EMP generates very large voltages in wires. The mechanism by which it destroys electronics depends on the electronics.
Solid state devices are destroyed by relatively modest voltages which break down the p-n junctions.
Vacuum tubes are much more resistant, as are some special solid state military devices. If you generate too much voltage on a bipolar junction RF transistor, it's gone. If you generate too much voltage on a vacuum tube, you'll be fine as long as you don't melt the plate or other electrodes. (You can literally run a vacuum tube so that its plate visibly glows without destroying it - if you don't keep it up too long).
Even electronically rugged devices which are connected to very long wires (powerlines, large antennas, etc) will be likely to destoryed by an EMP, as very large currents will devlop in that case, which will generate enough power to melt wires and other conductors.