[Elecraft] K3 and Expert 1.3K-FA amplifier power

[Bill Frantz]

Thanks Don. I got out my old Phillips 25 MHz scope and looked at 
the calibration sticker dated 1983. I decided to check the 
readout with a fresh alkaline battery and both the battery, 
checked with the DVM, and the scope looked OK.

I did the measurements on both 20M and 40M, since 40M is less 
than half the scope bandwidth and got the same result. 6.4v p-p 
times 10 for the probe 10.24W, well within the margine of error 
for my measuring equipment.

My conclusion is that this is a question for the Expert Amp people.

Thanks for your help.

73 Bill AE6JV

On 9/10/17 at 2:32 PM, donwilh at embarqmail.com (Don Wilhelm) wrote:

>Bill,
>
>I would say something is not calibrated correctly - either the 
>K3 wattmeter or the power determining device in the amp.  It is 
>difficult to say which one without doing some measurements.
>
>I would connect the K3 to a dummy load for the 1st test.  Get a 
>'scope out with a 10x probe.  The 'scope and probe should have 
>a frequency rating of at least 3 times the operating frequency.
>
>A coax TEE adapter inserted in-line will give you access to the 
>coax center conductor so you can read the RF voltage peak to 
>peak value.
>That RF voltage should be used to compute the actual power 
>developed by the K3.  Compare that to the power reading shown 
>on the K3 - you can use the formula {V(p-p) squared} divided by 
>400 to obtain the power in watts to a 50 ohm load.  Derivation 
>of that formula is "left to the student", but if you don't 
>believe it, you can always convert to RMS voltage and calculate 
>from there.  Formula reduction is required before plugging in values.
>
>Once you know the actual power when the K3 reads 10 watts, 
>remove the dummy load and connect the amp (leaving the TEE 
>adapter in-line).  Now send what the K3 indicates as 10 watts, 
>and again measure the RF voltage.  That will tell you whether 
>the K3 power indication is correct or the amplifier indication 
>is correct.
>
>73,
>Don W3FPR
>
>On 9/10/2017 5:10 PM, Bill Frantz wrote:
>>I have just set up an Expert 1.3K-FA amplifier with my K3 
>>(with most of the upgrades to become a K3S). My question is, 
>>when I drive it with 10W set on the K3 output power, it shows 
>>a drive level of about 30W and and output level of 950W. What 
>>is going on?
>>
>>Details: The two boxes are connected with about 6' of RG-58 
>>for the RF and a custom cable made by KC5PCB, Dan Tassell for 
>>the rest of the signals. The output is into a dummy load, 
>>although very similar measurements occur with my trapped 
>>vertical antenna. Tests were in the CW portion of the 20M band.
>>
>>The Amp is powered from a dedicated 120V circuit which shows a 
>>7 volt drop at 12.7A during CW key down transmission. (Voltage 
>>is measured by a Fluke 87 DVM and current by a Kill-A-Watt 
>>PA4400.) An external YS-60 power meter shows 800W, which seems 
>>reasonable agreement given the accuracy of RF power measurements.
>>
>>Why does the K3 show 10W and the amp show 30W? Is there 
>>something screwy here, or is it normal?
>>
>
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