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The output of the Si5351A direct is likely a little weak to drive the 10W Linear to full power 10W output. I'd suggest keeping the U3S PA and adjust the U3S bias pot (R5) for around 25mW +/-, i.e. whatever is necessary to get 10W (or the desired output, not exceeding 10W) from the Linear.
The QCX power meter isn't accurate and particularly not down at 25mW. And maxes out at 5.2W so won't be useful for measuring the 10W output of the Linear either.
My favored method of power measurement is 50-ohm dummy load and oscilloscope. But yes, unfortunately your 100kHz 'scope wouldn't be up to the job.
My suggestion just set up the 10W Linear as per the manual, and connect a 50-ohm dummy load and power meter... if you have the QRP Labs dummy load then use a DVM on the rectified output and reference the curve in the dummy load manual... then connect the stock U3S to it, and adjust U3S R5 until you get the desired output power (not more than 10W). Something like that...
73 Hans G0UPL
On Sun, Jul 14, 2019, 15:12 Thomas Fritzsche <tf@...> wrote:
Hi Hans, Hi OM's,