Regeneration of radio tubes
RADIO Monthly magazine for Technicians and Amateurs, 1st Year, May 1946, No. 3
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Difficulties in finding older types of electron tubes on the market and their high cost force us to consider the issue of restoring electrical properties to electron tubes which, due to long-term operation or short-term overload, have lost their emission capacity and are not suitable for use in radio receivers.
The subject of the article will be to provide an experienced radio amateur with a description of electrical methods for regenerating radio tubes. Of course, there can be no question of restoring the emission properties of electron tubes with defects of a mechanical nature, such as a burnt cathode, a short circuit between the electrodes or a bad vacuum. Only tubes with too low emission current can be considered.
The process of regenerating the cathodes of the receiving tubes is nothing more than an attempt to re-form the cathode, which consists in carrying out thermochemical processes on the cathode surface. As a result of thermal treatment, the so-called an active layer of a metal (eg thorium, calcium, barium, etc.) emitting electrons at a relatively low temperature (about 1000 ° K). This layer may be exhausted by temporary overload or as a result of long-term work. If there is a sufficient reserve of metal used to emit electrons inside the cathode, the electron tube can be reactivated. By analogy with the forming process, regeneration is carried out by heating the cathode to a temperature well above the normal operating temperature, generally distinguishing between two types of regeneration:
- overheating of the cathode without drawing emission current,
- overheating of the cathode while switching on the voltages of other electrodes.
The result of the regeneration process depends on the knowledge of the data on the method of forming the cathode of the reactivated vacuum tube. These data for various types of tubes and cathodes are different and mostly by companies producing radio tubes are protected as factory secrets. In addition to the cathode formation data, it is important to determine the degree of cathode wear. The state of wear can be determined by carrying out microchemical tests, during which destruction of the tube of the tube is unavoidable. Therefore, it is impossible to provide exact formulas regulating the reactivation processes of radio tubes. In any case of regeneration we are dealing with randomness. If the electron tube has a supply of electron-emitting metal in the cathode fiber, the regeneration process may be positive. Otherwise, the tube should be treated as useless..
After these preliminary remarks, we will discuss the appropriate methods of regenerating radio receiver tubes. Depending on the type of cathode structure, various regeneration methods are used.
1. Directly heated cathodes.
A) Thrusted cathodes.
This type of tubes can be recognized by a bright mirror covering part of the inside of the glass envelope (e.g. Telefunken tubes type RE 054, 064, 154 and others).
The cathode is heated with the filament voltage, gradually increasing over the course of 10 minutes from the nominal value to twice the value. We do not charge the emission current. Measurement of the anode current increase is a test of success of the regeneration attempt. In case of a negative result, we use the second method of regeneration. The electron tubes, with all nominal voltages connected, are heated with a filament voltage of 1.2 times the nominal voltage. When controlling the anode current, we make sure that the power dissipated at the anode does not exceed the allowable power. If the anode current does not increase, we lower the filament voltage to the nominal value, turn off the voltages of other electrodes and heat the electron tube for a few minutes under these conditions. Then we turn on the anode voltage and observe the anode current with the filament voltage gradually increased by 20%. Such attempts, if we especially care about a given electron tube, can be repeated several times until the desired effect is obtained.
B) Bar cathodes.
One way of fabricating barium cathodes is by heating barium by eddy currents and depositing it on the cathode filament. Electron tubes constructed in this way (eg RE084, 034, KC1, KL1) can be distinguished from others by paying attention to the special shape of the anode in the form of a box and the dark mirror inside the glass bulb, occupying most of the surface of the bulb. Before carrying out the regeneration process, we can visually determine whether a given tube is suitable for repair. For this purpose, the color of the cathode that glows under normal conditions should be observed. In the case where the color of the cathode is light, not dark red, regeneration due to the absence of an oxide layer on the cathode will not work.
There are three methods to reactivate this type of barium tube:
The method of burnout:
- After a few minutes after the electron tube has been heated, with normal voltage we continuously increase the glow voltage to 1.8 times. With this voltage, we glow the electron tube for about 10 minutes and return to the normal operating voltage.
- Similarly to point a, we increase the voltage to 1.5 times. The time of glow with this voltage is about 25 minutes, then return to normal working conditions.
"Load" method with increased filament voltage:
We switch on the nominal voltage for all electrodes and, with the filament voltage increased by 10% - 20%, we observe the anode current, regulating it in such a way that the permissible power dissipated in the anode is not exceeded..
With the filament voltage increased by 10%, we turn on the negative grid voltage and the anode voltage. By gradually reducing the negative voltage of the control grid, we increase the anode current to the state of weak anode reddening. (At the same time, slightly greenish lights appear on the electrodes, indicating the presence of barium vapors). After a few minutes of work under load, we gradually return to the conditions of normal work. After this test, electron tubes very often show a significant improvement in the emission current.
In addition to the barium cathodes described above, in practice we are dealing with paste-type barium cathodes. Tubes equipped with this type of cathode have a relatively small mirror inside the bulb at the base, while the length of the cathode is quite considerable. These will be battery and rectifier tubes (e.g. 1064).
RWe carry out the regeneration in the same way as in point 1 A) b) or by the "increased glow" method:
- 1.8-fold filament voltage for 20 minutes without emission current load,
- 1.2 times glow voltage at 0V grid voltage in 1-2 hours with normal anode current.
2. Indirectly heated cathodes.
The cathode active layer corresponds to pasty barium cathodes. During the regeneration process, the thermal inertia of these cathodes should be taken into account, with caution in annealing to avoid the harmful effects of the thermal grid current.
- Longer work of the cathode under load with a filament voltage higher by 20% - 25%.
- Work under load in the following conditions: filament voltage 1.2 times the nominal value, grid voltage equal to 0V, nominal anode current, test duration 1-2 hours.
- Regeneration test carried out as in point 3.
3. Cathodes with an unrecognized structure.
Due to the lack of data on the cathode structure, the regeneration of electron tubes is performed as follows:
a) Electron tubes after a long period of work.
Slowly increase the cathode filament voltage by 50% over the course of about 20-30 minutes. After a few minutes, we return to the nominal voltage. If the anode current measurement shows no positive effect, we increase the filament voltage by 80% and stay in these conditions for about 10 minutes, then return to the nominal conditions.
b) Tubes overheated (short period of operation).
With the nominal voltages connected on all electrodes, we overload the tubes according to the table:
|Time of heating in minutes
|Filament voltage multiplication factor
NOTE: During the test, the anode current should be controlled so as not to exceed the permissible power lost at the anode.
Finishing the discussion on the issue of regeneration of electron tubes, one should also pay attention to the list of electrical elements needed to carry out the above-discussed methods of repairing electron tubes. In any case, the following are necessary::
- AC and DC source,
- low impedance resistor, adjustable,
- accurate voltmeter - 2% class (suitable for current source),
- a control device measuring the emissions of the tubes after each test or during the test.
Below is a diagram of a universal device used to regenerate various types of tubes and to test the state of their emission.
The method of using this instrument is as follows:
- We regulate the mains voltage with the R1 resistor to the nominal value (e.g. 230V).
- Set the switch S1 to the nominal value of the filament voltage of the vacuum tube.
- Set the switch S2 to the "fikament test" position.
- The lighting of the neon lamp is a check for the continuity of the cathode fiber.
- Set the switch S2 to the "measurement" position. We regulate the resistance R2, the filament voltage on M2 to the nominal value.
- We read the deflection of M3. In the event of a large difference between the read value and the deflection for a new tube of this type, the tested tube is subjected to the regeneration process.
- Set switch S3 to the appropriate position of increased filament voltage, and switch S2 to the "regeneration".
- After the prescribed time has elapsed, switch S2 to the "measurement" position. We measure the increase in current.
In the case of indirectly heated tubes, the resistance R2 is used to regulate the deflection of the device. "K" switch - allows you to check the insulation between the cathode and the heater (in the "measurement" position, with the "k" switch pressed, the M3 deflection goes to zero).
The apparatus described above is not suitable for carrying out the regeneration tests with a load. It also has no additional devices to detect short circuits between the electrodes, etc.
It is the simplest device and thanks to this simplicity it is easy to build and operate.