After the success I wrote about last time I began poking around. I noticed that the driver, a BCX56, was contributing to the distortion at high output levels. I populated a second board, this time with what was an unknown SOT89 transistor taken from a FM92 sub-circuit. After some investigation it appears this transistor is a BFQ19. Mouser still stock them at around A$1 (US$0.75). However I have no more of them so I will experiment with some alternatives now that the proof of concept is working so well.
I also tried a different pair of output transistors (2SD1664R) and a different binocular core which I will write about later when I have a moment.
I doubt the binocular core was the reason for the different outcome. I could easily achieve 1 watt from 3.6MHz to 21Mhz. At the upper end I had to increase the drive but no distortion crept in. This new transistor costs just A$0.05 (US$0.04) a piece. That price is still a fraction of the cost of the thermal bridge but everything worked so well that this has become my new standard output transistor in this application.
A wider board with hopefully better thermal proprieties is planned.
73's
Richard
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