Unfortunately my two non-ww triphosphor F40s both have a bad filament (one has trouble connecting, the other is intact but just runs cold for some reason) so they don't dim well. So I put in an F34, however the ballast says for F40WT12 lamps. I can see how it wouldn't like it at full power, but if it is dimmed, is it okay to run? The ballast doesn't get hotter than normal or buzz or anything.

It does create an interesting start though, the F40 starts up normally and is brighter while the F34 hasn't start, the F34 on the other hand glows at the ends for a second before starting.

If the ballast says it’s good for energy-savers, I don’t see what the problem is.

Oops I fixed it, the ballast *didn't* include energy savers

Ah, I see. Probably not then.

Do have a way to measure the line current, such as with a Kill-a-Watt?  If the line current with the 34(s) is higher than the specified current for 40s, then it is not safe.  If it's lower then it's probably OK at least for a little while.  I'm basing that on the fact using F34s on an NPF ballast for F40s will result in them using more power and burning up the ballast in no time.  By contrast using F34s on an HPF ballast not rated for might make run slightly hotter and stress the capacitor, but usually won't result in an immediate failure provided the ballast was in good condition to begin with.  My impression is that HPF ballasts not rated for F34s will likely have a higher failure rate in the long term, but you can generally expect most ballasts to operate them for years without trouble.