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About the ins and outs of transmitter tubes

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 PersonsHISTORY

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: Filament, control grid and screen grid, from left. 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 WØMH 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