Valve Rejuvenation

[Fourlegsgood]

As per the title, has anyone tried this? Any successes? Any failures?

I am having a go with some tired 2A3 valves using the method of 135% heater voltage (ie 3.375V for 2A3) for 1/2 hr with no plate or grid voltage. So far I have tried a few and it has brought them back up to pretty much full spec current/gain from being completely on its last legs.
eg
Before 9mA, 0.3mA/V.
After 60mA, 4.4mA/V

I have done a search on here but can't find much discussion on this subject. There was some about CRT tubes but I am talking about direct heated triodes.

[bluepilot]

I've never tried it myself so can't say from first-hand experience. However overrunning the heaters risks burning the heater out. I saw a German site which produces the heating by using increased anode current. You'll find the site here:
http://www.jogis-roehrenbude.de/Regenerierer.htm
If your German is not too good, Google will translate it for you here if I've got the link right:
https://translate.google.fr/translat...-text=&act=url

[lesmw0sec]

I can accept that over-running the heater although risky, might well homogenise the coating, thus increasing the emitting area, but I am curious about the increased anode current method, which I would have thought might induce further stripping of the coating in the process.

[Fourlegsgood]

This is the source that I am using as a base for having a go.

The trick seems to be to get the direct heated cathode hot enough but not too hot in order to achieve a long term change / improvement.

http://www.antiqueradios.com/chrs/jo...uvenation.html

[trobbins]

One reason why some valves, especially input stage valves, are discarded is because of excessive heater induced hum.

A 1957 patent focussed on the heater ‘burn-in’ process used during tube manufacture. The data showed that 12AT7 heater-cathode resistance at ±100VDC was > 600MΩ for nearly all valves after aging with the patented technique, but normally there may be a significant percentage of valves manufactured with resistance in the range 10M - 300MΩ. I haven't seen any evidence that this technique has advantage for aged valves, but you never know ....

http://dalmura.com.au/projects/US2915355.pdf 'Insulating heater within cathode sleeves’, US Patent 2,915,355 submitted 1957 by T.H. Carlstrom from Sylvania.

[Hartley118]

I've sometimes managed to revive a low emission valve with the AVO Valve Characteristic Meter by over-running the heater at somewhat more than twice its rating for 30 minutes or so. I leave the HT on, but wind up the negative grid voltage to restrict the anode current within its rating to avoid destroying what's left of the cathode coating.

My most recent success was with an EL33 output pentode which exhibited very low anode current together with moderate grid-cathode leakage which would normally have destined it for the bin. With nothing to lose, 30 minutes of this therapeutic abuse cleared the leakage and restored near-spec anode current and gm. I wondered whether the improvement would be merely temporary, but a month or so later the valve still measures OK.

Quite how the therapy works, I'm not sure, but I imagine it may be burning off some internal contamination.

Martin

[Fourlegsgood]

Martin, thanks for your experience. I have been using the AVO VCM163, increasing the heater voltage but using the 'Circuit Selector' switch to keep the heater on and HT off. I have tried eight 2A3 valves so far and all have improved dramatically. Now it's time to see how long term that is.

Quoting from the link that I posted in post 4 for ease of ref.

"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."

[Herald1360]

It may be a questionable process, but what's to lose by trying it? The valve's already useless......