so ive been messing around with crystal controlled transmitters for a few months now, mostly just copying schematics from EMRFD and the old handbooks but i finally want to design something more from scratch. going for a 40m CW rig, nothing fancy, just a decent little QRP transmitter in the 5 watt range.

my main issue right now is the PA stage. ive been using a 2N3904 for driver stuff which is fine but when i start thinking about the final amp i keep going back and forth between using a IRF510 versus just stacking a couple of 2N2222s. the IRF510 seems like the obvious choice for power handling but i keep reading people saying the input capacitance is a pain at HF and you really need to think about the gate drive. anybody actually built with the IRF510 on 40m and can tell me if its worth it or if im overcomplicating this

also should i be worried about keying clicks at this stage of design or is that something i tune out later. im using a pi network output which i think handles some of it but not sure

IRF510 on 40m is totally doable, i built one a couple years back based loosely on a design from qrp-tech mailing list. the input cap thing is real but honestly if you just put a small resistor in series with the gate like 100 ohms it tames a lot of the ringing issues. dont obsess over it, just build it and see what the drain waveform looks like on a scope if you have one.

the keying click thing though, yeah you want to think about that earlier rather than later. shaping the keying envelope is way easier to design in from the start than bolt on afterward. a simple RC network on the keying line to soften the attack and decay goes a long way. you dont want to be that guy putting out splatter across the band when youre working QRP, kinda defeats the purpose. look up the W1FB keying shaper circuit, its simple and works well.

yeah the 2N2222 stack thing sounds appealing because you already have them but honestly getting 5 watts clean out of paralleled small signal transistors on 40m is more work than its worth imo. the IRF510 is like a dollar and the datasheet is well understood at this point. just go with it