[sunnydaze]

12ax7 in the 2nd gain stage of my JuicyMusic Tercel II phono stage.  A few days ago I saw it, researched it online, and discovered it's caused by a crack in the glass that causes it to lose its seal.  Tube is kaput.

OK, fine.  I replace it with the other tube from a matched pair.   Don't power on the unit and the next day I notice the exact same thing.  Never seen this in 20+ years of playing around with tubed gear.  And now it happens to 2 tubes on back-to-back days in the same position of the preamp?!     

Almost suggests that the preamp is causing the tube to go bad.   But is this possible if it hasn't even been powered on?  If not this, what are the odds of this tube failure coincidence?

Tube is NOS Amperex Orange World logo 12ax7.  Bought them as a matched pair, so I guess it's possible they are from the same batch.   As such, is it possible they both have the same defect and therefore have the same life span and are failing at the same time?  Seems unlikely since I used them as singles and they don't have the same total hrs of use.

Two good NOS tubes gone cloudy...now I'm afraid to insert another for fear of the same thing.  Just a coincidence?  Or is the preamp eating tubes?  If the latter, what needs to be repaired?

All input and advise is greatly appreciated.

Thanks,

- John

[gab]

put a cheap tube in and see what happens

[sunnydaze]

Yep, we're on the same page.  I planned to do just that.  Thanks.

[bentconvert]

Found this in another forum. https://www.thegearpage.net/board/index.php?threads/white-cloudy-preamp-tube.893981/

"The getter material is metallic barium, and it looks shiny and silver when the vacuum is intact. When the vacuum fails, and atmospheric oxygen enters, the barium rapidly oxidizes, and the barium oxide is a white color."

Not sure why it happened twice but according to folks on The Gear Page it has nothing to do with the amp.

[sunnydaze]

Yes, I saw that in my investigations.

Simply a tube failure seems a HUGE  coincidence given that it happened twice on consecutive days to 2 different tubes, and I've never seen it before in 20+ years with tubes.   Neither have any of my local tube-head audio buddies.

But I guess it's possible in the strange world of audio.   

[JakeJ]

Face it John, you suffer the luck o' the Irish.  It is rare but if the tubes are from the same batch it would make sense that more than one from that batch might fail.  Pretty high odds that one person gets two together.  I'll bet an entire dollar it is not the preamp.

[sunnydaze]

Hmmmm....you don't sound very sure there Jake!     

[sunnydaze]

Another thing I'm having trouble getting my head around.....

If it is simply tube failure and not related to the preamp, why does it only happen when the tube is inserted into the preamp?   The 2nd tube was sitting idle on the shelf.  It was clear when I inserted it.  The next day it was milky white.   Preamp wasn't turned on post insertion.

[woodsyi]

Your problem must be in the way you insert your rod into the socket.   

[sunnydaze]

Damn!  Never had that problem before!     

[Mike B.]

Tube is a goner. Replace it asap

[JakeJ]

Are the tube sockets really tight?  I'm guessing the seal around the pins is what is failing if there are no visible cracks and the tube wasn't dropped on a hard surface.  The seal around the pins may have been weak and ready fail?

[sunnydaze]

Jake.....I think you got it!   You just jogged my aging memory.     

Past few weeks I been moving lots of stuff around -- packing and such -- for a move.   I do kindof remember dropping a pair of tubes onto my wood floor.  Completely forgot about it till now, and also didn't notice which exact tubes.

I'm guessing the floor hit weakened things, and they only went milky after insertion because the wiggling into the tight tube socket finished the job and cracked the seal right where pin inserts on tube bottom.

Mystery solved....I think.  Thanks for all the input.

[rollo]

  Alien interference. Dam Greys.

charles

[sunnydaze]

Just to finish up.....

Yesterday I inserted a crappy current production 12ax7.  Played music for a few hrs, phono stage is fine, and tube did not get cloudy.

Thanks for all the input....especially to Jake "the memory jogger".   

[fado]

Sunnydaze: It can happen. I had two NOS  5AR4/GZ34 tubes from different suppliers apparently short out, one right after the other, in the same position in the same component so I assumed equipment fault. I tried several other less expensive tubes and the new one supplied with the component and they all worked fine. I have since used several different NOS tubes over a period of ~6 years without a problem.