A somewhat standard modification to vintage amp when the owner is looking for some modern features. I don't use an effects loop personally since the pedals I use are all distortion/volume based and go directly into the front end, before the amplifier's first gain stage (preamp). However if you are using effects like delay, you may want to insert it after the preamp. This allows the signal to be colored by the amplifier before the effect is applied, but before the signal is fully amplified.
This effects loop is based on the fully buffered Garnet design, which required installing and additional 12AX7, half used as the send buffer and the other half as the return buffer. Each has an adjustable level control giving the user maximum flexibility. There are a number of designs that use 1/2 of a 12AX7 or no tube at all. This layout, while not the easiest to implement, seems to produce the best results. I also installed a bypass switch to remove the loop from the circuit entirely if the user desires.
One bonus of this effects loop design is that it adds 2 gain stages that can be used for additional drive. When testing I could get a thick distortion tone without the use of any pedals.
This was a rather time consuming mod, requiring a lot of wiring and planning to fit all of the components into an existing layout. Luckily, there is a lot of empty space in a vintage Traynor amps. It would be harder to install this in a vintage Fender where space is more economical.