for AC15's and AC30's I use the equivilent of #1, #2, or #3 Groove Tubes. I'm talking about the rating system, not the tubes. The reason the tubes in that range are considered "early breakup" types is because it takes less input voltage to get max output. Cosequently they need a smaller negative bias voltage to bias properly. Which means in the end, if you stick 'em in there they'll work without overheating.
Maybe someone else can explain it better, but it works.