Rejuvenation of exhausted vacuum tubes

Gentle Fellows,

I performed a search on the topic's matter on the forum, and found with surprise that there seem to be no information so far. There currently is an interesting debate going on on the well known italian website Le Radio Di Sophie owned by fellow member Leonardo Mureddu. The topic started after the reading of this webpage (in english):

http://www.antiquewireless.org/otb/rejuve.htm

I did not try the suggested techniques personally, nor did I find any trace in the old books I have in my personal library (though the author claims those facts were known since the thirties). Any direct insight or comments by those who eventually tried it would be highly appreciated.

However things are, I guess this method would be worth trying, expecially with very old (and almost impossible to replace) tubes.

 

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Yes, I can confirm that the described methods work to rejuvenate different types of valves. There are some German books which describe this techniques.

But caution, there is one mistake in the article. The author claimed to use 350 % of filament voltage for max. 30 sec to clean the surface of a thoriated tungsten filament. In the literature max 2.5 times of filament voltage is recommended. So you can heat a 4 V thoriated tungsten valve up to 8 - 10 V (I personally would recommend max. 8 V) for a short time of 20 - 30 sec.

 I would not expect a 4 Volt valve to survive 14 V (350 %), but I did not run a test series on that.... 

In my valve workshop I activate new thoriated tungsten filament with 1.5 times of filament current. So a 200 mA filament is run at 300 mA which it can survive for minutes without problems.

Best regards

Rüdiger Walz

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Radio Friends,

When I was a kid, I would take used tubes to the local hardware store to test in their tube tester.  For whatever reason, I liked to play around and try different things.  Fortunately I never damaged the tube tester, as far as I know.

While trying various things, I found that many weak tubes could be rejuvinated by the following procedure:

1.  Increase filament voltage to 2X normal (or sometimes even more)

2.  Press the "test" button on the tester.  Watch the emission increase with time.  Stop when it saturates.  Too much of this "rejuvination" permanently destroys the tube.

3.  Return to normal filament voltage and retest.  Usually it's better than before.

Sometimes you can see some "sparking" inside the tube during Step 2.  I've noticed this is very similar to what happens when rejuvinating picture tubes using equipment designed for that purpose.  Overall, the tube tester procedure described above seems very similar to what a picture tube rejuvinator does.

I haven't had a reason to try this recently, but I would expect it to work sometimes using a normal tube tester.

Best regards,

Tom

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I thought rejuvenation was only possible for thoriated filament tubes? For this tubes no extra voltage is needed, just keep them with nominal filament volgate applied and no plate or screen voltage at all and they do rejuvenate.

One thing I have seen very often is that if you leave the tube on the tube tester with the filament lit for a few minutes, it usually measures better than it does in the first few seconds after warming up...

Mario

 

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Sure thoriated filaments get the greatest advantage by the rejuvenation process. Barium oxide filaments, however, can be helped by the process to some extent as well. Some italian fellows gave a try to the process with barium oxide tubes, and claimed the emission had undoubtably increased after the process.

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Original article link has changed and is probably found here: click here

 

But I have a question. Pure tungsten cathodes I can recognize by their high temperatures and glow. But how do I recognize oxide filaments and thoriated filaments?

I am particularily interested in Marconi 1937 battery operated 2V filaments. What are they made of?

Like KT2, VP21, X22, HD22

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Direct Filaments:

1. Pure Tungsten. Very Bright or else inefficent. As low as 70 hours life.
2. Thoriated Tungsten. Quite Bright, certainly used for very high power tubes long after 1940s, maybe still used on transmitter tubes or Magnetrons of Microwave Ovens. As bright as indirect heater.
3. Oxide coated Early types used Adize (?) Philips process, mucky and could contaminate other Electrodes. Only up to maybe end 1928 or 1929
4. Oxide coated only on filament, non-contaminating. Philips / Mullard from 1928/1929, others earlier.
    

Both types of Oxide are very dull and only visible at all in very poor light. Hence Dull Emitter.

Indirect:

Oxide coated cathode and various methods of insulating filament. Quite bright.  Types for Series use or Series and Parallel use have more insulation, thus may have slightly higher power consumption, and if a Rectifier (parallel or series needs up to 600V insulation in some cases) may take even longer to warm up cathode and be somewhat brighter than say an RF pentode with Indirect heater.

 

Rejuvination of Filaments

The best success can be with reactivation of Thorated Filaments, which must use fairly exact temperatures and time. Follow makers instructions exactly.

 

Under run or unused for a very long time Oxide filaments or indirect cathode benefit from perhaps 50%  excess filament current but only enough bias for less than 1mA or even 100uA to 500uA cathode current. Grid Bias has to be adjusted more negative as surface re-activates. This can take a few hours. The indirect heaters are slightly more robust than filament and reactivating cathode seems to be more successful than direct filaments, likely because they have more coating.

 

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Any low power 2V receiver tube after about 1927 is probably Oxide coated.

Also any 2V tube under 500mA is likely Oxide, actually are ANY Thoriated tubes as low as 2V? Are they not usually about 4.5 to 5.5V using a 6V LT via a Rheostat?

KT2 = Development of earlier Marconi PT2, new in 1931, they are virtually interchangeable. Both 200mA Only just visible filament with 60W lamp in room. Audio Output. Certainly Oxide.

VP21 = Low power RF IF Pentode. 100mA Filament. Replaces late 1920s & Early 1930s S.G (RF Tetrodes) such as S22, PM12 and 215SG etc (The S.G. all 150mA to 200mA oxide types). Certainly Oxide. Almost invisible filament.

X22 = 1937 Pentagrid (Heptode?)  Similar to Mullard/Philips FC2A, Oxide filament, only 150mA. The KK32 is essentially the FC2A with Octal base after 1945! Not Thoriated type. Pentagrid / Octode / Heptode and Triode - Hexode all generally need more filament current than same generation of RF Pentode as the g1 & g2 is an oscillator, and some other grid is RF mixer input. Higher over all current. In battery 2V and 1.4V receivers the Mixer/Osc is 2nd highest HT current consumption.

HD22 is essentially a variation of 1934 HD21, may be identical. One half of filament is for the two diodes, (the AGC anode  probably on the most positive end of filament for delayed AGC) and other 1/2 of V (point at top held by spring) is for the triode (audio pre-amp).  150mA as the filament is really 3 cathodes. Oxide type.

On later 2V and non-series 1.2V to 1.4 (i.e. Telefunken Y8 base Dx1x series) the RF/IF device takes lowest current and output tube highest.

Unless it's massive heater power and thus huge anode power and clearly visible you can nearly assume anything after 1926 or 1927 is Oxide (Dull Emitter if direct filament).

I'd like one of these :-) Battery model with these tubes.

If it's some other model you have can you request it and upload photos?

Good Photographs (Martin Renz, Germany).
Irfanview for Prepairing Photos (Konrad Birkner, Germany).
Irfanview for Schematics (F.-J. Haffner, Germany).
NEW: Formatting Pictures with Irfanview 4.33 (John Kusching, USA).
NEW: Uploading a Picture to RadioMuseum (John Kusching, USA).

 

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Thank you for very comprehensive reply!

As I understand, it highly unlikely to rejuvenate these tubes. Just checked KT2 I have. One has emission of 0,23mA @100V and other KT2 is even more dead with 0,07mA. While a working KT2 I have easily gives 11mA.

I don't get why these tubes have failed so deep? It is not possible that the last owner continued to use these tubes long after radio was dead? Or are there some tricks how tubes that have been laid idle for 70 years treated when powering up? 

The radio I am working right now is exactly the same on the picture. These pictures of Ilmatar-2 were uploaded by me.

Radio has been hidden for 70 years (1941-2011) and it was my hope that tubes were in working condition as probably these tubes were still all in working condition before radio was hidden, as radio was only 3 or 4 years old when it was hidden.

One thing that could work for a short time, is rising a cathode temperature. I measured currents at filament voltage of 3,2V, and that was 5mA for KT2 tube.

And one more thing I found. there seem to be a patent of how to rejuvenate cathode with a high voltage impulses.  how to rejuvenate with high voltage
 

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I would leave the tubes with 2.2V filament maybe 24 hrs (no HT). Then increase to 2.8V but with small HT (24V) and grid bias to limit current under 0.5mA. Check every ten minutes or 20 minutes and increase grid bias to keep current under 0.5mA. Do this about 4 hrs.

The tubes maybe with time the glass releases gas and oxide degrades. The small current avoids damage to surface (some parts may reactivate before others). The getter and also eventually any heated Nickel will also absorb any gas. The overheating without current encourages a fresh oxide/tungsten interaction if coating still exists.

If the 2V lead Acid LT cell was charged in situ while radio is on, then the filaments are seriously over run and worn quickly. Also the oxide coating reacts almost as readily with any vapour and certain gases perhaps seeping in at tiny quantity, not enough to make getter go white. Again heat and very little current helps reverse deterioration.

I have 1929 used tube (PM12) which is quite good and an unused "NOS" Boxed one (Mazda version) that is quite poor. It's mysterious!

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I wonder, if it would be useful to try and heat metal inside the tube along with getter with microwaves? 
 

After some search I found all the tubes being on the sale except HD22. So I think I can experiment with some of these dead tubes. But I will try to rejuvenate them before I try anything unproven. 

It is also interesting to notice that my working KT2 tube filament visibly starts to glow. It is dull red and faint, but visible. While none of the dead tubes filament becomes visible at normal 2V. Only after I have risen their filament voltage considerably filament sstarts to glow.

Yeah, thats a tube magic and mystery, I agree :)

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See this old article. TUBE REACTIVATION / REJUVENATION  It only relates to Thoriated Filaments, not oxide coated or pure tungsten.

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