Rival Sons - What is this vocal effect/treatment (and why does it annoy me after a while)?

 
 
Here's a couple of short excerpts from Electric Man, the first track on the new Rival Sons album Great Western Valkyrie:
 

 
 
Whatever they've put on the vocals sound very 'analogue', and to start with it seems to fit in with the retro sound the band are obviously attempting to emulate.
 
The effect is used across the whole album, it's apparent at all vocal levels, but the effect seems much stronger when singer Jay Buchanan is singing really loudly.
 
I must admit, after a few tracks it starts to get a bit tiresome, a bit wearing on the ears actually. Shame really, because it's a great set of songs.
 
Does anyone know what the process might be?
 
 
 
 
 
 
  

Doesn't matter how good something is, too much of it and you want out. Crosby Stills & Nash's three part harmonies sound beautiful to me for a couple of songs. By the third I'm thinking about throwing on some Naplam Death.

Good vibe though...very zeppy.