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Regeneration of radio tubes
RADIO Monthly magazine for Technicians and Amateurs, 1st Year, May 1946, No. 3
(Trioda website is not responsible for the content of the article)
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.