I had purchased one ( actually several) of those Chinese 70 watt linears. They use IRF530s for the RF transistors. I connected one of these to my uBitx years ago when I first bought my V3. I used it exclusively at that time on 20 meters and it worked fine, putting out about 50 watts. I have posted in the past some of my uBitx mods to improve power output. When I learned about all the intermod product issues with the V3, and trying some of the posted mods to solve the problem, I was not successful. It became clear that the V3 was not worth modifying anymore. I decided to try the Softrock SDR radio. I modified he Softrock so it could be used on all HF bands, and connected it the uBitx using the uBitx RF output stage on LPFs and retaining the Arduino to control the uBitx. The way the uBitx is designed, it is easy to have a different supply voltage (keep it under 25 volts unless you want to change the aluminum bypass caps). I changed the IRF510s to IRF520s and increased the output amplifier supply voltage to 19 volts. With this configuration, I have up to 52 watts rms output. I think that the same output and LPF filter modifications can be made on any uBitx and provide a similar output power. The only caveat is that the Softrock has 1 watt of drive. I am not sure how much drive power the uBitx provides to the final, so some additional mod may be required to reproduce the power level I have achieved, but a think a significant increase in power can be obtained with the drive as is. I have attached my schematics and design notes. The output stage is on page 7. With the uBitx, the microphone AGC may also be helpful. The SWR bridge could also be incorporated.
