I have been noticing something in the sound coming out of my Tremoverb but I can't quite put my finger on it. It sounds like a bit of static on top of the distortion.

I play the amp at least a couple times a week. Last time I replaced the tubes was April 2012. I suppose it is time?

I'm never quite clear on required tube replacement intervals.
Ah, forget it. I just placed the order. I suppose I was trying to reason myself out of shelling out the cash. 4 years is long enough.