The Electro-Harmonix 6L6 is a type of tube for a guitar amplifier.  I currently have a set of these in my 1976 Fender Twin Reverb guitar combo amp and they do the job well.  Installing them into the amp isn't hard at all as of course these fit perfectly since they are the right set of tubes.  I'm not sure which exact models that these tubes will work with, but I do know that a lot of Fender guitar amps need 6L6 tubes and you just check to see if these will work with yours.  While you do have a variety of options when it comes to 6L6 tubes out there, but I feel comfortable using these because I know that they are reliable and will work well all the time.  The only think that I can think of that people might feel uncomfortable using these is maybe because they want to use original tubes, as these weren't made at the time that older Fender amps like mine were around.  However, you should keep in mind that all of the tubes being made now are in this class and I would imagine that it would be pretty hard to find a set of older tubes that were made in the time period of your amp, unless you are quite lucky.  Either way, I think that there really isn't any difference at all and that it really doesn't matter if you are using 'original' tubes or not.  The amp will sound the way that it is supposed to sound either way.  If you need some new tubes for your guitar amp that needs 6L6s, try these out as they aren't expensive and are readily accessible.  These are reliable tubes made by a reliable company in Electro-Harmonix.  I definitely trust the gear that they make, and even if they aren't primarily a tube manufacturer, they know what they are doing across the board.