Care and Feeding of Alpha Power Amplifiers
 
In 1967, Eimac came out with a slim volume entitled "Care and Feeding of power Grid Tubes". It has become an essential read for anyone working with products engineered around power tubes, and has been through numerous printings, and is now available on-line: <http://www.cpii.com/docs/related/22/C&F1Web.pdf>. Many of RF Concepts products use a power grid tube for the amplifying element, and "Care and Feeding" is still a recommended read if you want to understand more about tubes in general. But, there is more in there than most people care to know, so in this article I will try to give some of the more pertinent results, and also how our experience at RF Concepts/ Alpha Power relate to that. Many excerpts from "Care and Feeding" are scattered in this article, and full credit must go to the Eimac engineers who put so much effort into this valuable work over the years.
 
The opening paragraph defines a power grid tube as "a device utilizing the flow of free electrons in a vacuum. It has an emitting surface called the cathode, and one or more grids controlling the flow of electrons. An element called the anode collects the electrons." More specifically, we are interested in "gridded tubes which handle large amounts of power, as contrasted to receiving type tubes".
 
The issue of the cathode of the tube frequently comes up. Basically there are two types of cathode- directly heated and indirectly heated. In the directly heated type, a high current passes through a metal filament causing it to get hot and emit electrons. Special treatment of the metal allows for copious amounts of electrons to be emitted. In the indirectly heated type of cathode, the  heating element is separate from the electron-emitting surface. This allows for more flexibility in design, and has numerous advantages. All Alpha tube amplifiers use the indirectly heated cathode type of tube. These tubes use an oxide cathode, which is a coating of barium and strontium oxides on a base metal such as nickel. These are laid down in a complex sequence, but once "activated" the cathode is capable of high emission, and runs at a temperature around 1000 Kelvin. According to Care and Feeding, high emission for each watt of heating power is one of the main advantages of the oxide cathode. Other advantages are high peak emission capability for short pulses, a low operating temperature and greater mechanical ruggedness compared to other cathode configurations.
 
Thoriated tungsten filaments are one form of  a directly heated cathode. A small amount of thorium is added to the tungsten wire, and during tube construction, by proper processing, is brought to the surface of the wire, and emission increases some 1000 times. The filament is also is exposed to carbide at high temperature, a process called carburization. This creates a skin of tungsten carbide on the filament, which reduces the evaporation rate of thorium and increases the life of the filament.  Directly heated filaments must operate at temperatures around 1900 Kelvin, much hotter and requiring more power to heat them up than the indirectly heated ones. For a thoriated tungsten cathode, peak and average emission are essentially the same. They also are generally less tolerant of vibration, and in most cases there are restrictions in a tube using such a cathode, which mean it can not be mounted with its axis horizontal.
 
For both types of cathode, it is important to observe the rated voltage. Over-heating an oxide cathode gives no worthwhile improvement in emission, but can shorten the life of the tube. This is one of the reasons why all Alpha amps have numerous primary-side "mains taps" on their transformers- it allows the user to set the amplifier up to match their ac supply voltage, and keep the heater voltage at the correct value.
 
Directly heated tubes are even more finicky in this regard. Eimac recommends that the filament voltage for optimum lifetime be determined in the field for each particular combination of equipment and operating power level. This is because the end of useful life for this type of filament occurs when the carbide "skin" mentioned above has evaporated. Theoretically a 3% (!) increase in filament voltage will result in a 20K rise in temperature, a 20% increase in peak emission but a 50% (!) reduction in life due to carbon loss. This works the other way too- reducing the temperature can extend the cathode life appreciably.
 
The take-home message from Eimac's "Care and Feeding" is that directly heated cathodes must be treated with more care than indirectly heated ones. Of course, the directly heated tubes are "instant on"- you can transmit a few seconds after applying filament power, while an oxide cathode takes 30 seconds to 3 minutes to heat up sufficiently, but this is a small price to pay for the other advantages of this type of cathode. The Alpha experience has been that the lifetime of an oxide cathode can be extremely long- we see amplifiers with tubes that have seen regular service in amateur stations for 25 or more years that look as though they would probably meet the original factory specifications. And this is without the user doing anything special to set the heater voltage other than selecting the correct primary-side tap on the transformer. In current production amplifiers, this tap setting is particularly simple, and is done by relays. In fact, the 9500 measures the ac line voltage and sets the tap for you. On the 8410, you move a jumper wire from one pin to another  to select the tap. We generally ship amplifiers with the tap set to the highest value- 240V. If you are running on 220 or 208, then you have to select the tap (on those amplifiers that don't do it automatically). This reduces the chance of the heater voltage being too high.
 
Typical tube life expectancy depends on a great many factors in addition to the heater. Operation below the maximum ratings will increase the lifetime of the tube. This is especially true with reduction in the anode dissipation of the tube. Alpha amplifiers are designed with margin for precisely this reason. But, there is no particular "wear out" mechanism, and you are not using up any "time" on the tube by having it on. So, if you go into the shack, turn the amplifier on when you turn the radio on- then the oxide cathode will be ready when you need it. Keep the airways cleaned out, tune for best efficiency and the tube may outlive you!