so ive been working on a simple AM transmitter for 40m, nothing fancy just a crystal controlled thing based on a design i found in an old ARRL handbook from the 80s. got the carrier looking pretty clean on the scope but when i modulate it the audio sounds really boxy and distorted, like talking into a tin can. checked my modulation transformer and the impedance should be in the ballpark, im using a 600 ohm primary into the final.

the modulator stage is just a single 6L6 running class A which i thought would be fine for this power level, maybe 5 watts carrier. tried adjusting the grid bias on the modulator and it helped a tiny bit but still sounds rough. not sure if its the transformer itself or if im overdriving the modulator or what. anybody built something similar and ran into this? i have a second 6L6 i could throw in there for push-pull if thats what it needs but wanted to ask before i start rewiring things.

push-pull modulator is almost certainly gonna help you, class A single ended is real inefficient and if you're anywhere near the limits of that tube you'll clip before you know it. but honestly the first thing id check before rewiring anything is whether your modulation transformer is actually rated for audio or if its just a generic output transformer somebody slapped in there. old handbook designs sometimes assume you already have specific parts and substituting stuff that looks close on paper can get you in trouble fast.

also worth checking your screen voltage on the final, if thats sagging under modulation it'll cause all kinds of weirdness that sounds like audio distortion but isnt really coming from the audio chain at all. throw a voltmeter on it while youre talking into the mic and see if it moves around. if it does you probably need a stiffer regulated supply or at least a bigger cap on the screen line.

had almost the exact same thing happen on a BC-era transmitter i was restoring, turned out one of the coupling caps in the audio chain had gone leaky and was shifting the bias on the modulator tube. the distortion sounded kind of boxy like you're describing. just something to rule out if the transformer and screen voltage stuff checks out.