Here is a simple, series, buffered effects loop. This is the loop that is currently installed in my J50 amplifier.
The loop features both send and return level controls. Depending on where you place this loop in your amp, however, the return level control might not be necessary. I think I prefer a 12AU7 in this position, but you can use a 12AT7 or 12AX7 instead.
This simple loop is precisely that: simple. It’s not the best way to incorporate a loop into your amp. One of these days, I’ll get around to installing a real parallel loop in my amp. For details on the best all-tube parallel effects loop around, see Kevin O’Connor’s book, The Ultimate Tone, Vol. One.