so i finally finished my first scratch built QRP rig, its a 40m CW only thing based loosely on the Heidelberg design but i kind of went off script on a few stages. took me about three weekends to get it to actually produce a clean signal, mostly because i kept screwing up the low pass filter section and getting harmonics everywhere. anyway its putting out about 4 watts into a dummy load which i'm pretty happy with for a first attempt.

my question is around efficiency i guess. the PA stage is getting pretty warm even at 4w output, like uncomfortably warm to touch after maybe 10-15 minutes. im running it off a 12v source and measuring about 1.1A draw during transmit so thats 13.2 watts in and 4 out, which if my math is right is like 30% efficiency? that seems low to me but i honestly dont know what's normal for a class E or class C type PA at these power levels. is that just what it is at QRP levels or am i doing something wrong with the bias or the transistor matching or what

using a 2N7000 in the final by the way, might be part of the issue but its what i had on hand

30% does sound a bit low but honestly with a 2N7000 in the final i'm not too surprised. that part works but it's not really optimized for RF - the output capacitance starts to bite you even at 7MHz if the drive and impedance matching aren't spot on. IRF510 is the classic swap for this kind of build, you'll see it in pretty much every QRP kit from the last 20 years, and a lot of guys get 60-70% efficiency out of it once the LPF and matching network are sorted. the warm PA is a red flag though, definitely worth checking your load line and making sure you're not running it into a mismatch. what does the waveform look like on a scope if you have access to one? flat top on the sine would tell you a lot

congrats on getting it working, three weekends sounds about right lol mine took longer and i had a kit. i dont know enough to answer the efficiency question properly but i will say the 2N7000 thing is worth looking into, i went down that rabbit hole once and ended up swapping to a BS170 for lower power stuff and it ran a lot cooler. might not be the exact same situation as yours though