Blown tube?

[dbishopbliss]

After 30+ years of using tube amps this is the first time that I have ever had a working amp suddenly stop working on me. I was playing, things sounded fine until there was no sound. No noise, no pops, no smoke (that I saw), just no sound. Long story short, I opened up the back and found a blown fuse and one of the power tubes looking different than the other (see photo). I thought they were the same but I bought the amp used and never really looked that closely. The getter in the tubes are different. Other than, its pretty obvious that one has a coating on the top and the other one is clear-ish; there seems to be residue. I thought they both had the coating before, but I might be mistaken. Did the one tube blow (for lack of a better term) and that burns off the coating?


I have installed another set of tubes and a new fuse and everything is working fine. Anything else I should look for?


UPDATE: From what I have seen here I'm guessing that the tube has an air leak. Is this something that just happens or did something cause it?

[HotBluePlates]

... one of the power tubes looking different than the other (see photo). ... one has a coating on the top and the other one is clear-ish; there seems to be residue. ...

They started outlooking identical.  The tube on the right has cracked glass, air entered, and evaporated the silvery getter-flash.

Usually (mis-)handling causes the glass breakage.  But it could be thin, defective glass and heat/cool cycle finally cracked it.

[WimWalther]

Though some people associate the shiny getter deposit with an old, worn-out tube, the truth is actually the opposite.

New tubes have shiny, opaque getter deposits. As the tube ages and thw gas seals leak or internal elements outgas, the deposit becomes whitish and less opaque.

In the case of your 'bad' tube, as HBP notes, it does seem that vacuum may have been lost entirely.. ending the life of the tube.

[glass54]

Correct Wim.
Some 12 years ago I serviced a Marshall Amp which had a faulty redplating Svetlana EL34 tube. When I removed the o/p tubes this one (pix attached stayed intact BUT with distorted glass. (Glass must be very tough)
Years later, a crack appeared and the getter went white.
I would call this resulting action the opposite of "letting the smoke out" 
Regards
Mirek

[tubeswell]

amp suddenly stop working … No noise, no pops, no smoke (that I saw), just no sound. Long story short, I opened up the back and found a blown fuse and…


 Anything else I should look for?

Apart from the vacuum loss in the tube, check that it didn’t take out the OT. PP Amps will run and pass signal on one tube if the OT is still working