so ive been working on a 40m CW transmitter for the past couple months, pretty basic crystal controlled design i pulled from a QST article from like the 80s. most of it came together fine, the oscillator stage is clean and i can see it on the scope no problem, but the output stage is where things get weird.

im using a 2N2222 in the driver and a IRF510 MOSFET for the final, which i know isnt the most elegant choice but i had a bunch of them in the junk box. the problem is im only getting maybe 800mW into a dummy load when i should be closer to 3-4 watts based on the design. ive already checked the drain voltage (its right), swapped the IRF510 twice thinking i killed one, checked the gate bias resistor values and they match the schematic. low pass filter is wound on a T68-2 toroid and i calculated it for 40m but honestly the wire count might be off, i counted twice but still not confident.

harmonics look okay on the spectrum analyzer at the club, nothing crazy there. just frustrating that the efficiency is in the tank and i cant figure out where the power is going. anyone dealt with this before with the IRF510 final or have a gut feeling what im missing?

the IRF510 can be really picky about gate drive. if your 2N2222 driver isnt swinging the gate hard enough or fast enough you lose a ton of efficiency because the FET spends too much time in the linear region during switching. check what your gate waveform actually looks like, not just the voltage level but the rise and fall times. if its rounding off a lot thats where your watts are going, mostly as heat in the FET.

also worth double checking your LPF, a miscounted toroid wont just hurt harmonics it can screw with your impedance match back to the drain and youll bleed power that way too. i built a similar rig a while back and was short by like half a watt just from having the inductance a bit low on one of the filter inductors. wound up measuring it with an LC meter and sure enough i was off by about 15%.

yeah what he said about the gate drive is probably it. i had the exact same IRF510 issue on a 20m rig i built a few years ago, spent forever chasing it. ended up being that the driver stage just wasnt cutting it and the FET was running warm as a result. you can sometimes feel the difference if the heatsink is getting hot fast even at low power levels.

one other thing -- whats your supply voltage and how are you measuring the output? just want to make sure youre not losing power in the coax between the rig and the dummy load or something dumb like that. ive done it lol