Perhaps it is not immune:
MYTH: Gear that uses vacuum tubes is EMP proof.
FACT: Sadly, no. There was vacuum tube equipment that was damaged in the 1962 tests.
EMP Myths | The Preparedness Podcast
http://www.thepreparednesspodcast.com/emp-myths/#sthash.IX3fDUt7.dpuf
Correct. In the 1962 high altitude EMP tests (Starfish Prime and others), vacuum tube radios connected to the power grid or external antennas suffered 'punch-through' failures of selenium rectifiers, and arcing damage to transformer insulation.
Older radios using vacuum tube rectifiers (Yaay 5U4s..) and not connected to the grid or antennas at the time of the event will not be exposed to the high induced voltage of an EMP event. The metal case on these old beasts are Faraday cages, by design. Without that shielding, the tremendous amount of electronic noise generated inside one of these radios would ruin reception in other equipment in the same space. (And yes, even within the radio, there are typically electromagnetic shields to prevent one art of the radio from affecting other parts. Brute force engineering at its finest!)
A modern solid state radio within a Faraday cage can also survive an EMP blast. Faraday cages are popular
DIY projects in the prepper community.
Where EMP phenomena rally excel is at roasting infrastructure. Long wire runs develop very high voltages, which take out substations and long distance transmission lines. The induced voltages on local lines produce massive power surges.
For the ham radio operators, antenna feedpoints would arc over and most feedlines (coax cable running out to the antenna) would fail from induced high voltages exceeding the breakdown voltage of the cable insulation. Carbonized insulation would short circuit the cables. (Ask how I know. :-( )
With no power, no antennas, and in most cases no equipment, they're off the air. That's why I mentioned that it would be up to the old timers to drag their AN/ARC-8 or similar boatanchors out of the basement. It might take them a few days to re-rig antennas and get an ancient dynamotor running again.
Maybe have your bad guys hit repeated EMP blasts to keep the radios down. There's only so much feedline and equipment that the hams and preppers could have stashed...
Take a look at "Directive 51" and "Daybreak Zero" by
John Barnes. (I haven't gotten to "The Last President" yet. It's in the 'to read' stack.)