There are generally a number of warning signs that one or more tubes need to be replaced:
- Reduced output or output that fluctuates
- High pitched squealing sound
- Significant hiss or hum
- Blown power fuse
- Glass bottle on a tube is no longer clear
I prefer not too wait for tubes to fail particularly when the amp is on stage. I play my main rig almost every day and I can tell very quickly if the output has changed or if the tone of the amp is different. That can happen anywhere between six months to a couple of years and I make a point of retubing at least the power tubes as soon as I hear a change in output levels or tone. If it has been more than a year or so since I last retubed the amp, I will generally replace the preamp tubes as well. Preamp tubes typically last much longer than power tubes.
My gig bag holds a backup set of tubes. If there is a tube failure on stage, I know that I can swap all of the power and preamp tubes quickly. I find it best during a performance or rehearsal to swap all of them out at once as it can take a bit of time to isolate a failed tube.
I have a new set of tubes coming in from thetubestore.com. They are a Canadian operation based out of Hamilton, Ontario. Great folks and their site has a lot of content and reviews on tubes. I am trying out their Northern Electric 12AX7 preamp tubes to see how they compare to the JJs that I normally play.