Why Are Tubes Mirrored?

Thanks Fran for explaining the getter ring. I had noticed that my 1960's miniature 9-pin vacuum tubes contain a "halo" ring near the anode (plate) which I now know is the getter ring. I learnt something new, thanks.

In the late 70s and early 80s, I was chief engineer of a small market UHF TV station that ran 30 kilowatts using klystrons. One ran at full power for the visual side and the aural side ran at about 3000 or so. The previous chief had turned down the filament voltage on the aural side to reduce wear. His background was not in klystrons, but tetrodes. They would do that to increase longevity in some high power tubes. Well, after I had been there a year or so, I noticed that the aural power was dropping. Raising the filament voltage slightly only made it worse. I called a company well familiar with my transmitter and they knew immediately what had happened. This klystron was gassing. The material that absorbed the gas was heated by the filament. Turning the filament voltage down cooled the material and after a few years, the gas caught up. I was instructed to slowly raise the voltage over the next few days or so and the problem would correct itself. It has been too long and I do not remember how long it took, but that did the job and full aural power was resumed.

The old blog post - http://www.frantone.com/designwritings/design_writings3.html#model-a

Fran Blanche

That's a beautiful looking 300B. You say it's from your HiFi? I would love to see a video about that amp!

Jason Thorpe

Wow! What memories those tubes brought back. Waiting 60 seconds for a TV to "warm up" because of all the vacuum tubes is something I'll never forget. And the smell of hot tubes in a dusty radio always seemed to be the same with every tube radio. Even the large local grocery store in the 70s had a tube testing machine in an obscure corner of the store where you could actually test tubes yourself and there would be a stock of the most popular tubes in a cabinet beneath the tester. As you mentioned in your video, if the vacuum seal is broken, the shiny getter matter would turn white. Me and my friends would raid the dumpsters of the numerous mom and pop TV repair shops in the early 70s to get the tubes. What would we do with them? Well, we'd go someplace and have a war and throw them at each other. They had a kind of nasty smell just after they burst on a hard surface. I remember people thought all their tubes were bad because of the mysterious shiny burned-like coating looked like something arced inside the tube. They thought the "White ones" were good. I was just a kid, and didn't really know why that area was apparently burned, but it seemed natural because of the ring just behind the shine. All tubes had that, working or not. Years later I found a book on tubes and learned why they used the getter. There was zero Internet in 1973, so I couldn't just do a Google search and get 1000 answers. I miss the carefree days where we all thought we were impervious to shattering glass splinters from tube wars. I think I'll walk back to my patio and throw a vacuum tube at the garage. After 2020, I need to blow off some steam!

Matt Wietlispach

Everyone needs an I love my Geiger counter tee shirt!

David Peaker

Well I never knew that. I have often wondered though when I saw tubes ( we call them valves in the UK) why they were silvered. Excellent video yet again. Thanks Fran.

Mike Hughes

she's a go-getter....!

Thanks Fran! I knew a little about getters but you cleared some things up for me. Take care, Sam

I've always wondered about that weird silvering inside of tubes and now I know! Thanks!

Circuitmike

Thank you for reminding me to charge my Geiger counter. I used to have an old AN/PDR-63, and now I have a wee little RadiaScan 701A.

Mark J. Blair

Great video, more tube videos please

That was fun. Reminds me of my tube training many years ago.

William Alsing

Used to work with prototype radiation detectors and electron-multiplier tubes. Flashed many a getter. There are passive getters that don't need to be flashed, but they must have only niche applications - I don't recall ever seeing one in the wild.

BobC