The great Tung-Sol tube failure mystery

[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.