Apparently Peavey used to sell an effect loop kit for those early model of the Peavey Classic 50, but no longer does.
Any chance someone has the instructions and the part list?
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This link provided at the bottom of the forum (PDF download link) has a schematic on the second page. It also includes the circuit board layout. You can get the values of the resistors and capacitors from there.
There are also instructions, though these are tailored for the kit itself rather than how to easily insert an effects loop into the amp. It appears that the kit ran as a little breakout board, through a 6-wire ribbon/bus, from a connector that was mounted as stock on the circuit board within the amplifier.
In case the link dies, here is the circuit diagram: