Whenever I play my Palomino there's this weird popping sound like every 5 seconds (I know that's not to descriptive but that's the best way I could think to describe it). Does this mean the tubes are going?(the amps a year and a half old and I've never changed the tubes)

If they are broken, what's a good combination of 4 el84s and 2 12ax7 to put in? I think I'm going to put a NOS 12at7 in valve 1 to smooth out the sound.