The 3500 is a stereo JC-1 is about the only difference worth noting in the electronic design. The 3500 has a ton more 2nd harmonic due to the International Rectifier devices in the driver circuit in lieu of the better mannered Harris devices in the JC-1 so the JC-1 is a much smoother amplifier without the glitz and glimmer in the 3500's sound. You are never going to get the 3500 as smooth as the JC-1s due to the devices as well as the layout.
Bob Crump
CTC Builders
Bob Crump
CTC Builders