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What's inside a 4CX250B tube?

nomadradio

Analog Retentive
Apr 3, 2005
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So what's inside a 4CX250B, Mr. Bones?

Is this a trick question? There's a vacuum inside there, right?

Sure, and some other stuff. This tube is different from those built with a glass envelope to, er "hold in" the vacuum. More like hold out the atmosphere. The anode is visible through the glass, surrounding the other elements inside.

This tube has a copper anode (*NOT* steel, mind you) that serves as the vacuum container. The vacuum is inside the anode structure, and stuff to increase surface area is bonded to the outside surface, like the wavy fins in an auto radiator.

Crack open a dud, and you'll see this.

TSjHOP.jpg


Crack open a perfectly-good tube and someone will probably want to wring your neck. The cylindrical fence structure is the screen grid. The wires are in fact gold plated. This is done because all metals do not spew electrons into a vacuum with the same level of enthusiasm. Naturally you want the cathode to spew as many electrons as possible. It's made of (I think) zirconium oxide, a substance that does this better than any kind of hot metal.

But a grid is supposed to control the flow of electrons squeezing through the space between grid wires, not spew them itself. Gold is at the opposite end of the "electrons per square inch" scale for what hot metal does in a vacuum. If this grid were to start spewing electrons, it would cease to control the tube's current and would cause it to pull all the current available, until something goes "POP!". The textbook name for this downhill-snowball effect is "secondary emission". All we want is the primary emission from the cathode, not from the other elements.

KzsDbL.jpg


The screen grid is mounted on a ring with slotted mounting holes for the three retaining screws. This allows a proper lineup, with each screen-grid wire in the "electron shadow" of each control-grid wire. Grid wires will inevitably block some of your precious electron flow, so this keeps it at a minimum.

9zPNmM.jpg


The control grid is inside the screen grid. The wires used to be straight. Got a little distorted as I pulled off the screen grid.

The inside surface of the anode always seems to look crusty when I look in there.

xjVtH7.jpg


I'm told that the shiny copper surface inside will chemically bond any stray gas molecules that build up or leak into the tube over the decades. This is called "active gettering" and requires you run the tube hot enough to activate this chemistry. I'm no metallurgist, but this story sounds plausible to me. Don't know just what else would explain it.

I decided that cutting deeper into the tube wouldn't reveal a lot so I stopped there.

So now maybe you'll be less tempted to crack open a perfectly good tube to see what's in there.

And no, it's not a "steel tube". Get out your magnet and prove it to yourself.

73
 

Captain Kilowatt

Professional Amateur
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And no, it's not a "steel tube". Get out your magnet and prove it to yourself.

73

Finally someone that actually gets it. I cringe when people call external anode tubes "steel tubes". Just another CB term that came from sheer ignorance I guess. Perhaps because the copper is silver plated is why people call them steel tubes. I have NO idea why they do but I really wish they would stop and just call them what they are, external anode tubes.
 

Shockwave

Sr. Member
Sep 19, 2009
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Not a good idea to crack that BEO ceramic insulator. Any dust particles from the cracked insulator, are highly toxic if inhaled.
 

Captain Kilowatt

Professional Amateur
Staff member
Apr 6, 2005
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Cracking it shouldn't be an issue at all. Cutting or grinding it open however is not a good idea because of the dust. I'm pretty sure however that one won't kill you anyway. Probably a hundred times safer than smoking.
 
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nomadradio

Analog Retentive
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Pretty sure the white ceramic is alumina, not beryllia. Just the same, I crack them outside in the open. I don't see a lot of potential for dust particles the way it fractures.

About the time you think you understand something, reality throws you a curve ball. Had the notion that if I had a helper crack open a few dozen of these tubes and pulled the screen grid from each, maybe there's a gold-recovery angle there, maybe?

Wouldn't you know it, the next 4X150 to get cracked has no hint of gold color on the screen-grid wires. Go figure.

Cd4D0b.jpg


Looks just the same up close. The anode has "High Ig2" written on it. Most likely why it flunked testing. Could the plating have simply been left off? Or is this grid made from some metal with low emission like gold?

mxFM0N.jpg


I won't be taking samples to a metallurgy analysis lab to find out.

This one takes the cake, though. The purple-shade ceramic isn't russky. It came from the RCA factory in Pennsylvania. The factory that was spun off as "Burle" when RCA was dismantled. The color of the screen grid wires looks like naked copper.

MwKxbx.jpg


The other two appear alongside for direct comparison.

hv6TdR.jpg


Apparently the nitty-gritties of a tube's internal design are not a rigid standard from one factory to the next.

The more I learn, the more I find out I don't know.

73
 
The nittie gritties can't be the same from one mfg to the next.

Otherwise you run into patent infringement.

Years ago the court systems upheld that if something went into a device called a socket that the device itself couldn't be patented. That is why Intel decided to create the 'slot' architecture.

Because the courts said you can patent a slot, but not a socket. Stupid, I know. But... This is why CPU motherboards are now proprietary every since the original socket 7 was put to bed.

Same goes for tubes. You can make a tube that goes into any socket. However, if you make that tube internals exactly the same as brand X, you have committed a crime. If you make it something that works just like Brand X, fits into the same socket, it's not patent infringment.

This is also why the Amperex 500Z, 4-400, 4-1000, etc where better tubes that sometimes required taming. They where not licensed copies. They where tubes that fit into the socket.


I never really understood the difference between a socket and a slot. But, there you have it, and the courts have reaffirmed it time and time again. Good thing Intel lawyers didn't read prior patent law pertaining to vac tubes (this goes back to the days of RCA, original Mullard, Eitel-McCollough (Eimac's original name).... Otherwise we wouldn't have AMD keeping Intel on their toes.



Yes, copper is used as a outgassing absorbent.

The original material required heat to work. A lot of it. And dangerous chemicals. The tantulum and / or graphite would absorb the gasses, but the plate material had to be red hot to work (this is why you should get your 'glass' tubes red hot for a few keys every month, and you should rotate your active tubes probably yearly!!!! To keep gas levels down). Copper requires a LOT less heat to absorb gas. Just filament heat will work. Ceramic tubes are both better sealed with the ceramic to pin interface and the tubes don't have to run at max dissipation to 'getter'. Just putting them in a socket works. Glass, you have to run to a red hot plate (this is NOT the case for sweep tubes. Just run a sweep tube for a couple hours in normal use and it will getter).


--Toll_Free
 
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