The great Tung-Sol tube failure mystery

[RalphH]

Have any of you ever had a rapid, catastrophic failure of new 12AX7 preamp tubes in under an hour of use? Can anyone provide any input of why this would happen?
As you can see from the photo, the tubes have cracked at the bottom and the vacuum has been compromised. One of them fell apart on removal.
All of this in under an hour of use! First, tube 1 (Preamp) failed after about 40 minutes. After swapping it out for an older spare, I continued on for another 10 minutes when the amp died again.
This time it was the other two newbies in the cathode follower and PI sections. The white tops where the giveaway of the tubes loosing their vacuum.
The amp has worked fine (and still does) with older 12AX7’s and 7025’s installed. I can’t believe that 3 brand new tubes with a good reputation can be faulty out of the box. Any ideas?

[Marcel]

There has been stories around for years that tube makers have been using thinner and thinner glass. It seems Tung-Sol have finally found that 'minimum thickness' needed to reliably contain a vacuum in a vacuum tube.

I'd be making a warranty claim ASAP through your retailer ... and if possible changing tube brands for the interim at least for the time being...

[Simon Barden]

JJs get the best press at the moment from my ex-Blackstar design mate. They did a lot of testing for their make of valve selection.

[DrNomis_44]

I've got three Genalex Gold Lion gold pin ECC83S preamp tubes installed in my Marshall MA100C amp and they've been really good so far, I bought them online from Evatco, they are a little expensive but the way I see it, it's worth paying a bit of extra money to get decent tubes for guitar amps since cheaper tubes are only going to give you trouble later on down the line....definitely don't skimp on tube quality.


https://www.evatco.com.au/

https://www.evatco.com.au/12ax7gg

[RalphH]

I've got three Genalex Gold Lion gold pin ECC83S preamp tubes installed in my Marshall MA100C amp and they've been really good so far, I bought them online from Evatco, they are a little expensive but the way I see it, it's worth paying a bit of extra money to get decent tubes for guitar amps since cheaper tubes are only going to give you trouble later on down the line....definitely don't skimp on tube quality.


https://www.evatco.com.au/

https://www.evatco.com.au/12ax7gg

The Tung-Sols got a good review (Cost $24.50 each from Evatco) and are used happily by lots of people so I don't think they are inherently poor quality. There must be something else going on for all of them to fail so rapidly.

A colleague offered the suggestion that the base of the new tubes are perhaps under stress when seated in the older 1960's sockets (which are supposed to be tighter than newer sockets) and when hot, they expand enough to fracture the glass. This could explain why the older used and NOS tubes are OK.
Anyone got an opinion on this supposition?
Cheers.

[DrNomis_44]

The Tung-Sols got a good review (Cost $24.50 each from Evatco) and are used happily by lots of people so I don't think they are inherently poor quality. There must be something else going on for all of them to fail so rapidly.

A colleague offered the suggestion that the base of the new tubes are perhaps under stress when seated in the older 1960's sockets (which are supposed to be tighter than newer sockets) and when hot, they expand enough to fracture the glass. This could explain why the older used and NOS tubes are OK.
Anyone got an opinion on this supposition?
Cheers.

It's possible, but a bit unlikely, that the gas seal around the pins may have been a bit substandard too, the pins are made from a metal that has the same expansion/contraction temperature coefficience as the glass used to form the base and envelope.

It's more likely that the glass envelope developed a leak, maybe a pinhole, it's hard to say, when the silver getter turns a milky white colour that's a sure sign that air somehow got into the envelope, for a tube to operate correctly there has to be a very good vacuum inside the glass envelope, this means that the pressure inside it is a lot less than on the outside.


You could try having a very close look at those faulty tubes with a strong magnifier to see if there are any cracks in the glass envelope, sometimes it's not very apparent.


I would also concur with what Marcel said about the thinness of the glass envelopes.


There's usually a few different ways that a tube can fail that I can think of:

1, The Cathode suffers from something called "Cathode Poisoning", this is where the electron-emitting coating on the cathode gets contaminated by absorbed gas, this leads to a reduction of emission, which is the ability of the cathode to give-off electrons which boil off the surface of the cathode when it is heated, rather like how steam boils off the surface of water when it is heated, note that there are other ways the cathode can be poisoned.

2, The heater can go open-circuit, or burn out.

3, The envelope can crack due to physical shock or heat stress, sometimes when output tubes go into red-plating (that is, the anode starts to glow red due to excessive current), the tube can get so hot that the glass envelope literally melts to the point that a hole develops which lets air in, that's a pretty extreme example.

4, Sometimes a tube can fail due to something called "Ion Migration" between two pins, this creates a conductive path between the two pins which can cause a short circuit between say the grid and cathode, or screen grid, the tube will cease to operate correctly.

5, Sometimes the glass itself can be very slightly leaky even without obvious cracks or holes, this causes a very slow ingress of gas over a very long period of time.

[RalphH]

I'm thinking the failure must be physically related. I have a few tubes which are electrically dead due to some of the reasons you describe, but they still have their physical integrity. All of the failed tubes have serious cracks in them; mostly radiating from the bottom. This is where the idea of the heat stress was born; that and the fact that the Tung-Sol's were a bit tighter in the sockets than the previous (and now current) tubes.
Understandably, I'm reluctant to install any other new tubes until I understand what the problem is. 10+ year old 7025's and 12AX7's are doing their duty for now.

[enikoy]

Have you spoken to Evatco about it? They seem pretty knowledgeable and good guys in my dealings with them.

Tung-Sols are my go to tube for guitar and hi-fi amps and for me have definitely been the best $/performance tube. Very interested to find out what happened.

[RalphH]

Have you spoken to Evatco about it? They seem pretty knowledgeable and good guys in my dealings with them.

Tung-Sols are my go to tube for guitar and hi-fi amps and for me have definitely been the best $/performance tube. Very interested to find out what happened.

Yes, have been having a conversation with Evatco. No mention of the Tung-Sols potentially being faulty. Check out the amp, etc...
The amp's performing like a trooper with Tube depot 7025's and NOS Mullards, so I'll just leave as-is for now.
When the cause is eventually determined, I'll share accordingly...

[DrNomis_44]

Yes, have been having a conversation with Evatco. No mention of the Tung-Sols potentially being faulty. Check out the amp, etc...
The amp's performing like a trooper with Tube depot 7025's and NOS Mullards, so I'll just leave as-is for now.
When the cause is eventually determined, I'll share accordingly...

Just a thought, has the amp ever been in transit recently?, ie, being transported from one place or another?, also, was the amp recently moved from a warm environment to a cold one after it had been powered down after use?, that can sometimes end in tubes cracking.