Vacuum tubes carry a character of their own and provide the player with the unique ability to change the voice of their amp, from warm to sharp, clean to grainy and from slightly overdriven to insane.

Most people seem to agree that old tubes are better than new, although many of the new ones seem to be pretty darn good too. A lot of people even use different tubes in different positions. What all this says is that replacing tubes because they stop working isn't the only reason for having a collection on-hand.

There isn't much of a guideline when it comes to tubes. Google "What tubes vintage amp" to get a feel for what others are doing. Some swear by British Brimar NOS tubes and others say they're flat sounding. Just about everyone seems to agree that RCA blackplates are the best 6V6 but some also agree that old Tung-Sol's are even better - who knows? It's just opinion, so the trick is to get one or two and start your own little collection.

I have a few listed that represent a reasonable cross section. Some new (recently manufactured), some new old stock (NOS) from the 50's and some used but in very good condition. I test every one I find in an amp and I guarantee each to work - if they are DOA send them back I'll refund you 100% including postage.  NOS and Vintage tubes are running out and they're fragile, but if properly cared for have a very long shelf life so they do make a reasonable investment.