Back in the days when the klystron was the tube of choice for high power UHF transmission, industry insiders felt UHF stations should be grateful they had a piece of the spectrum at all. VHF was spoken for. If the klystron wasn't very efficient, the high electric bills÷much higher than those for VHF for the same city grade coverage÷was the cost of doing business.

Then along came pulsing, and that made the klystron a bit more respectable. As Rich Schwartz, director of Marketing at Axcera, said, "I can remember, when at ITS, we pulsed a transmitter for a PBS station in New York, and that saved the station $5,000 a month. It was a fun technology to sell!"

Along came the Klystrode from Eimac in the late 1980s, and UHF stations drew a collective breath of electric power bill relief. That kick-started the IOT revolution.

According to Marconi, the concept of the Inductive Output Tube (IOT) was first brought about by Andrew Haeff back in 1938, but limitations in available technology combined with the lack of any real push toward energy efficiency meant that it wasn't until much later that this idea became a reality.

While the industry focused on the new IOTs, few paid careful attention to efficiency numbers in VHF, especially for solid state.

While putting together an update on transmitters, I talked with Jim Adamson of Larcan about solid state efficiency, especially since the company's LDMOS had been accepted as the device of choice. I implied that solid state efficiency must be very good, since there were no filaments and the power supplies were different.

Adamson caught me by surprise when he said that although an individual solid state device may be reasonably efficient, ganged together with supporting circuitry the solid state transmitter was no more efficient than the old klystron-equipped transmitter.

But solid state continues to be a strong contender in transmitter sales because, unlike IOT-equipped transmitters, no one device can shut down the transmitter. Since they're modular designed transmitters, those modules can be changed while the transmitter is on-air. Add to this their excellent reliability, and it's no stretch to see why they're still selling.

The New Breed
One of the first of the new breed of UHF tubes was conceived by Marconi's EEV operation. Dubbed the ESCIOT, this tube is a three-stage depressed collector IOT designed to greatly improve efficiency while maximizing simplicity and cost-effectiveness. A number of these devices were in sockets at various transmitter booths at NAB2002 and are expected to be in service by year's end.

Thales Electron Devices (TED) is also on the cutting edge. As Sales Manager (Thales Components Corporation) Jim McVea said, "Our design uses a Îsimple,' straightforward, single depressed collector similar to that used by Mr. Haeff back in 1939.

"It provides the single greatest improvement in efficiency over a standard grounded collector IOT. Additional collectors do provide for higher efficiencies, but with a higher cost and level of complexity in the tube, production procedure, and in-transmitter power and cooling supplies.

"A patent-pending accessory allows cooling of the tube, cavity, and preamplifier with water and glycol...which plugs in the cavity for easier maintenance."

Of course, Northrop Grumman (formerly Litton)'s Constant Efficiency Amplifier (CEA) has also gained notoriety for being one of the most efficient devices available today. This unique design is now operating in test beds at just over 60 percent efficiency, while the other new tubes are closer to 50 percent.

So is the Klystrode still being used? Yes. The latest K2D110W model can handle 110kW in digital service or 75kW in NTSC service. You'll recall that the first Klystrode model went on the air at WCES-TV back in 1988 in a Comark transmitter, earning an Engineering Emmy in 1991. It's still offered under the Eimac banner.

Apples And Oranges
According to transmitter manufacturers, in the newest generation of VHF tubes, there is a great move forward in efficiency. I talked with Axcera, Harris, Thales Broadcast & Multimedia, and others about hopes for implementing these new products into their transmitter designs. All manufacturers insist they are running these tubes through their paces in test beds. They will try all the new tubes. And here's where transmitter manufacturers play a larger-than-life role in tube designs:

Just as Comark and Eimac went through the fire and brimstone of making the first Klystrode work successfully at WCES-TV back in 1988, transmitter manufacturers today look at tubes under their own conditions, in their own sockets. And they invariably make recommendations on how to change the tube to make it work better and make it more compatible with their specific systems.

In fact, as Dave Glidden at Harris put it, his company and others also hope to convince tube manufacturers to tackle the compatibility question, so that the newer tubes aren't so unique that each requires special transmitter changes.

Meanwhile, transmitter manufacturers have been bombarded with questions about the "real" efficiency of a new device in a new transmitter. They insist that power supplies, heat exchangers, pumps, and other related power-hungry devices must be taken into consideration when you're looking for bottom line efficiency. Efficiency, no matter how you calculate it, is tied to operating cost. That's the real bottom line.

As Axcera's Schwartz said, before you can understand the savings you'll realize with the new tubes, you need to calculate your power versus cost per kW/hour ratio. Then you calculate again using the new transmitter/tube combination, giving you quantifiable information.

With kW hour costs ranging from 4 to 12 cents, the annual cost savings can vary widely along with the power used at differing levels. However, it's safe to say that over time the savings will put a large dent in the cost of any transmitter using the new tubes.

It may be several quarters before the new tubes make their way into the field. The big jump will problably come when stations gear up to run their DTV transmitters at full power.

Still, the evolution of these tubes is enough to make Mr. Haeff and UHF station owners smile.