[Elecraft] K2 power output level fluctuation

George Kuehn imjustgeorge at comcast.net
Thu Apr 23 20:43:57 EDT 2015


I know from the forum that a K2 will show a power increase over a given setting if left in tune for an extended time as a consequence of the final transistors heating up.  That makes sense from what I’m seeing here.  The effect is greater at higher power settings although not proportionally higher as my set level increases.  I’m just wondering if the amount of shift I’m seeing is normal. 

Here’s what I’m seeing:  With my K2 operating into a zero SWR 80 watt (nominal) dummy, the observed increase both on the K2 display as well as on my computer monitored W2 wattmeter tends to be 1.5 watts or less over a ten second period.  The initial increase happens quite quickly and then slows.   It is difficult to confirm, as I don’t want to leave the rig in tune for an extended period, but it seems like the system stabilizes or at least increases very slowly once it passes the initial increase. Operating CW at 12-15 wpm into the dummy, the power output fluctuates but doesn’t exhibit a tendency to rise more than noted above, given a 50/50% sending/receiving behavior.

My observations were made at 7.100 MHz with power settings at 2, 5 and 10 watts.  The K2 was connected by short length coax leads through the 200 watt W2 Wattmeter  Directional Coupler into a dummy consisting of a set of 4 series-parallel 50 ohm Caddock MP820 thick film non-inductive resistors mounted on a fan-cooled heatsink (nominal 80 watt rating) that shows zero SWR from 160 meters through 24 MHz and 1.1 SWR thereafter though 6 meters as measured with an MFJ 259B Antenna Analyzer.  The resistors remain cool to the touch even after extensive testing.

So, do I need to be concerned about the power increase behavior I’m seeing?

I do wish to operate digital modes such as PSK31, Hellschreiber, and similar modes where the duty cycle is higher than casual CW or SSB phone.  Assuming normal power level control in the K2, is there a power level setting recommended to protect the final in this type of operation?

In the future, I plan to homebrew a linear amplifier to give me PEP output in the 80 to 100 watt range primarily for SSB.  I was hoping not to have to create gain control back into the K2 from the amp as is done in the K2/100 setup.  Any recommendations from those who run a homebrew linear?

George Kuehn
N9AUP


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