Baking vacuum tubes

[adrianywu]

One of my Acapella plasma tweeters became noisy about 3 weeks ago. There is a static noise that occurs at random, sometimes with showers of them that last for a few seconds, sometimes just periodic pops. The problem seems to be getting worse over time, and has become impossible to ignore. Since these tweeters each uses a PL509 pentode, I reckon this is the symptom of a gassy tube. I remember reading an article by Morgan Jones, the author of the book Valve Amplifiers. He explained that the vacuum in many NOS tubes goes "soft" with age, which means some air leaks in over the years. The gas molecules become ionised by the electron beam, and the positively charged particles get attracted to the grid. This causes grid current, and if the grid circuit has a high resistance, a significant noise voltage will be induced. He did experiments with some more than 30 years old NOS tubes, measuring the grid current before and after baking. The theory goes that heating up the getter will reactivate the chemical reaction that absorbs the gas molecules. Indeed, he showed that the grid current is reduced by an average of 80% after baking. Therefore, I decided to give it a try. Since I was going to expend all that electricity, I might as well get all my NOS tubes that I might use in the foreseeable future treated. I put them in the oven at 120 degree Celsius for 12 hours and then let them cool down. I then reinstalled the tube. Except for the first few minutes after turning on, when the tube was getting warmed up, there was no more noise throughout the next 2.5 hours of listening. It seems to have worked ! I will see how it goes over the next few weeks, and I hope this has prolonged the useful life of the tube, since NOS stock seems to be dwindling and there is no new manufacture for this tube type.

 

[Republicoftexas69]

One of my Acapella plasma tweeters became noisy about 3 weeks ago. There is a static noise that occurs at random, sometimes with showers of them that last for a few seconds, sometimes just periodic pops. The problem seems to be getting worse over time, and has become impossible to ignore. Since these tweeters each uses a PL509 pentode, I reckon this is the symptom of a gassy tube. I remember reading an article by Morgan Jones, the author of the book Valve Amplifiers. He explained that the vacuum in many NOS tubes goes "soft" with age, which means some air leaks in over the years. The gas molecules become ionised by the electron beam, and the positively charged particles get attracted to the grid. This causes grid current, and if the grid circuit has a high resistance, a significant noise voltage will be induced. He did experiments with some more than 30 years old NOS tubes, measuring the grid current before and after baking. The theory goes that heating up the getter will reactivate the chemical reaction that absorbs the gas molecules. Indeed, he showed that the grid current is reduced by an average of 80% after baking. Therefore, I decided to give it a try. Since I was going to expend all that electricity, I might as well get all my NOS tubes that I might use in the foreseeable future treated. I put them in the oven at 120 degree Celsius for 12 hours and then let them cool down. I then reinstalled the tube. Except for the first few minutes after turning on, when the tube was getting warmed up, there was no more noise throughout the next 2.5 hours of listening. It seems to have worked ! I will see how it goes over the next few weeks, and I hope this has prolonged the useful life of the tube, since NOS stock seems to be dwindling and there is no new manufacture for this tube type.

Thank you for sharing and I found this very interesting.

 

[adrianywu]

A bit of follow-up. It has been a little over two weeks since I re-installed the baked tubes. I have been listening almost daily since then. So far so good. The dreaded static noise has not recurred. Unless the simple act of removing and re-installing the tubes cured the problem, otherwise I can conclude that baking the tube did get rid of the gas molecules, resulting in a quiet tube once more. I hope to find a smaller device that can keep the temperature at 120 degrees celsius for 12 hours so that I don't have to waste so much energy using a home oven. For example, these cheap food dryers are perfect for baking tapes with sticky shed syndrome, but they don't get up to high enough temperature for tubes.

 

[Mendel]

This is a very interesting report! I have read similar reports online of people “baking“ tubes to cure gassy tubes that become noisy or have trouble holding bias. Maybe a way to extend NOS tube life.

 

[adrianywu]

This is a very interesting report! I have read similar reports online of people “baking“ tubes to cure gassy tubes that become noisy or have trouble holding bias. Maybe a way to extend NOS tube life.

Glad you find it useful. Worth a try if you are going to throw the tube away anyway. I have a tube curve tracer that can measure grid current, and a proportion of my NOS tubes failed the leakage test even before use. I have not measured them again after baking. So far, the PL509 has remained quiet.

 

[sparkie]

I hope to find a smaller device that can keep the temperature at 120 degrees celsius for 12 hours so that I don't have to waste so much energy using a home oven.

I use my electronics oven that is a toaster oven I got from the thrift store and modified it by taking out the mechanical timer and installing a switch. I initially created my oven for soldering/re-flowing SMT boards, but I've used it twice to bake out gassy tubes. Its a little aggressive, but it does work.
Most of the time with NOS tubes, running the heaters without plate voltage for a week fixes low emissions from storage. Gassy tubes you have to bake it out. No ways around that.

 

[djsina2]

Very interesting. I have a pair of tubes that have become noisy. So 120C for 12 hours is the recommended bake? How did you place them in the oven?