How do you know when a tube is dead?

[Happy Ghost]

Hi All,

I had a question? How does one determine when a tube needs to be replaced? Is it at the absolute end when it just refuses to even light up? Or when you notice consistently significant degradation in music quality? I would be interested to know people's experiences on this especially involving 12AU7 driver tubes.

Thanks,
Atul

[elmoe]

In my experience volume will start to get lower, and you will hear artifacts (loud fluttering that comes and goes) when the tube starts to under perform.

[Happy Ghost]

Okie.. So a bit more detail from my side.. I had a new production 12AU7 and initially it sounded horrible but later the sound changed and it was actually beautiful.. Now over the last 2 days it sounds just plain. All the instruments are muddled, absolutely no separation, the mids were shot.. It sounded like I was just listening to my DAC at high volume with a slightly muddled sound.. I am pretty bummed out at the change in my fortunes I changed the driver tube and the sound quality changed. So I am sure the problem is with the 12AU7. Time to replace you think? Or could it be a problem in my build? Some loose connection?

[Paul Birkeland]

A voltage check would be a good idea to check on your build.  I have had a Crack or two come in over the years that had good DC voltages but some iffy solder joints, and they did sound pretty terrible until we got all the suspect joints flowed out. 

A new production tube that's brand new should last a very, very long time, but some of the Russian ones will need 100 hours or so to break in and sound like they are supposed to.

[Happy Ghost]

I will check on the voltages again.. But if the sound is good with a diff. set of tubes then it is a problem with the new production tube I am guessing.. I thought "burn-in" was a myth.. Guess not

If anyone has experience with how the sound signature changes/burn in times for a Genalex Gold Lion 12AU7, I will be interested.. 100 hours does sound like a awful amount of burn in

[Paul Birkeland]

Yeah, I would leave that Genalex tube running for a week before making any judgments.

[Happy Ghost]

There should be no problem running the amp continuously for a long period, correct? No overheating etc. that I should be worried about?

[Paul Birkeland]

Provided you don't have the amp running in a small box or something like that, it should be just fine.

[Happy Ghost]

Hey Paul..

On a more general note do you recommend burn in for tubes both new production or NOS? What are some possible burn in timings for some of the popular brands like say RCA or Tung-Sol's. I would love to hear of your experiences with tube sound pre and post burn-in.

I've never really bothered to burn in any of the tubes I've tried so far. I think I might be missing a trick here..

Thanks,
Atul

[Paul Birkeland]

The burn in issue with Russian tubes seems to be somewhat unique, and when we used to provide EH12AU7s with the Crack, the voltages at terminals 1 and 5 would consistently be high on a fresh 12AU7 until it got some run in time.

With old production tubes, sometimes they need a few hours of operation to come out of hibernation, but I haven't noticed anything consistent.

[Happy Ghost]

New production EH12AU7's were shipped with the Crack before.. I didnt know that.. Which means that those tubes are not bad at all.. I'm tempted to check them out myself now..

Russian power tubes like the "Winged C" benefit from a burn-in too?