So I've been wanting to do this for a while. The Blues Junior doesn't have a lot of clean headroom, and IMO sometimes it's hard for time-based pedals in front of the amp to sound good (again, IMO, probably it's just my preference). Personally, I struggle a bit finding a good use for my delay/verb/trem pedals when I use my Blues Jr. With a BJr, turning it up loud means you are going to get dirty. Nature of the beast. So... my theory is that I can put an effects loop before the master volume and my time-based pedals should work nicer with the amp when I turn it up loud. I wanted a tube-driven buffered serial loop with a switch so I can turn it on and off. The Blues Junior has an unused triode (V2A) and I could have used that. But I had already modded my amp and use V2A as a cathode follower to drive the tone stack. So I need to add a tube. Since my plan was to add a tube, I looked for circuits that used both sides of the tube I would be adding. One side for the buffer, the other side for a recovery stage. Some ideas for circuits: There was a schematic I found from a Weber kit but it didn't look like it would work. Valve Wizard - Cathode follower page has an idea for a buffered loop at the bottom http://www.blueguitar.org/new/schem/misc_mod/fx_loop.gif I actually tried both circuits, and settled on the Valve Wizard version. I also tried inserting the loop at various points: Before the amp's reverb. After the amp's reverb. Separate from the amp's reverb. My amp's circuit board looks like it took a beating from all that but I wasn't worried about it. I got my amp used for pretty cheap. Here is a sketch of the circuit integrated into the Blues Junior schematic (my version is the old Green Board): After trying different placements, I settled on putting the loop send and return between C16 (V2B coupling capacitor - C8 on the cream board version) and R40 (430K). Thus, the amp's reverb comes after the effects loop. Note: The reverb on Green Board versions is after the Master Volume, while Cream Board versions the signal for reverb is taken before that 430K (R18 on cream boards) resistor and returns after it. My Green Board was previously modded to handle the reverb just like the cream board does. So after the V2b coupling cap, the signal is sent to the loop buffer (1/2 of a 12AU7) which drives the send. Send goes out a switching jack so if nothing is plugged in then the signal goes to the return jack. From the return jack the signal goes through the recovery stage (the other half of the 12AU7) and the signal is returned to a point on my board where it is picked up at R40 and also sent into the reverb circuit. I don't have lots of pictures because a lot of this was trial and error. With trying different spots to get and return signal, and building the loop circuit several times with different components, there was no way to document the process. But I'll post a few pics of the final result in the next posts.