If the 390 is on, the thermistor will be hot so it will not protect the other set when it is turned on. I put my CL-80 in a box on the back of the 390A covering the power filter pins too. Cheers de Gord. -- Gordon L. Hayward, Ph.D., P.Eng., Associate Professor, Biological Engineering, School of Engineering, University of Guelph, Guelph, Ontaro, N1G 2W1.