How can you tell when tubes are shot?

Tube preamp or amp are not supposed to run 24 hours a day unless they have a stand-by feature.

Any recommendations on replacement tubes for the Audio Research PH3 phono preamp? Thanks in advance.

Thanks for all the great tube advice. The reason that I run my stuff all the time is that I actually get to listen to music from the time I get up until I hit the sack. Plus I use the record out on the REF 1 to feed an A/D convertor which feeds a great jazz station from Seattle (KPLU) through out the house. I have ordered replacement tubes from Audio Research because I guess I just feel more comfortable getting them from the folks that built the stuff, plus the little rings are cool. I do use a glove whenever I am handling tubes so as not to leave any marks, kinda like installing halogen bulbs.

Let us know how it sounds.