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Test Cards for L1 and L3 tube testers (L3-3)
(Last updated: 19-Jun-2016 15:02 )

 

6SN7 REJUNIVATION CARD

With tube rejunivation (regeneration), there is a lot of mystery. The only literature I know about, is some articles in radio magazines from the past.

The 211 tube.

I once found an original manufacturers instruction inside a NOS box of General Electric 211 (VT4C) tube, how to rejunivate 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), rejunivating 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 for 6SN7. 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 rejunivation. Whereas Chinese new made 211 suffer from all kind of other problems, and they get permanently bad within 1000 hours often.

TV picture tubes.

Cathode ray tubes rejunivation 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 less easy. Procedures are different for indirectly or directly heated tubes, and different for Thoriated Tungsten and for Barium coated tubes. Moreover, tube rejunivation 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, rejunivation can not repair physical damage, and rejunivation attempts on such tubes will not work, or have a negative effect even.

Physical Damage.

When taking apart a 6SN7 that was not possible to rejunivate, 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 cathode sleeve becomes visible underneath, mostly in the center of the tube. It should be clear that such a tube can not be rejunivated 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 rejunivated partially, as long as some smallest residue of material is still there. This is really remarkable. However what is very difficult to rejunivate, is a cathode with darkened burn spots on it. This darkening seems to be an emission killer, and is hard to remove. It can not be blasted away by high temperature, as that will only make it worse, and will chip off larges parts of the cathode. These observations were made by opening up tubes, and do the experiments on only one cathode of the two triodes that are 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.

Conclusion.

Rejunivation has best chances with tubes that have still a reasonable good condition, like 75% Anode current at normalized condition (which is for 6SN7 250V Anode, and -8V on the grid). 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 can do additional damage, rather than repair 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 rejunivation 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. Yet, it has be said about the 211 (see above) that rejunivation was so much recommended, General Electric put even an information sheet about this inside the boxes of military versions (VT4C).

How rejunivation is done with 6SN7.

Chances on successful procedure exist, when the tube is physically undamaged. So we try to improve the composition and structure of the cathode surface, and the cleanliness of the grid. The methods we have for this is:

  • Temperature shock of the cathode
  • Temperature of the whole tube
  • A process called electrolysis

It will not be described here, how each of those processes work physically. It will be only describes how to do the process.

  1. 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 rejunivation result with this initial result. After this, use the rejunivation 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 rejunivation card again, and do the heater cleaning steps, as indicated on the card. Already with the rejunivation card, you may (or may not) see an increase of plate current, before and after. Also here, any improvement is a positive sign.
  2. 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 plate current of minimum 35mA. This is for both systems together, as both anode plugs are inserted together. (don't try with only one plug!). Use 6.3Volts heater. A weak tube will not supply this 35mA, meaning it doesn't get hot enough, but you need to be patient. 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 you are above 35mA or when nothing changes any more. If it stays below 35mA, go to the next step.
  3. Below 35mA, and when nothing changes, the tube doesn't seem to get hot enough to self clean. So we need to increase the heater voltage to 6.5Volts, in order to boost emission for a short time, in order to allow a higher plate current. The tube may now come in such a condition where it begins to rise plate current slowly. This may take a few minutes to begin. If the increase process will not begin, you can increase the heater voltage to 6.7 Volts. Or even use 7 Volts if 6.7 Volts will not work. So to say, to make something happen. However the higher the voltage, the higher the violence, and the higher the risk on damage. So try at lower voltages first, and be patient. So when you see something begins to happen, at lower voltage, do not interrupt this, as long as you are below 35mA.
  4. Note: Electrolysis, this is the slow and steady increase of plate current described above. Like 0.1....1mA per minute. When it begins slowly, and then faster, that is a good sign. This process restores the solid Barium layer, but it presumes cleaning was done in the previous steps. In the above steps, the grid was already cleaned. There are some substances that can collect on the grid, which change the bias of the tube.
  5. As soon as you can, reduce the heater voltage, in the direction of 6.3V again. Naturally this will initially cause a drop in plate current, but the repair of the tube now starts, when you see the plate current begins to recover slowly still. Let it recover, and try if it it keeps on recovering at slightly lower heater voltage too. So any recovery process, you can maintain at possible lower heater voltage is preferred. So you need to get away from higher than 6.6V heater voltage as soon as possible.
  6. If all works well, the tube can be used somewhere near 35mA at 6.3V, and probably becomes stabile.
  7. Try to exercise the tube such that it can do so even below 6.3 Volt. So try 6.2...6.1 Volt. This will initially cause a reduction in plate current. After thermal balance, watch if the plate current has a tendency to go up again. This may take some minutes, and begin very hesitating. Then go faster. If this is so, try to go as low as possible, but below 5.9V. Always when you see recovery, do not interrupt it.
  8. If you see the tube slowly loose emission again, below 6.3V, it was not ready for it yet. So exercise the tube further at higher heater voltage if needed.
  9. At a successful recovery, you will find the tube willing to draw reasonable current still somewhere below 6.3V. If so, keep it in that condition for one or more hours, but always be careful the tube increases. At any continuous decrease of plate current, the heater voltage should be increased, until recovery begins. Generally you are unsuccessful if this requires a heater voltage above 6.3V.
  10. At some point you will conclude this is the best you can do.
  11. Now take the normal 6SN7 test card, and see if the tube is above acceptance level again.

Please do not ask for Rejunivation cards for other tubes. I will post them if available.


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