If the mix pot goes all the way to 100%, then turning it all the way to 100% is approximately a serial loop. I say approximately, because the dry input is still connected and can still cause oscilations and other weirdness. To get around this and turn the loop into a true serial loop, you'll have to rewire the mix pot. There's three wires connected to the mix pot. The middle one is the mixer output, and the outer two wires are the wet and dry inputs respectively. Just solder the middle mixer output wire to the outer wet input wire and tape off the outer dry input wire to keep it from shorting out on anything. This will hard wire the "mix" to 100% wet, so all the preamp signal flows through the FX loop jacks, i.e. serial loop. The mix pot will do nothing after the mod. Loop mixes are usually wired up with 0% at full counter-clockwise rotation and 100% at full clockwise rotation. Looking at the mix pot from the back (knob facing away from you) and with the solder lugs and wires at the top, the wet wire should be the right outer wire. If for some reason you pick the wrong outer wire to solder to the middle wire, you will simply bypass the loop completely. So if you do the mod and your not getting any FX in the mix, try the other outer wire. It won't hurt the amp if you pick the wrong one. Just be VERY careful inside the amp. Tube amps have lethal voltages (400+ Volts!) lurking inside even after the amp is turned off and unplugged. If you touch the wrong thing, you can end up really dead, really quick. If you don't know how to safely discharge the filter caps, I'd recommend taking the amp to a tech and have them do the mod. Better safe than sorry!
