Since 1993 Copyright Notice

With tube Rejuvenation (regeneration), there is a lot of mystery. The reason for this is, sometimes it may work, and sometimes not at all, and people think it only needs some kind of a trick, or secret method. But there is no such a single method. I just list up some of the points here, unsorted.

• Gassy tubes. Gas can have two reasons: Vacuum leak, or outgassing of the metals. Against both things there is nothing you can do. There is of course the end phase where the tube fails completely in the circuit. But before that, you observe is a small reduction of plate current, and lower transconductance. A gas test with such tube gives a small reading only, nothing serious, and some good tubes many have the same reading, while they have no gas at all. However, over time it progresses, and at some point you may have the idea to rejuvenate the tube. Well, this is not going to work, unless there is Zirconium getter in the tube, additional to the Barium getter. All other tubes gets only worse from it. The assumption that Barium getters will remove the gas, is wrong. If they could, they already would have done so. The only hope you may have, these getters become active when they are above 180 degrees. And that is really very hot. Tubes which are not capable of that glass temperature, at the place where the getter is, and that is the majority, those will not re-activate the getter. If the getter is already disappearing a little but, the tube has lost it's high vacuum. It stil works partially, at low vacuum, but you can't change that into high vacuum any more.
• Zirconium getter may give you some hope. This is a dark grey material, powdered on the anode, or soaked into graphite anodes, or present as a tiny wire mesh getter, powdered with Zirconium, at a place where it gets very hot, as done with the famous C3g tube for instance.
• In general though, a gassy tube, with lots of hours on it, it is the end.
• The cathode, when has lost a part of the emission, may be rejuvenated, provided there is some active material left, in deeper layers, and provided we can migrate this to the surface.
• In some other cases, the cathode can be poisoned by gas attached to it, or by metal or carbon deposits.
• Thoriated Tungsten Cathodes and Barium cathodes are rejuvenated different.
• Thoriated tungsten can be rejuvenated multiple (!) times. The radio active Thorium evaporates from the cathode, which process speeds up by gas residue. However deeper inside, is a lot left, and it needs only to be migrated to the surface by a short period of over heating, without anode voltage. Instructions for this, you will find with the 211/845 tube Rejuvenation cards, which are in the L3-3 tube tester card set we sell. This is a risky process, because Thoriated Tungsten with a lot of use on it, has become very brittle. So the wire may break, and you need a bit of luck. But this is still better than throwing the tubes away. I once found an original manufacturers instruction inside a NOS box of General Electric 211 (VT4C) tube, how to rejuvenate this tube, and they wrote this could be done three times. Given, lifetime of properly used VT4C is 10.000 hours (exceptional is 40.000 hours), rejuvenating it three times gives unbeatable life time. This is a thoriated tungsten tube, 100 Watt dissipation, it can withstand a few Watt grid dissipation. The heater contains radio active substances to some content, and the anode is soaked in Gettering material. All of this does not apply to modern tubes. So in case you own a weak historical VT4C, don't be very optimistic, as most of the time (now 80 years after production) problems are related to bad vacuum, and not to lack of Rejuvenation Whereas Chinese new made 211 suffer from all kind of other problems, and they get bad within 1000 hours.
• Barium Cathodes, if used correctly. and they reach the end of life, there is nothing you can do about it. The internal depot came to an end.
• Barium Cathodes, if under heated, suffer from two problems. First, there is no re generation from the inside. So the outside layer evaporates, and the emission goes down quickly. Simply use the tube at correct conditions will make it recover, provided the emission was not lower as 30%. Second, due to lower temperature, any unwanted material will not evaporate, and collect on the cathode. This is called poisoning. Against this problem, a short over heat period may help.
• Barium Cathodes, if over heated, are sure not poisoned, but the balance between evaporation and regeneration was lost, and there was too much evaporation. Just use normal conditions may bring the tube in good condition again, provided the over heating did not last very long. If some Barium deposit is left, it will migrate to the surface, and the tube may come back to life again.
• Chrome tops, or chrome getter tubes. For good reason, such tubes are very popular. They look a lot nicer, but the getter is not chrome, but an aluminium alloy. This is part of a much older process, not used any more approximately until 1945. These are much better quality with respect to long storage. They just don't seem to suffer from that. But, when they are worn out, I was never able to rejuvenate them. Probably such tubes with Thoriated Tungsten cathode, you can rejuvenate them by an over heating impulse, but I never had such a tube on the bench, and I just don't know. For the Barium cathode Chrome Tops, I can tell you, it doesn't work.
• TV picture tubes. Cathode ray tubes Rejuvenation is another story. This is considered rather "maintenance" , because these cathodes run so extremely little current, and there is so much physical mass and chemical substances inside a CTR tube, any poisoning may happen. Such thing as grid poisoning will not happen, as a CRT has no conventional (wire) grid. Wear out of the cathode is very low at this current. Meaning, the cathode has enough Barium depot still, and no material will be chipped off. So all we need to do, expose the heater to a thermal shock, to blow the contamination off, and you're done. So in the end, replacing a picture tube because of worn out cathode, was seldom needed. The CRT usually outlives all other tubes in the TV.
• With amplifier tubes, the situation is not easy. Procedures are different for indirectly or directly heated tubes, and different for Thoriated Tungsten and for Barium coated tubes. Moreover, tube Rejuvenation for gassy tubes is another than for low emission tubes, and repair a tube that had instable bias due to grid emission is a specialty of itself. Yet all of this can be done with some chances on some degree of of result. We do need to be aware, Rejuvenation cannot repair physical damage, and Rejuvenation attempts on such tubes will not work, or have a negative effect even.
• Physical cathode Damage. When taking apart tubes, that are not possible to rejuvenate, normally damage of cathode is visible with the bare eye. The cathode surface is not nicely homogenous, and not nicely bright colored. Instead of that, there is darker color, the appearance is like "burned" indeed, and has a bad appearance in general. Cathode material can be chipped off, and the naked cathode sleeve becomes visible underneath, mostly in the center of the tube. It should be clear that such a tube cannot be rejuvenated back to 100%. Though some small improvements may be possible even on such a tube. From many experiments I did, my conclusion is, a chipped off cathode can be rejuvenated partially, as long as some smallest residue of material is still there. This is really remarkable. However, what is very difficult to rejuvenate, is a cathode with darkened burn spots on it. This darkening seems to be an emission killer, and is hard to remove. It cannot be blasted away by high temperature, as that will only make it worse, and will chip off large parts of the cathode. These observations were made by opening up tubes, and do the experiments on double triodes, doing only one cathode of the two inside. So indirectly by that method, a "before" and "after" condition can be compared. If a 6SN7 has two dark burned cathodes, I tried to clean one of the two by burning it away. The result was always negative, so inside I found one burned but intact cathode. The one with cleaning attempts was still burned, but additional a lot of the material was chipped off. Yet amazingly, sometimes this was am improvement still. In general though, any process, using a harder and harder treatment, in order to "make something happen" most of the time will cause only more damage.
• Golden rule: Whatever you do, when you see recovery, NEVER interrupt it. If you stop it, you may not be able to start it a second time.

• Temperature shock of the cathode. For this, first we need to remove old residue off the cathode which may have gathered during storage. First, use a normal test card, and make note of the test results. This is important, as we need to compare the Rejuvenation result with this initial result. After this, use the Rejuvenation card, and do not insert the anode plugs at first. This will set the grid voltage and anode voltage to zero Volt. Set the heater voltage to 6.3 Volt, and run the tube for 6 hours that way. This will initially remove some dirt from the cathode. Take the normal test card again, and see if you had some improvement. If so, this is a positive sign. Now take the Rejuvenation card again, and do the heater cleaning steps, as indicated on the card. Already with the Rejuvenation card, you may (or may not) see an increase of plate current, before and after. Also here, any improvement is a positive sign.

  Temperature of the whole tube. The whole tube is now heated up as far as it will take for the time you are doing this. This will require maximum dissipation. When you see a small increase, slowly, but steadily, the repair process has started. Let it run, do not stop it, do not change anything as long as it works. Stop when there is not further improvement, since maximum dissipation is not very good for the tube.

• A process called electrolysis. This causes a slow and steady increase of plate current described above. Like 0.1....1mA per minute. It t begins slowly, and then faster, that is a good sign. This process restores the solid Barium layer
• Grid cleaning. There are some substances that can collect on the grid, which would change the bias of the tube. So the tube will draw less current. In general, this may happen with tubes, made of not very clean materials, with little outgassing done, cheap getters, too hot glowing cathodes, and all other kind of sources for contamination. In short: This is a problem with cheap tubes. It t is a very difficult and delicate process, to clean the grid by out glowing it. This is done by not connecting the anode, and use the cathode + grid as a diode. The grid becomes the anode of this diode, and can be heated up this way. It can be glowed dark orange for a few seconds. This needs to be done in the dark, with a lens, so you can look inside, and see the grid glowing. There is a 300B grid cleaner card with the card set. However this may NOT be done with gold plated grids, as the gold will come off, and damage the cathode.

Conclusion.

Rejuvenation has best chances with tubes that have still a reasonable good condition, like 75% Anode current at normalized condition. Such tubes need no hard treatment, and the balance between "repair" and "additional damage" is a good one. The tube may come recover to 85% and sometimes even 100%. Tubes However, that are below "bad" which means below 60% plate current, need a much harder treatment, which will do some additional damage, while repairing existing problems. So it can normally not be brought back to 100%. However, with some luck, and not too tough treatment, you can get it back at 70%, which is the level is a "good" test result. Given, most Rejuvenation leaves also some damage, it should be clear this will nit last as long as a new tube, and repeating it will not be possible many times.