2. amps-preamps
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Tube newbee question

After 30+ years of solid state I now have a bran new Octave V70se currently running sed winged KT88's. I noticed 1 tube glowed a much a brighter blue than the other 3. 2 nights ago (day 5) I also noticed the blue would flicker with the bass. I turned the volume from a 9 o'clock position to about 10:30 when 1 cornor of the tube seemed to get super bright - then a bright blue flash occured with the Octave going into protection mode. Also heard a thud/pop from my right speaker. Recycled power and verified that the bias was set corectly - which it was. I left it in for a couple more hours and it still works but has a much brighter blue glow to it. Octave included a spare KT88 and the next moring I replaced that 1. Well that spare tube also has a brighter blue side. Note I will not put that original back in. Today I swapped that tube position to another and it still has a brighter blue side in the different spot. Today is day 7. So for now I have to assume I had 1 bad tube. My question is seeing a KT88 or even 6550 that glows a brighter blue than the rest a problem to worry about? The last thing I noticed is that both tubes that glow blue brighter needs to have the bias turned lower. The tubes are supposed to be a match quad set.

I also have a matched quad set of EL34's and a matched quad set of 6550's on order. All are factory sed winged c's. I want to eventually pick a favorite tube which is why I got the other sets.

Last my Octave dealer forwarded my concern to Octave and we are waiting for a response. I would like to get any opinions if this is something to worry about.

THANKS
xti16

Showing 1 response by rrog

rrog
1,958 posts
 
The blue glow in tubes is not normal. If the blue glow suddenly appears it means the tube has developed a vacuum leak. If the blue glow has always been there it means the tube did not have a good strong vacuum to begin with. The strength of the vacuum does affect the performance of the tube.

The bright flash that shut down your amp was most likely a short in the tube. Without a protection circuit your amplifier could have suffered severe damage.

I had a tube short in my Audio Research M 300 which had no protection circuit. It blew resistor pieces all over the room and damaged the circuit board in the area of the resistor.

If Svetlana tubes are all showing signs of low vacuum or they are not holding vacuum I would look for another brand. I have had good luck with Chinese KT88 tubes when they are properly sorted.
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