gosh, I remember when I owned Fourier Panthere OTL Monoblocks, I believe r 16 of those Russian 6c33b output tubes per side. I was having noise issues which I was trying to diagnose by swapping one by one. You had to also power down and wait several minutes before re-powering up due so to let the Caps drain out.
I had my first tube fireworks display in my system, it caused my Condo complex to lose power momentarily.
I have since stuck with tubes but went the simple 4 tube designs for a while until i found as Stanwal said, proper design in my Air Tight 6 tube per side monos.
but to answer the question, so far my output tubes have hummed, made noise like static or rushing water when they were in need of replacing. if a tube also does not bias within range, another sign of an aging or bad tube. tubes can also turn bright red before they fail....often one is not able to see the tubes when in operation due to faceplates, but I always take a peek at them after they have reached full bias point to make sure they are not glowing red
I had my first tube fireworks display in my system, it caused my Condo complex to lose power momentarily.
I have since stuck with tubes but went the simple 4 tube designs for a while until i found as Stanwal said, proper design in my Air Tight 6 tube per side monos.
but to answer the question, so far my output tubes have hummed, made noise like static or rushing water when they were in need of replacing. if a tube also does not bias within range, another sign of an aging or bad tube. tubes can also turn bright red before they fail....often one is not able to see the tubes when in operation due to faceplates, but I always take a peek at them after they have reached full bias point to make sure they are not glowing red