I never really figured this out

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pentium
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I never really figured this out

Unread postby pentium » Mon Jul 02, 2007 6:48 pm

Pretty much every vacuum tube I have ever seen has a dark and sometimes shiny location on it. No matter where it is in the tube, there is always a round disc in close proximity.
It can't be soot and I doubt that either the heat or the electron emmissions are causing the glass to go translucent. What is it? I sometimes see it grow larger in size as the tube gets older and in some cases it goes white when the tube cracks or it's vacuum is lost.

I was packing a box of tubes away and the question came to me.
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Re: I never really figured this out

Unread postby zizban » Mon Jul 02, 2007 6:57 pm

Its heat, I believe.

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pentium
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Re: I never really figured this out

Unread postby pentium » Mon Jul 02, 2007 7:13 pm

Then what prevents the entire tube from evenly getting this effect?
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Re: I never really figured this out

Unread postby anotheradamdickson » Mon Jul 02, 2007 7:21 pm

pentium wrote:Pretty much every vacuum tube I have ever seen has a dark and sometimes shiny location on it. No matter where it is in the tube, there is always a round disc in close proximity.
It can't be soot and I doubt that either the heat or the electron emmissions are causing the glass to go translucent. What is it? I sometimes see it grow larger in size as the tube gets older and in some cases it goes white when the tube cracks or it's vacuum is lost.

I was packing a box of tubes away and the question came to me.


Also most/all tubes have something called a 'getter' which reacts with the latent O2 that's left from the evacuation process and is released from oxides when heated/over time. The 'little round thing' is a 'U' shaped channel that is filled with the substance that reacts with the O2

Without something to react with the excess oxygen - the performance of the tube would be bad, and get much worse over time. Most of the reaction happens with the first few minutes/hours of the tubes life, as soon as the heater gets the stuff up to reaction temp. After that the 'heat' and material from the heater that sublimes on the glass will change the color a little bit, not much though. The reaction greatly slows after the first few hours and then only happens very slowly over the life of the tube as oxygen leaches out of any oxides in the metal that has been released (very very slowly)

Big tubes that are still used for high/huge/insane power TV/FM broadcasting have a metal chamber that can be opened to for repair, they have a consumable reactive agent that must be changed each time the chamber is opened, and then pumped out. As the reaction with oxygen takes place you can see the performance of the tube go up, however the metal lined tubes can withstand a _much_ higher vacuum so there's much less gas left anyway

Once any gas reaches a critical point in a vacuum tube it goes 'gassey' and the tube looks like a red-neon light (and performs so badly the equipment often stops working). Once all the gas ionizies it starts conducting and brings the impedance at the grid down from megaohms to ohms. Often while a tube is starting to go gassey it will start leaking the heated current/voltage to the grid and it will amplify the (often) AC heater voltage and modulate it into the signal that it's supposed to be amping

Tubes are still cool though, they can take a hell of a beating and keep ticking, they can't take O2 or gasses they. I don't imagine the Traveling Wave Tubes and Magnetrons will find semiconductor replacements in all their uses for a very, very long time (think microwave, however TWT will go out of style first for most uses)

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Re: I never really figured this out

Unread postby choza » Mon Jul 02, 2007 8:45 pm

and this is why i love this place!
great explanation mate :mrgreen:

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Re: I never really figured this out

Unread postby anotheradamdickson » Mon Jul 02, 2007 8:49 pm

choza wrote:and this is why i love this place!
great explanation mate :mrgreen:


And I thought people just tuned me out when I babel hoping everybody will tell everyone else what they know and then everybody will know everything - then Nekochan can take over the world - THE WORLD I TELL YOU!!!!


[flash=]THE WORLD[/flash]

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Re: I never really figured this out

Unread postby anotheradamdickson » Mon Jul 02, 2007 8:50 pm

anotheradamdickson wrote:
choza wrote:and this is why i love this place!
great explanation mate :mrgreen:


And I thought people just tuned me out when I babel hoping everybody will tell everyone else what they know and then everybody will know everything - then Nekochan can take over the world - THE WORLD I TELL YOU!!!!


[flash=]THE WORLD[/flash]


Umpf it's not [flash=]flashing[/flash]

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Re: I never really figured this out

Unread postby zizban » Tue Jul 03, 2007 7:15 am

My grandfather worked on the earliest electronic computers and when he died we found crates of retro 50's vacuum tubes in his storage.

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Re: I never really figured this out

Unread postby jan-jaap » Tue Jul 03, 2007 7:36 am

anotheradamdickson wrote:Umpf it's not [flash=]flashing[/flash]

Thank god :)
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Re: I never really figured this out

Unread postby akimmet » Tue Jul 03, 2007 8:36 am

Yep, anotheradamdickson is correct, that is the getter.
Well anyway as you already know when the silver spot turns whit that means the tube has cracked somewhere and air has entered the tube.
However cool just any old tube is, get some equipment with gas regulator tubes or mercury rectifiers for an even better light show.
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Re: I never really figured this out

Unread postby pentium » Tue Jul 03, 2007 9:04 am

Thanks guys.

[flash=]This can't flash, lol.[/flash]

:roll: :roll:
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Re: I never really figured this out

Unread postby nekonoko » Tue Jul 03, 2007 9:08 am

pentium wrote:Thanks guys.

[flash=]This can't flash, lol.[/flash]

:roll: :roll:


That's for inline Abode Flash files, not flashing text.
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Re: I never really figured this out

Unread postby anotheradamdickson » Tue Jul 03, 2007 5:10 pm

zizban wrote:My grandfather worked on the earliest electronic computers and when he died we found crates of retro 50's vacuum tubes in his storage.


WAIT!!!!!! Don't throw those out. Depending on the tubes audio-idiots will pay top dollar. I'm not insulting audiophiles however I am insulting audio-idiots that have been know to pay $500 for a block of wood because of some alleged 'scientific' effect it had on the sound. Consumer sound is way *way* too consumer driven for my me not to melt down when I see people blowing so much money on fluff, then ignoring the stuff that would really make a difference. But if they're the right tubes you can use these to up the price and still be selling something of value

If they are output tubes or belong in vintage audio/video equipment you could be sitting on a gold mine. Well sitting on may reduce the value greatly :). Sitting beside would be safer

OEM tubes from the 40-50-60-70's have be known to get hundreds of dollars EACH! (if they're 'New Old Stock')
Used tubes don't get much, if they're working perfectly and are highly desired you could still get many tens of dollars a piece. If their just crap tubes he kept when he was fixing TV's then their pretty much worthless, if it's tubes used for highend audio equipment then people will pay through the nose for them

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pentium
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Re: I never really figured this out

Unread postby pentium » Wed Jul 04, 2007 9:46 am

That's for inline Abode Flash files, not flashing text.

I kinda thought that.
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Re: I never really figured this out

Unread postby Frapazoid » Wed Jul 04, 2007 1:28 pm

choza wrote:and this is why i love this place!
great explanation mate :mrgreen:


Seconded!

I go on here daily just to read the topics. There's like a dozen threads I'm keeping up with just to read all the fascinating stuff people write here. Some are really good like this one and the one about the guy trying to get Linux working on his O2...
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