Bill Smith <ko4nrbs@yahoo.com> asks >What is involved in replacing the 3-400 tubes in the >Swan Mark 1 with 3-500ZG? IIRC, it would be plug-and-play except for the sockets (which ARE different but have good resale value) and possibly the geometry of the slightly larger tube envelopes. However -- the 3CX800 IS plug-and-play, if memory serves me. Jim N6OTQ