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About the ins and outs of transmitter tubes

by Mark Persons
Radio World Article
February 1, 2015

Do you know or have you wondered what is inside a transmitter tube? There is a huge base of tube transmitters out there, and you might be tasked with repairing or maintaining one. The job of tuning and keeping a tube transmitter working will be a lot easier if you know what is inside.
Fig. 1: Two well-used 4CX5000A tubes, one with its bottom concentric contact rings showing.
Photos by Mark Persons

Electron tube technology dates back to about 1910. The British called them valves because that term is an apt description of what tubes do.

Today, a typical 20 kW FM broadcast transmitter uses about 400 watts of RF drive to a single tube. That tube has a gain of about 50 (34 dB) to develop 20 kW of RF at the output. It is RF drive that controls emission in a tube, like a valve would in a water pipe.

Radio transmitter tubes today use a coaxial design. That is to say, the tube elements are arranged in concentric circles or cylinders around a central axis.

It was the team of Bill Itel W6UF and Jack McCullough W6CHE who developed power tubes starting in 1934. Their company became Eimac, now known as CPI, which folks today recognize as the premier power tube supplier.

Initially, tubes used glass as an insulator between working elements of the tube and the outside world. Much like a standard incandescent light bulb, electron tubes need a complete vacuum or the tube's filament will burn up (oxidize) in short order.


The numbering scheme describes what is going on.

Many know the 4CX250B tube. It was preceded by the 4X150. The "4" means it is a four-element tube.

The X separates the elements from the 150, which is the maximum power in watts that the tube can dissipate (turn into heat) safely under CCS (Continuous Commercial Service) conditions. In other words, the tube can do this 24 hours a day provided there is adequate cooling.

Another rating is ICAS (Intermittent Commercial and Amateur Service). The tube can withstand higher dissipation for a short period of time under that definition. Then there is the 4CX tube. The "C" indicates ceramic is employed as the insulator instead of glass and can withstand higher temperatures. A 4CX250B can be used as a direct substitute for a 4X150 because they are the same tube, but with different insulators.

So that brings us to the suffix.

Fig. 2: The inner workings separated from the anode.
An A, B or C is the original tube with design changes. If it has an F1, that means "flying leads." These are heavy filament wires that make it possible for a transmitter manufacturer to design and build a transmitter that does not have an expensive tube socket. The leads often connect directly to a filament transformer. There will be a lead on a control grid too. These are found in AM, but not FM transmitters, because the leads would be too long for 100 MHz circuits.

Common tube types in use today include 4CX15,000A, and five flavors of 4CX20,000 with an A, B, C, D or E suffix. Each is slightly different and usually not compatible with another.

Some tube types have a number after the suffix letter signifying another variation on the tube design. Newer numbering schemes came from military naming, for example: YU-148. They are often a second number for the same tube. In this case, it is the 3CX6000A7. A bit confusing.

Air is pumped out of tubes when they are built or rebuilt. Each tube has a small metal pipe sticking out of the top. The tube is turned upside down and connected to a vacuum pump. The idea is for gravity to help air molecules fall out of the tube for the best possible vacuum while pumping. It is that critical! The metal pipe is pinched off providing a good metallurgic seal before a protective cap is added.


There are many places where air can get into a tube. Ceramic insulators are bonded to metal rings, which are connections to elements within the tube. The ceramic to metal seals can develop a slow leak that is sometimes not detected at the factory.

Running a tube too hot can cause a seal failure. Poor cooling from plugged air filters or dirt on cooling motor squirrel cage fan cups will dramatically reduce cooling. A misaligned tube socket can put undue sideward pressure on tube seals causing failure too. When run right, tubes are very robust and reliable. Treating them poorly can result in early failure.

Fig. 1 shows two well-used 4CX5000A tubes, one with its bottom concentric contact rings showing. A 12-inch ruler is included to give readers a sense of size. Most ceramic tubes have white insulators. These are the Svetlana brand tubes from Russia. Their ceramic is pink in color. Anode, grid and filament contact rings are silver-plated copper. The idea is to make good electrical connections to a tube socket. The ones shown here are tarnished from several years of use.

Fig. 3: A screen grid joined to a contract ring.
Fig. 2 illustrates the inner workings separated from the anode. The filament, control grid and screen grid cluster is referred to as the "stem." Those parts are assembled and aligned before final assembly when the anode is added in production. The stem portion weighs in at just 2 pounds, while the anode, which is silver pated copper, tips the scale at 6 pounds in this case.


Fig. 3 shows a screen grid joined to a contract ring. This tube type has six ceramic seals. There are lots of places where a seal can leak.

Fig. 4 shows a filament, control grid and screen grid, lined up to show the small size difference between them. Once inside each other, there is not much room for things to go wrong. These elements are fragile, too. You can see that the filament came apart as I was disassembling the tube for this article. Don't try this at home! Sometimes a tube element wire will break during use and short to an adjacent element. Ouch, end of tube!


It all starts in the center with the filament. Think of the filament as a light bulb, which gets hot and emits light.

Fig. 4: (l-r) Filament, control grid and screen grid.
In this case, it gets hot enough to emit electrons too. Those electrons will be attracted to the outermost element of the tube, known as the anode, often called the plate. There could be 10,000 volts or more difference between the filament and the plate. Those negatively charged electrons are attracted to the positively charged plate, aided by being boiled off the filament at high temperature. It isn�t complicated once you understand what is going on.


Use pliers to safely remove metal staples from flaps in tube shipping boxes. Ripping a box open ruins the flaps, so shipping it out for rebuilding is difficult. Also, staples can tear your skin if they are still in part of a ripped box.

Keep the boxes on hand, even if they are empty. You will need them eventually. Boxes are engineered to safely cradle a tube in foam to keep it safe from damage. Shipping a tube without a proper box is a bad idea.

Keep your hands off the ceramic too. Oil and dirt from your fingers can create the potential for an arc-over path.

In the concluding part of this article, we will discuss how to make a tube work correctly in a transmitter. Stay tuned. It makes perfect sense.

Mark Persons W�MH is Certified Professional Broadcast Engineer by the Society of Broadcast Engineers and has over 30 years experience. He has written numerous articles for industry publications over the years. His website is

You can also see this article at Radio World Magazine: 

February 17, 2015:  Mark: 
I read your piece on transmitting power tubes.  Nice work!  Right up your alley.  Jim Barry in Destin, Florida.

February 17, 2015, from LinkedIn:  I continue to enjoy your article in Radio World.  I never realized that tubes were evacuated upside down to get some gravity assist in the process.  Is that true, or were you just pulling our legs?  The article about the stereo 19kHz pilot getting broadcast on an AM transmitter brought back memories of long ago before I went overseas with VOA in '89.  I discovered that happening on one of my contract stations while they were taking an off-air FM sports network feed using a cheap run-of-the-mill audiophile FM receiver.  As I recall, it put a very noticeable 1 kHz beat note on the second adjacent stations' signals. Charles Lewis, KY4P, West Jefferson, North Carolina.  Tube question answer: Evacuating tubes in an inverted position is true and is the best way to do it.  Mark.

February 13, 2015:  Hi Mark, Just wanted to say I checked out you article in the Radio World magazine this month (Feb 1st, 2015).  Good Stuff my friend!  Never knew what the inside of a ceramic tube looked like before this.  And then there's the electron theory of operation inside between the filament and plate.  On Behalf of all of us RF numb skulls everywhere;  Thanks!  Andy Bursaw, WKLK Radio, Cloquet, Minnesota.

February 12, 2015: Great article in Radio World on tubes, Mark.  I've never seen the inside of a tube!  Also, good info on the naming scheme.  Thanks.  Dan Houg, Station Engineer KAXE Grand Rapids, Minnesota, and KBXE Radio Bemidji, Minnesota.

February 8, 2014: Just got my February 1st issue of Radio World yesterday.  Good story by the way, looks like some work to break the tube apart.  Dave Cox, Brainerd, MN.

February 5, 2015: I've framed your article and it now hangs in an honored place on my office wall.  Best, Charles S. Fitch, P.E., Avon, Connecticut.

February 4, 2015: Hello Mark, Thank you for the most excellent Radio World tech tips article.  Interesting, informative to the extreme, and of personal interest to me.  After retiring from government work, I became chief engineer at KVMR in Nevada city and KMYC in Marysville California.   Now retired in Florida.  Again, thanks for showing the tube innards.  Jerry Snaper, KG6FDM, Plant City, Florida.

February 2, 2015: Dear Mr. Persons: Saw your article on the 4CX5000A tube, published in Radio World this afternoon.  EXCELLENT !  This is the type of educational articles I'd like to see published more often in Radio World.  Keep writing them.  I want to tell you that I appreciate your time to disassemble and photograph the 4CX5000A tube.  Never saw what one looked like inside.  Thanks for your time to write the article in Radio World.  Sincerely, Kent Verbeck.

January 30, 2015: I just received my February 2015 Radio World publication.  Page 18 & 19 "about the ins and outs of transmitter tubes" by Mark Persons, W0MH.  Great article with the history of, and photos showing the Eimac 4CX series tubes (4CX5000A) with cutaways and disassembled tubes.  The Tube Collector organization is very alive with hundreds of collectors/members.  I'm sure they will enjoy your article.  They have a monthly collector magazine covering everything tube.  They are a good bunch.  73, John Dilks, K2TQN  Former Editor Vintage Radio Column, QST Magazine 2000-2014.

January 30, 2015: Thanks for the great article.  I passed it along to a student IT employee who has shown some interest in transmitters and all other things broadcast.  Yours in service, 73 DE KA�GKT/7 Stephen Claasen, Senior Broadcast Engineer, Arizona Public Media

January 30, 2015: Hi Mark: I enjoyed your article on high-power tubes.  I particularly enjoyed the photos showing the �innards� of a 4CX5000.  Although I�ve worked on transmitters, used ceramic tubes were always sent back for rebuild so I�ve never actually seen inside one.  Thanks!  Bob Weller P.E.


See you down the road a bit. I'll leave the soldering iron on for you.  Mark Persons, W0MH.

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