Buzzing speaker noise and blue light in an output tube on start-up, ST-70

Upon powering my stock ST-70, a buzzing noise came out of left channel speaker, and glancing at the amp I noticed V3 glowing a pulsating blue glow inside the glass.  I unplugged immediately, now writing to those with more knowledge and experience than I.

What does this symptom/failure point to?  Thanks!!

I lament that my matched quad of winged C tubes may be no longer a foursome!  Advice on next steps are appreciated.

It sounds like the tube with the blue glow is now deceased as it is most likely gassy or has an air leak due to cracked glass. You can try switching it to another socket, but it is highly likely that the tube will do the same thing in another position. How many hours are on the tubes?

Thank you! That was my suspicion, the blue light being gas inside what should be a vacuum.

Not many hours on this quad at all. Less than 50, if I had to guess. I had the amp in my garage, where the daytime temps are above 130F. I suspected the temperature swings may have caused the tube to spring a leak.

The new generation of tubes I don't believe have the life expectancy of an original Mullard or Amperex tubes. I just had an Electro Harmonix EL34 fail in one of my ST70's last week. I doubt the tube had more than 75 hours on it and it shorted internally. In my other ST70 there are a quad of real English Mullards that have been in service for years and still test good. The positive side to the Russian tubes is they are fairly cheap compared to NOS tubes and they sound decent.