it is approaching 10%.    That throws another variable in power 
measurements.

I stopped frettin' over power values a long long time ago.

73
Bob, K4TAX



On 11/9/2018 4:56 PM, Jim KO5V wrote:
> Don and Bob,
>
> The K2 and amp are both powered from the amp's supply.
>
> I used a known good DVM to measure the voltage across R7, and it agrees with old Beckman. I now think my old kit-built Ten Tec power meter may be badly out of calibration. I have a dead LP-100 that is on its way back to Larry for upgrade and calibration, so I'll just wait until it is back to finish calibrating the KPA's power setting.
>
> Here are what I measured and calculated at the approximate center of each band, in cw, when the K2's LCD read 100W:
>
> 10M: .067V, 13.4A
> 12M: .049V, 9.8A
> 15M: .064V, 12.8A
> 17M: .065V, 13A
> 20M: .048V, 9.6A
> 30M: .056V, 11.2A
> 40M: .052V, 10.4A
> 60M: .056V, 11.2A
> 80M: .054V, 10.8A
>
> The current seems to vary significantly between bands, but maybe that's normal?
>
> Thanks again. 73,
>
> Jim KO5V
>
>
>
> -----Original Message-----
>> From: Don Wilhelm <donwilh at embarqmail.com>
>> Sent: Nov 9, 2018 11:37 AM
>> To: Jim KO5V <ko5v at earthlink.net>, Bob McGraw K4TAX <rmcgraw at blomand.net>, Elecraft List <elecraft at mailman.qth.net>
>> Subject: Re: [Elecraft] K2: KPA100 fan issue + current question
>>
>> Jim,
>>
>> That 11 amps would sound more reasonable to me if you can say that the
>> base K2 is being powered separately, but I would expect something closer
>> to 50% efficiency rather than 66%.
>>
>> 73,
>> Don W3FPR
>>
>> On 11/9/2018 12:42 PM, Jim KO5V wrote:
>>> Thanks Bob,
>>>
>>> That makes me feel a bit better. I'm still going to remeasure it with a newer meter and see if the measurements check.
>>>
>>> 73, Jim KO5V
>>>
>>>
>>> -----Original Message-----
>>>> From: Bob McGraw K4TAX <rmcgraw at blomand.net>
>>>> Sent: Nov 9, 2018 10:29 AM
>>>> To: Jim KO5V <ko5v at earthlink.net>
>>>> Cc: Elecraft List <elecraft at mailman.qth.net>
>>>> Subject: Re: [Elecraft] K2: KPA100 fan issue + current question
>>>>
>>>> At 13.8 volts, 11amps is about 152 watts input. 100/152 = .658 * 100 for about 66% efficiency.
>>>>
>>>> All in all , looks about right to me. AKA - I wouldn't be concerned.
>>>>
>>>>
>>>> Bob, K4TAX
>>>>
>>>>