Solid State Guitar Amp Forum | DIY Guitar Amplifiers

I would like to send a line-level signal from one amp's 1/4" mono line out jack to another amp's stereo FX return, which is a single 1/4" TRS jack, with the tip being one stereo side and the ring (pretty sure, anyway) being the other. I guess the sleeve is common...anyway, I want to have this mono signal drive both sides of the stereo power section equally, and I'm really shaky on knowledge of line level signal measurements and impedance relationships in that realm.

I have tried plugging a mono 1/4" instrument cord into the TRS FX return partway, until I heard the signal coming out of both speakers. It seemed to work well, but I'm not sure if there's supposed to be some sort of transformer isolation DI signal splitter doodad you're supposed to use for this kind of thing, versus just connecting the two hot terminals of the TRS to the one hot coming from the source amp. Thoughts?

Do I need to post a schematic to get an answer to this question?

Don't worry.
Line send/receive in different amps usually try to accomplish the same, so they at least *try* to be compatible.
You should be *very* unlucky to find incompatible amps.
In your case connect:
1) Hot Mono to hot left + hot right (tip + ring).
2) Ground Mono Plug to ground Stereo Plug

Thank you!