I have been working on a number of upgrades for the uBitx. One is an external amp. A number of people, including myself ordered an amp like this one: https://www.ebay.com/itm/70W-SSB-linear-HF-Power-Amplifier-For-YAESU-FT-817-KX3-Ham-Radio-DIY-Kits/401280684714?epid=579537921&hash=item5d6e3152aa:m:mTAXYtpMu9k-yfYI6Z1m1hQ
For the price it is a good deal, but it doesn't include the heat sinks or the needed low pass filters for each band. I currently have the unit running with my uBitx only on 20 meters. I temporarily but an appropriate filter on the board while I work on my uBitx daughter board that will be a collection of several improvements. I have attached my version of the original schematic and also my revised schematic. I made one major change and a few minor ones. The major change is that I use two IRF520s in parallel instead of the original IRF530s. This came about accidentally, but it is a could change. When I put the amp together and started testing it, the FETs quickly self destructed, before I could shut it off. I looked through my parts been and found that I have a number of IRF520s but no IRF530. I checked the data sheet, and found that when you put 2 in parallel, the total capacitance increases by about 20%. But the thermal impedance for 2 in parallel is lower for the two, and the temperature rise in each is lower because the current is 1/2 for each, and the on resistance is not much higher in the IRF520. The bottom line, is you get much better thermal performance and can use a smaller heatsink. To the amp itself, I found that I get better performance by changing the 150 ohm feedback resistors to 235 ohm (to 470 in parallel) and I made the output transformer secondary 4 turns instead of 3. I got slightly better output power, and I believe a better antenna match, since my antenna is closer to 70 ohms then 50. Lastly, I changed R2 to 4.7K from 10 K and put a 6.8 volt zener in series with it so that the PTT can be put in parallel with the uBitx PTT switch. If you want to use the amp for both SSB and CW, then you will need to or the two lines together with a couple of diodes. I'm not running CW now, except for testing, so I didn't bother with it.
When I finish my daughter board design, I will post it. Probably in day or so. The amp is tested, the daughter board will be just the design at this point, with some parts breadboarded.