How can you tell when tubes are shot?

I like Sugarbrie's take which is an easy way to tell. Another thing to watch out for (if you are not used to switching/comparing tubes) is to allow them to cool all the way down before handling/changing them. Not doing so can damage or take a lot of life away from them (especially non Russian 6922 types many of which are very frail). Another good practice is to clean the glass globes and pins (I use electrician's alcohal which is cheap) and use a clean piece of cotten to handle and install the tubes which will keep skins oils and such off of them.

Hi Ntscdan; I keep my SF Line 2 in standby when not in use, and I found out the hard way when to replace it's tubes. After a year or so of use, the sound (music) becomes harder, brighter, more "brittle", and fatiquing-- but this happens gradually-- at least with the Sovtek 6922s. When I finally realized what was happening and replaced them with a new set, it was like having a new pre-amp. Now I replace them every 12 months or so. Cheers. Craig

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.