About the durability of electron tubes
- Category: Radioamator i Krótkofalowiec
- Published: Thursday, 01 July 2021 10:59
- Written by Grzegorz Makarewicz
- Hits: 1507
About the durability of electron tubes
(Radioamator i Krótkofalowiec 1970/02)
Currently, over three million TV sets are registered in Poland. These are almost exclusively receivers based on electron tubes and it can be assumed that in the near future the electron tubes will not be completely replaced by transistors. Taking an average of 15 electron tubes in one TV set and adding to them electron tubes operating in radio receivers and tape recorders, you will get approximately 70 million electron tubes systematically used in these devices. Due to their huge number, the "life" time of the electron tube is interesting. The factories usually guarantee the operation time of this type of electron tubes from one thousand to several thousand hours. This does not mean, of course, that an electron tube (operated in proper conditions) cannot "end" before this time elapses, or work for a much larger number of hours.
The probability of correct operation for the guaranteed period of time is in the range of 90% to 98%. Knowledge of the wear processes of electron tubes imposes certain rules of their operation, the observance of which significantly extends their service life.
The durability of an electron tube is mainly determined by its cathode heating and the cathode itself. The temperature of other electrodes depends to a large extent on the cathode temperature, i.e. the speed of all physicochemical processes taking place, which increases with increasing temperature. Commonly used electron tubes usually use oxide cathodes. Such a cathode is usually made of a thin nickel cylinder covered with a layer of barium oxide or strontium oxide. In the process of forming and activating the cathode, the oxides are partially decomposed.
The released oxygen is pumped or absorbed by the getter, while the alkaline earth metal atoms remain inside the crystal lattice of oxides. As a result, a certain amount of free barium or strontium atoms is formed. These atoms play an essential role in the mechanism of action of the oxide cathode. Pure barium oxide is a dielectric, but the presence of free barium atoms gives it semiconductor properties with the presence of free electrons to be emitted.
Due to the high temperature of the cathode, an intermediate layer is formed between the cylindrical metal support and the oxide layer. It consists mainly of barium orthosilicate Ba2SiO4, which has a relatively high electrical resistance. The thickness of this layer increases during the operation of the tube, and the resistance itself adversely affects its operation. This resistance produces a certain feedback (similar to the resistor connected to the cathode), leading to a decrease in the anode current and gain. The durability of the electron tube is significantly influenced by maintaining the appropriate cathode temperature. Overheating or under-heating is detrimental to the oxide cathode and reduces its life.
At higher temperatures, the evaporation of barium and its oxide is more intense, and the intensity of interlayer growth increases. At lower temperatures, the cathode becomes more sensitive to "chemical poisoning" and ion bombardment, and its transverse (through) resistance increases due to its negative temperature coefficient. However, this applies only to a significant underheating of 20%, while when the filament voltage is reduced by 5% to 8%, the life of the cathode increases significantly. As the mains voltage can often fluctuate in the range from + 10% to -20%, the electron tubes can work in very unfavorable conditions. With the above-mentioned changes in the mains voltage, the electron tube with a filament voltage of 6.3V will be either very overheated (7V) or very underheated (5V).
Therefore, it would be advisable to power the TV set via a good electromagnetic voltage stabilizer (RiK No. 4/1969).
In the initial period of operation of the receiver, it is even worth reducing the supply voltage by 5% to 8% by connecting a small resistor (15Ω / 10W) in series between the stabilizer output and the receiver. The low voltage reduction also has a positive effect on the durability of the cathode heater itself.
Alund, which insulates the heater fiber from the cathode cylinder, undergoes changes during the operation of the tube. The tungsten from which the filament is made diffuses into the insulating layer, creating aluminum tungsten with much worse insulating properties than alundum.
Hence, at higher temperatures, and especially when the cathode has a positive potential with respect to the heater, the probability of cathode-heater breakdown increases.
During the operation of the vacuum tube, its filament heats up to a high temperature of 1200oC ÷ 1400oC. This leads to heating of all other elements and electrodes of the vacuum tube: cathode, grids, screen, anodes and housing. In addition, the heating of the tube elements increases due to the power dissipated on the electrodes as current flows through them. The heated elements of the vacuum tube become a source emitting various gases contained in them. The released gases have a detrimental effect on the cathode of the tube, oxidize the active layer, which leads to a reduction of the emission current, which determines the slope, gain and output power of the tube. This applies especially to the tubes used in TV receivers: PL500, PL36, PCL86, PCL85, PL84, PCL82 etc. Therefore, a slight reduction of the anode voltage is highly recommended to increase the working time of the tube. So-called "heat pipes" can also be used to reduce the heating of the tube by applying metal cups with ribs to the tubes.
Modern TV sets, and more often also radio receivers, almost always have semiconductor rectifiers in the power supply system. After connecting the device to the network, full anode voltage appears immediately at the cathodes that are just warming up. The anode voltage is even higher during this period, because the system does not consume any current yet. This situation is harmful for the device's electron tubes. The service life of the tubes will therefore be adversely affected by the frequency of switching on the device.
This disadvantage can be eliminated by various methods. The easiest way is to install an additional switch in the circuit after the first electrolytic capacitor of the power supply, by means of which the anode voltage is manually switched on only a few minutes after the receiver is connected to the mains. The cathodes of the tubes will then reach a sufficiently high temperature. Switching on the anode voltage can also be delayed by means of an additional vacuum tube with a suitable relay, or by means of a bimetal switch.
The proposed use of TV sets significantly extends their lifetime. My Delta AT-550 type TV set (stabilizer and additional switch) works for three years without damaging the vacuum tube.