Thermal Runaway

[hasserl]

I was reading this older thread http://music-electronics-forum.com/t27434/ and the post from Rhodesplyr where he wrote:

I know that 6V6s and 6L6s technically have the same maximum grid-leak resistor values on the datasheets, but my experience has suggested that 6L6s are slightly more prone to thermal runaway when run hot than 6V6s.

That got me thinking about thermal runaway, and if my understanding of the issue was the same as his. I did a google search on the term, which brought up a wicki page on it, which agreed with my understanding; that is that the condition of excessive heat leads to increased current flow, which causes more heat, leading to more current flow, etc. This is thermal runaway. So, if tubes are subject to thermal runaway, what is it that causes increase in current flow as the temperature increases? As I understand it, current flow thru a tube is controlled by the voltage relationship between the cathode and the control grid; and on a pentode or beam power tubes also by screen grid voltage. So what does increased operating temperature do to these elements that results in an increase in current flow?

And, what is there about 6L6's that make them more susceptible to this? Or is it your experience that 6L6's are no more susceptible to this than 6V6's? For that matter, is there a general rule of thumb about which tube types are more thermally stable and which are less stable?

Or, is the term "thermal runaway" used just more as a generic term for conditions where tubes are operated too hot, too much dissipation, for the tube to remain stable? I.e. the tube cannot dissipate as much heat as is being generated, and it eventually reaches the point of failure due to the heat generated; not that the heat causes increased current flow resulting in a true runaway condition?

Thanks,
Hasse

[Steve Conner]

When the control grid of a tube gets too hot, it starts to emit electrons, just like the red-hot cathode does.

This electron flow (sometimes called "reverse grid current") passes through the grid leak resistor and develops a voltage across it which opposes the bias voltage. The tube is biasing itself hotter.

When a tube goes into thermal runaway you can see the control grid voltage collapse right down near zero as the plate goes red. In an amp with parallel tubes it will drag down the bias to the other tube too.

[hasserl]

Thanks Steve, that helps. so, any input on the issue of some tubes being more susceptible to this than others?

[R.G.]

There is another effect of overheat as well. When tubes are evacuated, all the gas molecules can't be removed. Some are semi-permanently attached to surfaces inside the tube ("adsorbed"). As the tube gets hotter, they get more thermally agitated and can be shaken loose. They get ionized by the electron flow, and wind up making the "vacuum" more conductive by moving charge with the ions, and in the opposite direction from the electron flow. This too affects grid bias and can make the tube hotter, making more emission from any place that will emit, making it hotter...

Ion bombardment of the cathode can actually knock off some of the metal-oxide coating on the cathode and let it condense on the grid and other places, so they also boil off electrons when they get hot. This adds a lot to the grid-emission effect as well.

[hasserl]

When the control grid of a tube gets too hot, it starts to emit electrons, just like the red-hot cathode does.

This electron flow (sometimes called "reverse grid current") passes through the grid leak resistor and develops a voltage across it which opposes the bias voltage. The tube is biasing itself hotter.

When a tube goes into thermal runaway you can see the control grid voltage collapse right down near zero as the plate goes red. In an amp with parallel tubes it will drag down the bias to the other tube too.

Thinking more about this, I'm struggling with seeing how the plate getting hot would cause the control grid to get hot. The control grid resides right next to the cathode which runs far hotter than the plate would get under elevated dissipation conditions, and the heat from the cathode doesn't cause the control grid to get hot. But run the plate hot and the control grid then gets too hot and starts boiling electrons? That doesn't seem right.

I'm not doubting it happens, as I've had at least a couple of EL84's over the years do something very similar; the control grid became conductive and drew the bias voltage down to zero, and the tube running in parallel to it red plated. But this wasn't caused by the tube over dissipating, the bias in both cases was set so the tube before failure was running very mildly. I'm just struggling with envisioning how this happens.

[hasserl]

Well, an hour spent searching the net and I did find this jewel: Tubes 201 - How Vacuum Tubes Really Work And it does describe both conditions mentioned by Steve and RG:

Grid Current

Since the grid has no physical connection to anything (as the circuit symbol shows), it is natural to think of it as being electrically isolated. In fact, this is not the case, because of the flow of electrons and ions inside the tube.

Current flow to and from the grid arises for two reasons. The most obvious is when the grid is positive, which causes it to attract electrons. Less obviously, when the grid is negative, it attracts positive ions resulting from collisions between electrons and gas molecules. These cause the grid to become less negative. It is because of this effect that the grid must never be left truly floating but must always be connected through a resistance, typically for a small tube not more than 1MW. Without this, the voltage of the grid will gradually creep up, reducing negative bias and increasing current through the tube and leading to a runaway which, at worst, will destroy it through overheating. Tube gassiness is measured, in tube testers, by measuring this small current (typically a few nA), which is in direct proportion to the amount of gas in the tube.

At the right voltage, these two currents cancel out. This normally occurs at around –0.5V (depending slightly on plate voltage), which is slightly positive relative to the virtual cathode. If the grid is simply left disconnected, it will float at this voltage.

Positive grid current is deliberately used in some cases, for example in high-� transmitting tubes. When it occurs, Child’s Law (or the more accurate formula taking account of initial electron velocity) gives, not the plate current, but the sum of the plate and grid current. This is because total current is controlled by the field in the immediate vicinity of the cathode, which is due to the combination of the two other electrodes. To calculate the actual plate current (and hence also grid current), it is necessary to know how much of the electron stream flows to each of the two electrodes. This is given from the following formula:
where: d = current division factor

The current division factor is slightly greater than the shielding ratio of the grid. If grid current curves are available for a tube, it is easy to determine its value from them.

Unfortunately this tidy formula only gives the value for the primary current. Once the grid is more positive than the plate, secondary emission will start to occur, resulting in a secondary current flow from the plate to the grid and reducing the effective grid current. This is, for all practical purposes, unpredictable. Depending on the tube, the secondary current may even exceed the primary current, typically at around 30-50V, resulting in a second stable voltage for a disconnected grid at the point where the primary and secondary currents exactly balance.

Positive grid current results in heating of the grid, for the same reason that the plate gets hot (i.e. due to the kinetic energy of the arriving electrons). Small tubes are not generally designed for this, but power tubes and especially tubes designed for positive grid operation have substantial dissipators attached to the grid structure. Overheating of the grid is bad for two reasons. Firstly, since it is not designed to run at a high temperature, it does not have any way to retain its tension as the metal expands. This results in changing geometry and in the worst case melting or a short to the cathode, which is instant death. Secondly, the grid is generally contaminated by oxide from the cathode, and if it gets hot then it will start to emit electrons like the cathode, resulting in a substantial secondary current.

But again, I don't believe either of these conditions are the result of running the bias "too hot".

[tubeswell]

I think of the grid-cathode bias voltage as an 'electron brake' which inhibits the flow of electrons from the cathode to the plate. If you lose the bias voltage, its like taking the brakes off completely and the electrons will then just go holus bolus toward the plate, and the plate won't cope with this, and quickly gets beyond the design point for dissipating the energy that is being fed into it, whereupon it then goes into meltdown, taking the envelope with it.

[Chuck H]

its like taking the brakes off completely and the electrons will then just go holus bolus toward the plate,

Right. I think I read that. holus bolus is the technical term for it

It's pretty ugly. With the grid acting like a cathode the tube acts somewhat like a diode. Except it's supposed to be gated in it's current allowance. So the tube begins to pass as much current as it can relative to the load, only it can't handle all that current so it typically melts instead. How much damage becomes a matter of what internal components melt first. I saw a thread here once where a tube had literally melted a hole in the glass!!!

[Steve Conner]

Well, when the plate gets hot it heats up everything else in the tube too. Not much, but enough to make a difference. Even things that are hotter than the plate will be heated (or rather, cooled less) because they'll find it harder to radiate their heat away.

High current tubes have a larger cathode area, and high transconductance ones have a control grid of fine wires placed as close to the cathode as possible. So this combination is asking for thermal runaway and needs a low impedance grid circuit to keep it stable. You should see that something like an EL509 will have a lower spec for grid circuit resistance than a 6L6.

These tubes often have a gold-plated grid, but it gets contaminated with cathode material as the tube ages. And you can see two large cooling fins in the space above the plate, these attach to the control grid support rods to help it radiate heat.

The other stuff mentioned above is also true.

[Enzo]

I'm struggling with seeing how the plate getting hot would cause the control grid to get hot.

Think of it as a symptom rather than a cause. Correlation is not necessarily causation. Like a blowing B+ fuse is not the cause of the associated power tube failure.

[JoeM]

Here's a photo, maybe the one that Chuck saw.

[Chuck H]

That's not "the" one. But it's a good one. Wow!

To think that it operated beyond it's dissapation rating long enough to generate that kind of heat!!! Now that's a tube.

[dai h.]

Hasserl's point about the grid being so close to the (heated by the heater towards the inside) cathode was interesting (g1 must get hot being so close). From what I gather, it's apparently the *degree* of (over)heating that matters.

reading this (old Matsushita technical info sheet) :

http://www.ne.jp/asahi/myamada/tube/shicopy/6ca7_2.gif

"The glass bulb and electrodes for an output tube experience substantial increases in temperature. If the rise is excessive, gas and electrons can be emitted from the metal parts. As a result the degree of vacuum can be lowered and tube performance compromised through grid emission. Therefore, glass bulb temp. needs to be kept under 250 degrees Celcius, and electrodes besides the cathode need to be kept under 600 degrees Celcius. And the lower the better (which goes without really needing to say). ...Also the grid posts are made of a special copper alloy to enhance heat transfer and the grid wires are gold plated to lessen the worries in regards to grid emission. Therefore a relatively high value of grid load resistor may be used."

(The left side drawing is sort of a cutaway from one end of an EL84 (byline says "EL34 is extremely similar"). The "sideways elongated barrel" is the cathode, next is g1 and so on.)

pic of grid posts, grid fins in EL34 (you can see how it corresponds to the drawing with the "elongated barrel" shape cathode, etc.) :

http://www.ne.jp/asahi/myamada/tube/...7/6CA7_M_P.JPG

[tubeswell]

I saw this a while ago on youtube Overloaded EL34 - YouTube

[hasserl]

Hasserl's point about the grid being so close to the (heated by the heater towards the inside) cathode was interesting (g1 must get hot being so close). From what I gather, it's apparently the *degree* of (over)heating that matters.

reading this (old Matsushita technical info sheet) :

http://www.ne.jp/asahi/myamada/tube/shicopy/6ca7_2.gif

"The glass bulb and electrodes for an output tube experience substantial increases in temperature. If the rise is excessive, gas and electrons can be emitted from the metal parts. As a result the degree of vacuum can be lowered and tube performance compromised through grid emission. Therefore, glass bulb temp. needs to be kept under 250 degrees Celcius, and electrodes besides the cathode need to be kept under 600 degrees Celcius. And the lower the better (which goes without really needing to say). ...Also the grid posts are made of a special copper alloy to enhance heat transfer and the grid wires are gold plated to lessen the worries in regards to grid emission. Therefore a relatively high value of grid load resistor may be used."

(The left side drawing is sort of a cutaway from one end of an EL84 (byline says "EL34 is extremely similar"). The "sideways elongated barrel" is the cathode, next is g1 and so on.)

pic of grid posts, grid fins in EL34 (you can see how it corresponds to the drawing with the "elongated barrel" shape cathode, etc.) :

http://www.ne.jp/asahi/myamada/tube/...7/6CA7_M_P.JPG

Dai, that's good info that backs up what Steve and RG posted earlier. Thanks for posting.