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

[Don Wilhelm]

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?
>