Antique Sound Lab Hurricanes - Triode or Pentode ?

I think the simple answer is just look at the numbers. I find 480 for each tube to be the best. So just make sure under all circumstances the number is around 480.

I will do so. My question was wether bias changes when going from triode to pentode. My concern was that the pentode bias would be much more than the triode bias, causing the tubes to run much greater than 480 on the first turn-on, perhaps resulting in lost tubes. I guess I will find out.

Some initial observations:

1. Bias goes up slightly when moving from triode to pentode - 480 became 500.
2. Substantially higher gain in pentode mode - at least 6db more and probably higher.
3. Better dynamics in pentode mode.
4. Better focus in pentode mode.
5. Slightly brighter balance in pentode mode.
6. Slightly more depth in triode mode.

Your observations I share. As I haven't tried the triode mode for many months your points 5 and 6 remind me that sometimes I find my system (currently set as pentode) to be on the bright side. Perhaps I should try again the triode mode for the tweeter to see whether I will get a better tone colour. Thank you very much.

That sounds like a worthwhile thing to try, but keep in mind that the gain is much less in triode, requiring you to have separate volume on each amp or set of amps. I was quite suprised at how much more gain there was in pentode mode. My previous ASL amps (MG-SPM40, SE 4xEL34 monos) did not exhibit this much difference in gain between pentode and triode operation. That may have something to do with the push-pull design of the hurricanes vs. SE of the SPM40s.