Are Sylvania 6l6GC safe to use in Diezel Herbert

[H3000]

I have some Sylvania 6l6GC and want to know are they safe to use in a herbert
I want to try two 6L6GC and four EL34
I read that some tubes like KT88/6550 can run the amp too hot and I do not know if that is the case with Sylvania 6l6GC

I also read on the forum somewhere about the recommended preamp tubes for the Herbert can these be bought from Diezel or is there a reccomended supplier I can get them from

 

[Peter Diezel]

The 6L6 have a lower rating concerning anode and
screen grid voltage. It´s mostly 450V. Most types
will stand a higer voltage, some types not. I know
that the 6L6-JJ is running relativ safe.