[AMPS] Tube Rejuvenation

[km1h@juno.com]

The following may be of interest to us "big tube" guys also. I am
forewarding verbatim from the Boatanchors List.

73....Carl   KM1H

--------- Begin forwarded message ----------

The following material is copyrighted 1978 by the California Historic
Radio Society.
REJUVENATION OF VACUUM TUBES
Author: Lane S. Upton

< Permission to reproduce this here has been granted>

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.
Copyright 1978 California Historical Radio Society, all rights
reserved.No part of this publication
may be reproduced in any form, or by any means, without prior written
permission from CHRS,
except that you may make "fair use" of quotations of text fully
attributed by you to the source
(CHRS Journal) and author.

CHRS
PO Box 31659
San Francisco, CA 94131


--------- End forwarded message ----------

--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com