Re: QSX 10W HF Linear PA kit

Hans Summers
 

> Congrats for the amplifier. Surely interesting for TX homebrewing 
> at a reasonable cost. Is there any evidence that it would improve 
> the PA of the uBitx?

Evidence, no. I don't have a uBITX and the Linear kit has only just become available this afternoon. 

Theoretically: yes, it would improve the PA on the uBITX - a lot of care was spent on the board layout, keeping traces symmetric and leads as short as possible (look at the 1mm or so between the IRF510 body and the PCB, for example!). It has very flat gain from 2-30MHz and can even produce 10W on 4m band (70MHz) though gain is 8dB down by then... during testing this amplifier could not be made to exhibit any instability (oscillation). It also has a much larger heatsink than the stock uBITX amp, and the heatsink is grounded which also prevents feedback. 

BUT - the problems of the uBITX are not all in the driver and final PA stages. Earlier stages are overdriven and don't have flat gain with frequency. Spurs are generated but not filtered. Simply replacing the final two stages of uBITX only fixes some of the problems, not all. 

So yes, this amp would improve the PA... but at the end of the day it can only amplify what you give it: if what you give it is already unclean, it won't fix that. 

73 Hans G0UPL

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