Why does red plate spell early death?

[EC8010]

Another source of water vapour (for poisoning the cathode) is the mica wafers that support the electrodes. All mica necessarily contains water, so if you heat it excessively (with a red-hot anode) it will outgas - that's why high reliability valves use ceramic electrode supports.

As Wavebourn points out, a very few anodes are designed to operate dull cherry red because they are coated with getters such as zirconium. Usually, they're graphite anodes, and they always have ceramic electrode supports.

[kevinkr]

Take a look at the 8005 sometime - this glows cherry red in operation and the first time I saw this in a MAC MI-250A (IIRC) I was more than a little disturbed even though the service manual stated quite clearly that this was normal. In this tube running with some plate glow actually allows the plate chemistry to capture and recombine gas floating around in the envelope. I think there are also certain Eimac triodes that operate this way - a bit counter-intuitive if you ask me..

Makes me wonder if whether with careful operation long out of service power triodes like these might slightly improve their vacuum once restored to operation? I have heard this alleged, but have not experimented to find out if true or not. Baking at high temperatures "allegedly" helps too, but I am wondering whether this is all urban legend as I can't imagine you could get things hot enough for this to work without compromising the glass and seals?

Comments?

[richwalters]

Originally posted by JohnAtwood
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.

- John Atwood

Well written- expanding- this is one of the reasons why not to exchange a KT88 designed o/p stage with 6550's unless well below the power/B+ ratings. On paper the tube power ratings seem identical but the 6550 has a softer vacuum than the 88. So that's why in fixed bias high power stages, make sure the 50K grid leak resistance isn't increased. Tough work for driver stages.

If you force the 6550, the symptoms of anode current runaway become noticeable regardless of the amount of neggy volts applied and the grid (g1) is no longer in control. Most tubes posess this when pushed beyond ratings, some more than others.

richj

[HollowState]

Originally posted by kevinkr

Makes me wonder if whether with careful operation long out of service power triodes like these might slightly improve their vacuum once restored to operation? I have heard this alleged, but have not experimented to find out if true or not. Baking at high temperatures "allegedly" helps too, but I am wondering whether this is all urban legend as I can't imagine you could get things hot enough for this to work without compromising the glass and seals? Comments?

From personal experience I can tell you that to some extent this is true. Small transmitting type tubes can sometimes be degassed by simply running them. It all depends on how gassy they are. Some, like the one Tubelab George put up pictures of, that are deep blue/purple inside are way too far gone. Others less gassy can often be brought down by running them for a time. Sometimes it will take hours to recombine the gas, and other times it will take only a few minuets. I've actually watched the grid current fall (using my big rack tester) as they ran. Blowing forced air across them helps in the process. And sometimes cycling them through hot runs and then cooling them down to be heated again will be needed to get the gas down.

Certain Eimac tubes like the 4-250A, 4-65A, 304TL, 4E27 and many others, big and small, were built with a tantalum plate that was designed to run cherry red during normal operation. The red hot plate is the getter in these tubes. Eimac even coined the term "Pyrovac Plate" for this action.

Some tubes like the 813 and European versions of the above mentioned types (and others) employed a carbon or graphite plate that when hot provided gettering action. I have even been able to cook out gas in some external anode tubes like the 4CX250B and 8122 for example. While these tubes have only a copper anode, baking them under power can sometimes help. It really all comes down to just how much gas you are dealing with and how much time and effort you're willing to spend. Several times I wasted hours trying to degas a tube only to find that it was weak or the cathode had become poisoned.

Victor

[SY]

Kevin, Morgan Jones wrote an article some years back (Electronics World, Nov 2000) where he did some experiments on baking tubes that have been long in storage. IIRC, he also mentions this in VA3.

FWIW, I used some D3a that he had baked and they seemed a bit quieter than "fresh from the old box" ones, but they suffered a loss in cathode emission at a rather young age. It may not have been the baking, I have no controlled tests to indicate that and the baked tubes were from a different batch than my unbaked ones, so I'd want to try it on something cheaper and easier than more of my precious D3a.

[Elias]

Hello,

Can it happen in over heated tube that something melts and drops and makes short circuit inside the tube, for example connects anode voltage to input grid which would lead to input capacitor exploding? That would not be very nice.

- Elias

[Wavebourn]

Here is probably the original case where cryogenic treatment and break-up hours' myths had started from...

[Bandersnatch]

hey-Hey!!!,
The Tantalum anode tubes designed to run red/orange hot can indeed be de-gassed. This process needs some careful attention, as the cathode is also hot and it seems to me that that element can also get wrecked by gas( Thoriated Tungsten ). I've fired the cathode, and rapidly applied plate voltage....lots of blue glow and kicks in the current limits of my supply but quickly the anode glowed, and the gas stuck to it.

For the grid circuit issue, seems that the lower the value the better, ending at a grid choke or an IT secondary. The 813 limit is 30k, and for my apps, a grid choke will do much better. Same with 50's, Eimac transmitting tubes, etc.
cheers,
Douglas

[Geek]

Hey George,

Doesn't a red plate just mean the "right cooking temp" for a Tubelab stress test?

Cheers!

[Ty_Bower]

Yes, but notice that George only does that kind of stuff to the cheap tubes.

Thanks to everyone who had something to offer to this thread. There's been a lot of good responses, and it's been truly educational.