As per this question, I bought myself a very nice Ibanez 7 string which I have tuned as A-D-A-D-G-B-E (giving a very nice fat sound which works with the kind of music the band plays)
I didn't think to ask what the string gauges were - but in general I play either Ernie Ball Slinky 9's or 10's on my 6 string guitars.
What should I buy for the 7th string in these two scenarios?
If you like the 9's or 10's Slinky from Ernie Ball, then you should go for the "7-String Super Slinky" (2623) with an additional .052" string or the "7-String Regular Slinky" (2621) with an additional .058" string. But, since you're using a dropped E and B string, normal gauges can get quite "sloppy". In this case, (and if this is bugging you) you should go for 'heavy bottom' strings like the "Dean Markley 2504C LTHB", which are pretty normal 10's string expect that the low E- and B-string are thicker (.052" and .060") which should work perfect for your needs. Additional you can make your own set by using a normal 6-string pack and add a single fat string (brands like Pyramid or d'Addario sell single strings).
I hope this helps.
