Tube rejuvination

[thaumaturge]

In broadcasting, a fairly standard practice for squeezing an extra bit of life out of a very expensive tube whose emissions have dripped off is to up the fillament voltage say 10% and let it cook about 12 hrs with no HV applied. This seems to somehow free up a bit more cathode material. At multiple thousands of dollars each even an extra six months of time is worth a little effort.

My question is has anybody tried a similar practice with audio tubes and if so what were the results?

Doc

[maglemer]

I've tried raising the tension even more (50%) but just for shorter periods, 30-60 seconds. It works pretty well on some tubes, but has no effect on others, depending on type and quality, I guess.

[woody]

I believe this just works with directly heated tubes. The higher heat causes some of the thorium to come to the outside of the cathode where it has been depleated.

[DF96]

The same trick was used for indirectly heated CRTs.

[thaumaturge]

Thanks for your replies. I know the cathodes are different in smaller glass envelope tubes. I knew that the main cathode material was thoriated tungsten. In broadcast tubes I've seen amazing results from the rejuvination process. Saw a Klystron that was basically dead come back to life and live another year and a half. At $36,000 ea. that was significant.
Doc

[ikoflexer]

Got some direct heated transmitting tubes and a bunch of them have the following issue. With the heater on and 0V on the grid, when I bring up B+ there is very little current passing through. At around 300V on the anode a purple/bluish glow appears at the bottom of the tube and the current through the tube goes up abruptly. Looked around the net and found some advice on tube rejuvenation which involved baking the tube for 12 hours or so at about 100C, which I did, last night. But it didn't help even one tube. The tube needs 5V/7.5A for the heater, so I also tried to give it 10V for a few minutes. Tough tube, the filament didn't die; the plate was nice and red.

Not sure what else I can try. Any advice?

[thaumaturge]

Run the filament up to 5.5 volts and let it cook a day. No HV on. Check for emmission then repeat. But it may not be emmission but a gas problem. Depending on the tube, you might check to see if they are rebuildable. Some are. Surprisingly you get better results out of rebuilt tubes because they run much longer without gassing up because all the components are pretty much through outgassing. Basically they just replace the filaments then resuck a vaccuum and reseal.
Doc

[ikoflexer]

I'll give it a try. The tube is 4E27.

[HollowState]

Originally Posted by ikoflexer

Not sure what else I can try. Any advice?

It would help to know the type/number of the tube in question. However, the symptom you describe is a large amount of gas that most likely makes the tube unredeemable. However, with certain smaller types, I have had fair to good results with this treatment. First, make a diode out of the tube by connecting the grids and plate together. Next, light the tube filaments to the normal level. Then, apply 120 volts AC across the tube (cathode to plate) in series with a 60 or 100 watt light bulb and let it run for several hours. If the bulb grows dimmer, the gas is recombining inside. You may also need to apply forced air cooling with an external fan and chimney setup depending on the tube type. Use an isolation transformer if you're squeamish about the AC.

edit: I just saw that it's a 4E27. Those are very old. If the bottom of the tube has green glass, these are always leaky and almost never work out well. And if they're anything other then Eimac, forgetaboutit.

[thaumaturge]

My question answered, this thread now up for sale... grabs... whatever.
Doc