Amplifier designers confuse often data sheets maximum possible values, with recommended use values.

THIS PROCESS MAY NOT BE USED FOR GOLD GRID TUBES. Such do not need it, and when you do it anyway, the gold wil damage the cathode.

This process is RISKY, and should only be used as a final attempt, befor throwing a tube away. I was lucky with it several times though. In particular, you can not repair a grid current problem this way, when the cause is gas, or dirt on the mica, or leakage inside the socket. It may help only against grid current, caused by grid emission, and the tube must be good for the rest of it.

One of the problems, overheating can cause, is small migration of Barium, from the cathode to the grid. Like when anode voltage is maximized, and anode current also, and heater voltage is out of tolerance even. If this damages new tubes, the shouting is always loud. It becomes even more painful, when RCA NOS tubes, or other top quality items are damaged. Because then we know it was the amplifier, but the amplifier builder will never admit it, and will try to blame the tube seller.

The defect mechanism: The small Barium residue on the grid, will cause so called grid emission. This is real emission, but the problem with that is, the grid current resulting from it. This will offset the tube bias, and the anode current will increase. Now this happens with a tube, which "by design" is heated to it's limits. This great heat will migrate even more Barium to the grid, and the anode current begins to increase, Further and further, it is an avalanche effect. This results in the initially good tube, to out glow it's anode bright orange, and the fuse will blow. This effect however, migrates even more Barium to the grid, and the tube keeps in running away thermally. Though every time, the tube works good, until this happens.

The repair: Sometimes however, such a tube can be repaired, by glowing the grid cherry red. This is definitely not good for the grid, but the Barium will evaporate. This is done, by using the grid as a diode. The anode has little function here, but we keep it negative voltage. Like this, the anode will attract the Barium Ions, by electrostatic force.

The result: Afterwards, mostly the tubes have somewhat reduced transconductance, but still within specs, and a long burn in may improve this, provided there is enough life in the tube left. So a low emission tube, even suffering from grid emission, has little prospect to be repaired. Yet a strong emission tube, but with thermal runaway due to grid emission, make come out of the process without grid emission again, but with slightly less emission as it had before. But at least it is good again.

The test circuit used here, is very unconventional, REVERSING grid and anode. Like this, the grid will be heated as if it was the anode. Whereas the anode, from the outside, is given a negative voltage, on order to catch positive ions. First, we want to catch those, and second we do not want to damage the cathode with those. It is remarkable how much power the gird can take, it is several Watts.

You need to look extremely careful inside the tube, and really be able to see the grid clearly, use a good lens for this, or whatever it needs, so you can see it. Glow the grid, the same red color as the cathode. How long to to this, and at what voltage, is written at the test cards.

The risk is, to glow the grid too hard, and it will be damaged, Also it may exhaust some gas. So heat the grid, but not over heat it.

Afterwards the tube needs a long burn in, so the getter can absorb the dirt which was removed from the grid, and initially absorbed by the anode surface. Slowly this will evaporate from the anode, and finds it's way to the getter, provided the tube is not gassy, and the getter is still good.