[Elecraft] K2: KPA100 fan issue + current question

[Jim KO5V]

Thanks Bob,

Yet another variable! I just measured my dummy load at 54 ohms (cold). Maybe I should just declare victory and move on!

73, Jim KO5V  


-----Original Message-----
>From: Bob McGraw K4TAX <rmcgraw at blomand.net>
>Sent: Nov 9, 2018 8:51 PM
>To: Jim KO5V <ko5v at earthlink.net>, donwilh at embarqmail.com, Elecraft List <elecraft at mailman.qth.net>
>Subject: Re: [Elecraft] K2: KPA100 fan issue + current question
>
>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
>>>>>
>>>>>
>
>