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.
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.
