The preamp tubes last a long time and generally don't need to be replaced nearly as often as the power tubes. The 6550's and KT88's that most Sunn stuff used as power tubes are very robust and very long lasting tubes, so they don't need to be replaced as often as you might think. I had a '67 200S several years back that I bought fromt he original owner. It had Tung Sol 6550's in it and they were the original tubes. It sounded good, but one of the tubes blew because it was worn out. I put JJ KT88's in it, and it sounded WAY better. Since then I've noticed when a tube begins to get tired by how it loses the lows and highs and starts to sound weak. If you can't tell, or don't have the experience to know, then you replace them when they fail pretty much. If you gig all the time and use the amp a lot, then you can probably get at least a year and maybe more out of most 6550's/KT88's nowadays. Sometimes you can get 5+ years with those tubes too. It just depends on the amp you have them in and the brand you get.
