Rejuvenation of Vacuum Tubes

With the ever increasing scarcity of old tubes, it is becoming more important that we try to save as many as possible. Over the past two years I have been experimenting with the rejuvenation of these older tubes. This work has been based primarily on present day techniques used at Eimac and on data given in various books published in the 1920's Using the methods described herein, I have had approximately 85% success in returning inactive tubes back to usable transconductance. The failures have primarily been due to filaments being burned out during application of the excessive voltages required. The tubes which failed either had filaments that had been weakened from long hours of operation, or were marginal at the weld joints. None of these failures were opened for investigation as they are still valuable for display purposes.

The primary failure mode of these older tube types is a loss of electron emission from the filament or cathode. With the wide inter electrode spacing used in these tubes, a short is very rare except in the case of a broken filament wire where the oxide has flaked from the filament or cathode and has touched the grid. The loss of electron emission typically shows up in the tube tester as a weak tube or one which will not raise the meter needle. If a tube tests normal and does not show any erratic indication on the test meter, no attempt should be made to improve it by rejuvenation.

The equipment required for rejuvenation is relatively simple. In addition to a tube tester, a variable filament supply is required with a meter of reasonable accuracy for measuring the applied voltage. In place of a separate filament supply, a filament voltmeter may be connected to the tube tester and the filament voltage switch and "line" adjustment used for voltage control. For the thoriated tungsten filaments it is preferable that no grid or plate voltages be applied during rejuvenation. With the oxide emitter tube, voltages should be applied during rejuvenation. The removal of plate and grid voltages can be readily accomplished by the construction of an adapter socket with filament connections only. The voltage applied to the filament during rejuvenation mush be carefully controlled to the values given. The accompanying graph shows the results of various voltages applied to a thoriated tungsten filament during rejuvenation. It shows that a voltage lower than the recommended value will eventually result in a fairly good tube, while too high a voltage will result in a tube which will remain weak.

Emission loss is generally due to contamination (poisoning) of the emitting surface. The vacuum and the original outgassing of the elements in these older tubes was not near the present day standard, therefore, they contain considerable residual gases. The poor emission usually is the result of either the emitting surface being poor in storage, or, immediately upon being heated the filament/cathode was poisoned by the residual gases which had condensed on the emitting surface. The function of rejuvenation is to drive off these condensed gasses and to replenish the electron emitting layer on the surface of the filament/cathode.

Vacuum tubes have essentially three basic types of emitters. These are: pure tungsten, thoriated tungsten, or a directly or indirectly heated oxide. The type of emitter in a given tube can be determined by its operating color at rated filament voltage. The pure tungsten filament operated bright white, the thorated tungsten filament runs orange to yellow, while the oxide emitter operates in the dull red region.

The pure tungsten filament needs little rejuvenation as its operating temperature makes it self-cleaning. Operation at 110% of rated filament voltage for up to 30 minutes should clean them up. This type of filament was used in such tubes as the UV200, UV201, and in many types of transmitting tubes.

The thorated tungsten filament is probably the major one to be dealt with by the collector. This filament is a composition of tungsten and thorium with the tungsten acting as the heat source while the thorium is the emitting source. This filament was used in tubes such as the UX200A, UX201A, UV99, UX99, UX120, UX210, and in many of the later (and present day) transmitting tubes. Two methods are used for rejuvenation of these filaments. If a tube is only weak or gives erratic readings, the first procedure should be tried. If a tube is completely dead (but the filament lights up) then the second procedure should be used. 1) operate the filament at 135% of the rated voltage for 30 minutes. Test the tube, and if the tube has improved but is still not to rating, continue for another hour. If at the end of this time the tube is still not up to specification, use the following procedure. 2) In this procedure the filament is run white hot to strip the emitting surface completely clean, then the surface is restored using the above procedure. Operate the filament for 15 to 20 seconds at 350% of rated voltage with no other voltages applied. Then, operate the tube under the conditions given in the first procedure. Test the tube every 30 minutes, and if the tube is not up to rating after two hours, it has reached the end of its useful life. Note: Do not attempt to test the tubes at the end of the first step, as there will be no emission.

Typically the oxide emitter consists of a layer of strontium and/or barium oxide deposited on a heated surface. In the directly heated type, this layer is placed directly on the surface of the filament. Typical of this type are Western Electric tubes such as the VT-1 and VT-2 and the WD11, UX226, and UX280. The indirectly heated cathode is the more modern type of emitter consisting of a metal sleeve with the oxide layer on the exterior and the filament mounted in the interior. The indirectly heated cathodes include the ac heater types such as the 24, 27, and the Kellog tubes. These tube types should initially be operated at the rated filament voltage for at least one hour and then checked for quality and stability. If they still are not satisfactory, then the following procedure should be used. With the tube in the tube tester, increase the filament voltage to 120% of rating while carefully watching the plate current or tube tester meter reading. The meter reading will slowly increase, hit a peak, then start to decrease. At the point of maximum reading, reduce the filament voltage back to rated value. Continue to operate the tube at rated filament voltage for at least four hours, then test. When two tests spaced one hour apart provide the same reading, the tube is rejuvenated as much as possible.

The rejuvenation of the old tubes can be very rewarding especially considering that some of the would otherwise be in the junk box. It does take some time for this work as there are no short cuts, but it is something that can be done without constant attendance. While not all the tubes will come up to 100% or rating, at least many tubes can be brought up to the point of being usable. As these old tubes become more scarce this may be the only way we will have of getting the old sets operating.