Extending Tube Life
Reprint from Eimac Bulletin #18 – CPI, Eimac Division, San Carlos, CA
In recent years, station managers have seen a substantial increase in replacement costs for power grid tubes. The blame can be placed on higher manufacturing costs due to inflation, volatile precious metal prices, and an uncertain supply of some exotic metals. The current outlook for the future holds little promise for a reversal in this trend toward higher prices.
One way to offset higher operating costs is to prolong tube life. For years, station engineers have used various tricks to get longer operating life, with greater and lesser degrees of success. Success can be maximized, however, by understanding the various factors that affect tube life, and implementing a program of filament voltage management.
A number of factors can aid maximum tube life in your transmitter. For example, are the maximum ratings given on the tube manufacturers data sheet being exceeded? Data sheets are available upon request from most companies. Most tube manufacturers have an engineering department to assist in evaluating tube performance for a given application. Make use of these services!
Is the final power tube of the transmitter capable of delivering power in excess of the desired operating level? Or is the demand for performance so great that minimum output power levels can only be met at rated nominal filament voltage?
Figure 1 can be used as a basic guide to determine if a given transmitter and tube combination has a good probability of giving extended life service. Extended life service is defined as useful operating life beyond that normally achieved by operating at rated nominal filament voltage. The amperes/ watt ratio is obtained by dividing average plate current by the product of filament voltage and filament current. If the amperes/watt ratio falls in the “good” to “excellent” range, excess emission is sufficient to permit filament voltage de-rating. At a lower filament voltage, the filament temperature is lowered, thus extending tube life.
A typical FM transmitter on the market today may have an amperes/watt filament ratio of 0.002 to 0.003. This equipment would be considered an excellent choice to achieve extended tube life.
On the other hand, if the amperes/ watt ratio falls in the “poor” range, it is unlikely that filament de-rating is possible, due to limited emission. Note that this guideline should be used only for thoriated tungsten emitters only, and does not apply to oxide cathode-type tubes.
Are all tube elements metered in the transmitter? Elements should be metered for both voltage and current, and meters should be red-lined to define operation within safe limits. More modern transmitters may incorporate a micro-processor controlled circuit to monitor all pertinent parameters.
In addition, the following controls are necessary for an effective filament voltage management program: power output metering for an FM transmitter or a distortion meter for AM; accurate filament voltage metering (iron-vane instrument is preferred over the average responding RMS calibrated type); the filament voltage measurement must be made at the tube socket terminals, filament control, capable 0.1 volt secondary voltage changes; and a filament current meter (desirable but optional).
A means must be provided to hold filament voltage constant. If the filament voltage is permitted to vary in accordance with primary line voltage fluctuations, the effect on tube life can be devastating. An acceptable solution is the use of a ferroresonant transformer or line regulator. This accessory is offered by some transmitter manufacturers as an option and should be seriously considered, if a tube life extension program is planned.
Once the transmitter has been placed in operation, tube life is in the hands of the chief engineer. The first action to prolong tube life falls into the category of routine maintenance. Most transmitter manufacturers have a routine maintenance schedule established in the equipment manual. This procedure must be followed carefully, if operating costs are to be held to a minimum. During routine maintenance it is very important to look for tube and socket discolorations, either of which can indicate overheating.
Look for discoloration around the top of the cooler near the anode core, and at the bottom of the tube stem where the filament contacts are made. It is possible for discoloration to appear in the areas mentioned if the transmitter has to operate in a dirty environment. If this is the case, the tube should be removed and cleaned with a mild detergent. After cleaning, the tube should be rinsed thoroughly to remove any detergent residue and blown dry with compressed air.
If the discoloration remains, this is an indication that the tube has operated at too high a temperature. Check inlet and outlet air ducting and filters for possible air restriction. It may be necessary to verify that the air blower is large enough to do the job in the present environment, end that it is operating at rated capacity.
With the tube removed, the socket should be blown and/or wiped clean, and carefully inspected. Any discoloration on the socket finger stock, caused by overheating, could contribute to early tube failure. A finger stock that loses its temper, thorough prolonged operation at high temperature, will no longer make contact to the tube elements. A well-maintained socket will score the tube contacts when the tube is inserted. If all the fingers are not making contact, more current flows through fewer contacting fingers, causing additional overheating and possible burnout.
Filament Voltage Management
The useful operating life of a thoriated tungsten emitter can vary widely with filament voltage. Figure 2 describes the relative life expectance with various filament voltage levels. Obviously, a well-maintained filament voltage program will result in longer life expectancy. Improper management, on the other hand, can be very costly.
For a better understanding of this sensitive aging mechanism, the filament itself must be understood. Most filaments in high-power, “gridded tubes are a mixture of tungsten and thoria with a chemical composition of W + THO2. A filament made of this wire is not a suitable electron emitter for extended life applications until it is processed. Once the filament is formed into the desired shaped and mounted, it is heated to approximately 2100° C, in tbe presence of a hydrocarbon. The resulting thermochemical reaction forms di-tungsten carbide on the filament’s surface. Life is proportional to the degree of carburization. If the filament is over-carburized, however, it will be brittle and easily broken during handling and transporting. Therefore, only approximately 25% of the cross-sectional area of the wire is converted to di-tungsten carbide. Di-tungsten carbide has a higher resistance than tungsten; thus, the reaction can be carefully monitored by observing the reduction in filament current as the carburizing process proceeds.
As the tube is used, the filament slowly de-carburizes. At some point in life, all of the di-tungsten carbide layer is depleted, and the reduction of thoria to free thorium stops. The filament is now de-carburized and is no longer an effective electron emitter.
The key to extending the life of a thoriated tungsten filament emitter is to control operating temperature. Emitter temperature is a function of the total RMS power applied to the filament. Thus, filament voltage control is temperature control; temperature varies directly with voltage. As the emitter temperature rises, the de-carburizing process is accelerated and tube life shortened. Figure 2 shows that the useful tube life can vary significantly with only a 5% change in filament voltage.
Of great importance to long tube life is the temperature of the elements, and the ceramic-to-metal seals. Element temperature can be held within proper limits by observing the maximum dissipation ratings listed in the data sheet. Seal temperatures should be limited to 200° C, at the lower anode seal, under worst case conditions. As element temperature rises beyond 200° C, the release of contaminants, locked in the materials used in tube manufacturing, increases rapidly. These contaminants cause a rapid depletion of the all-tungsten carbide layer of the filament.
When a new power tube is installed in a transmitter, it must be operated at rated nominal filament voltage for the first 200 hours. This procedure is very important, for two reasons. Fit operation at normal temperature allows the getter to be more effective during the early period of tube life, when contaminants are more prevalent. This break-in period conditions the tube for operation at lower filament voltage to obtain longer filament life. Secondly, during the first 200 hours, filament emission increases. It is necessary for the life extension program to start at the peak emission point.
A chart recorder, or other device, should be used to monitor variations in primary line voltage for several days of transmitter operation. The history of line voltage variations, during on-air time, must be reviewed prior to derating filament voltage. Plan to establish the de-rated voltage during the time period of historically low line voltage, as this is the worst-case condition. If line variation is greater than +/ – 3%, filament voltage must be regulated.
Record output power (FM), or distortion level (AM), with the tube operating at rated filament voltage. Next, reduce filament voltage in decrements of 0.1V and record power (FM), or distortion levels (AM), at each decrement. Allow one minute between each decrement, for the filament emission to stabilize.
When a noticeable change occurs in output power, (FM) or the distortion level changes (AM), the derating procedure most stop. Obviously, operation at this point is unwise since there is no margin for a drop in line voltage. It is safer to raise the voltage 0.2Vabove the critical voltage at which changes are observed to occur. Finally, recheck power output power or distortion to see if they are acceptable at the chosen filament voltage level. Recheck again after24 hours to determine if emission is stable, and that the desired performance is maintained. If performance is not repeatable, the de-rating procedure must be repeated.
The filament voltage should be held at the properly derated level as long as minimum power (FM), or maximum distortion requirements (AM), are met. Filament voltage can be raised to re-establish minimum requirements as necessary. This procedure will yield results similar to those shown in Figure 2, to achieve as much as 10% to 15% additional life extension. When it becomes necessary to increase filament voltage, it is a good time to order a new tube. Filament voltage can be increased as long as the increase results in maintaining minimum level requirements.
When an increase fails to result in meeting a level requirement, filament emission must be considered inadequate, and the tube should be replaced. Don’t discard it or sell it for scrap! Put it on the shelf and save it. It will serve as a good emergency spare and may come in very handy some day. Also, in AM transmitters, a low-emission RF amplifier tube can be shifted to modulator use, where the peak filament emission requirement is not as severe.
Review the following steps you can take:
1. Investigate the manufacturer’s ratings on power tubes in your present, or planned purchase, transmitters.
2. Check that your transmitter has sufficient headroom.
3. Look for instrumentation in your next transmitter. Are all tube elements monitored for voltage and current?
4. Whether your transmitter is new or old, start a filament life extension program.
Remember that each time you replace a power tube, the recommended derating procedure must be rerun.