Hoffman Fender Blues Junior Conversion with effects loop

[jeffs]

Added a Riot Devices effects loop based on a 6N17B subminiature tube. 

The narrow space and lack of headroom to the back panel required mounting with angle brackets instead of an easy horizontal or vertical configuration.

I inserted the loop after the treble control, so needed to add a grid resistor for the next gain stage at the output of the board.

I also added a DC return path so the bias current wouldn't be forced onto the shields.