Why does red plate spell early death?

[Ty_Bower]

Biasing tubes too hot typically results in red spots glowing on the plate. It is common advice to run tubes below maximum dissipation, or their life will be shortened.

Exactly what physical or chemical phenomenon causes the accelerated failure under high dissipation conditions? Does the cathode just burn off its emissive coating that much faster? Does the heat warp the plates? Or do the plates offgas at high temperature, ruining the vacuum?

[nhuwar]

It can ruin the vacuum in tubes by releasing trapped gases in the metal that weren't baked out.

Alot of infra red radiation being pushed inward the the grid and cathode and if the grid cant take it then it could very easily melt.

You can also start boiling off metal causing it to sublimate and condense on everything else in the tube.




Nick

[JohnAtwood]

All metals have gas "dissolved" in them when they are made. Part of the processing of a tube is to heat-up all the internal elements as hot as possible (without damaging them) to drive out the gas while the tube is still connected to the vacuum pump. However, this process of diffusion takes time, and for mass-production can only be done for so long. At the end of this out-gassing phase, the tube is sealed and then the getter is "flashed" which captures nearly all the remaining gas as well as leaving a coating on the bulb that will absorb some gas in the future.

When a tube is over-heated, usually evidenced by red or orange plates, more gas is liberated from the hot metal. Reactive gasses, such as oxygen and water vapor, combine with the barium oxide on the cathode and "poisons' it, thus reducing its electron emission capability. At some point, the emission isn't enough to supply what the tube needs, and it goes "flat".

The presence of gas in a power tube can also cause positive grid current to flow. This happens because at high voltages, gas molecules ionize (an electron separates, leaving a positively-charge atom), and these positive ions get attracted to the negative grid. As they accumulate, a grid current starts flowing. If there is a high resistance in the grid circuit (i.e. the grid resistor), the grid voltage starts to go more positive, causing the tube to draw more current, causing the plate to get hotter, thus releasing more gas. The end result can be a spiral to violent death. This is why it is important not to use too high a grid resistor value in power amp circuits.

The positively-ionized gas molecules are also attracted to the cathode, and because of their high mass, can actually physically damage the barium oxide layer. This is called "cathode stripping".

As mentioned before, the getter flash can absorb some gas molecules, but as it only has a certain capacity to do so. That is why power tubes that are driven hard or have been used a lot have an eroded flash.

The getter flash can cause problems in really high-voltage tubes, such as transmitting tubes, so many of them, such as the Eimac glass tubes, have no getter. This requires that they be brought to incandescence while on the vacuum pump, and be pumped for a very long time (sometimes 24 hours or more). This costs money, so isn't considered feasible for consumer tubes.

I hope this helps,

- John Atwood

[kevinkr]

Excellent description John!

[nhuwar]

Ya that oretty much put mine to shame


But much more informative.




Nick

[JohnAtwood]

no problem Nick. I started writing my post before I saw yours posted.

You bring up a good point - the sublimation of metals. From what I have read, this usually happens first from the getter flash. The effect of this is to liberate more gas and to cause leakage paths where the metal gets deposited. Another reason why flash getters aren't used in high-power transmitting tubes. Many high-power tubes use materials such as zirconium on the plate, which absorbs gas when hot.

- John

[tubelab.com]

John's description is indeed correct. I will add that many new production tubes, and some old ones were not built with the best attention to component purity and vacuum quality. It takes less "glow" to contaminate the vacuum.

The grid current issue is very real and will lead to runaway, dead tubes, and blown parts. Pay attention to the "maximum grid circuit resistance" spec in the tube manuals. Reduce the resistance far below the maximum if you intend to push a power tube near or above the limit.

Just how much contamination can be generated by a glowing plate? Several years ago I proved that you could "outgass" enough ions from a glowing plate such that an ionized plasma could be generated inside the tube. By careful manipulation of the current limiting I kept the plasma going until the glass began to soften. When the glass began to soften it bulged OUT!

I have since lost this tube, but I plan to repeat the experiment again some day.

[Miles Prower]

Originally posted by Ty_Bower
Biasing tubes too hot typically results in red spots glowing on the plate. It is common advice to run tubes below maximum dissipation, or their life will be shortened.

Exactly what physical or chemical phenomenon causes the accelerated failure under high dissipation conditions? Does the cathode just burn off its emissive coating that much faster? Does the heat warp the plates? Or do the plates offgas at high temperature, ruining the vacuum?

Unlike transistors, VTs like to run hot. However too much of a good thing is a bad thing. In addition to what's already been mentioned, excessive heating can also stress the glass-to-metal seals, leading to the vacuum's escaping. Not a good thing.

Unless the spec sheet says it's OK, red plates are best avoided.

[Salas]

833 OK

[Wavebourn]

Some military tubes were built with nickel or even graphite anodes, so they could run red without suicide.