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LF: PA problem

To: "[email protected]" <[email protected]>, "[email protected]" <[email protected]>, "Discussion of the Lowfer (US, European, & UK) and MedFer bands" <[email protected]>
Subject: LF: PA problem
From: N1BUG <[email protected]>
Date: Tue, 7 Nov 2017 17:33:07 -0500
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I wonder if anyone can help me with this.

I built one of these:

http://qrp-labs.com/ultimate3/u3mods/lfamp.html

It's not what I want but it's what I can afford. In all cases, I was driving it with 250 milliwatts at 137 kHz. It was running into a perfect 50 ohm resistive load (dummy load good through 2 GHz).
At first I had a DMM between the 13.8V power supply and the PA to
measure current. I had preset bias to about 25 mA current with no RF
drive. Output was about 19 watts and efficiency was only 30%. By
changing the bias I was able to get 22 watts out. The bias point for
maximum output was below the point where it draws any idling current
without RF drive. Efficiency increased to 50%.
Next I removed the DMM. Power output dropped off. Re-adjusting the
bias I was able to get 15 watts. Bias was still below where it
starts to draw idling current. Efficiency seemed better as the amp
was running much cooler.
Then I shortened the cable from power supply to PA. Power dropped
again. I re-adjusted bias but this time had to make it quite a lot
higher to peak the output. I didn't have an ammeter in line but from
earlier tests I know it was now well into the region where current
is drawn without RF drive. I was able to get 20 watts. At the
instant I removed RF drive the FET died! I'm guessing oscillation.
In all cases, the output looked clean on an oscilloscope. I was
using a scopematch to measure power.
I am only guessing but it seems like maybe the PA is not properly
decoupled from the power supply? Or perhaps it is just unstable. Any
suggestions? I need to make this work if I am going to be on 2200
meters this winter. I have one more FET and will order a couple more
but I would like to have some idea what to try with the next one.
73,
Paul N1BUG

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