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Old 20th April 2012, 03:38 AM   #1
yero is offline yero  United States
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Default another tube fact or fallacy

In audioxpress april 2004 Graham Dicker suggests with tubes that were out of service for an extended time should run just their filaments for a couple of days before testing. He says "often a tube tested new from the box will test low, but after a few days of filaments only, it will often re-test 100% and be perfectly okay" Can anyone confirm this practice to be effective? And what about used tubes, will this technique work also.
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Old 20th April 2012, 05:00 AM   #2
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This is a good question, and my answer is sometimes yes and sometimes no. This is another situation that applies more to industrial and transmitting tubes with thoriated tungsten filaments, and the need for deeper vacuums, then to small receiving types. But "sleeping sickness" can affect both new and good used tubes. It is very rarely helpful for weak tubes. And it is sometimes difficult to actually know if a tube is weak or suffering from "inactivity" when it's history is unknown. If a known new/good tube is testing low when you are pretty sure it shouldn't, then you have little to loose besides some time and a little electricity.

The following is from an ITT industrial power tube data sheet:

Filament recovery and processing - Occasionally a thoriated tungsten cathode which appears to have lost it's emissive capabilities may be reactivated by applying filament voltage only in accordance with one of the following schedules.

A. Apply 110% of rated value of filament voltage for a few hours or over night.

B. If the emission fails to respond after schedual A, run at 30% above rated voltage for 10 minutes, then at 10% above normal for 20 to 30 minutes.

C. In extreme cases, where A and B have failed to give results, and at the risk of burning out the filament, run at 75% above normal for 3 minutes followed by schedule B.

This procedure is not effective in cases where the protective layer of tungsten carbide formed on the surface of the filament wire during manufacture has been severly depleted.
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Last edited by HollowState; 20th April 2012 at 05:04 AM.
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Old 20th April 2012, 06:34 AM   #3
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I am afraid it is the way to poison cathode by gases. We discussed already baking of tubes, when getter flash is heated instead of cathodes.
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Old 20th April 2012, 11:43 AM   #4
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It all depends on what is wrong, so what is necessary to fix it. A weak cathode (low emission) can sometimes be rejuvenated by overrunning the heater for a while. Only worth doing for rare or expensive valves.

A slightly gassy valve (high grid currrent) can be baked in an oven to encourage the getter. I would imagine you might be able to achieve a similar effect by running the valve with full heater voltage and lowish anode voltage so that it heats itself. The low anode voltage might limit the cathode damage caused by positive ions until the gas has been absorbed.
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Old 20th April 2012, 01:56 PM   #5
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its a yes. you can however the effects vary from valve to valve .

best to do this on a testpad whit adjustable voltages and currents . as you might burn grids if you cook em""

Ive had a EL84 tube wich came out of radio that had not been turned on for a very long time . it gone from aprox 27 milliampere to 35 running it at 9vdc heater for a few mins .

v4lve

Last edited by v4lve lover; 20th April 2012 at 01:59 PM.
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Old 20th April 2012, 03:47 PM   #6
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back in the day when tube testers roamed the drugstores, there were tube rejuvenators; little boxes that you plugged the tube into that promised to fix em up like new... I think some of them just overheated the cathode for a little while to try to burn off the interface coating. Maybe sometimes it would work; I never tried it. I think it was most common to do with CRTs but there were ones for receiving tubes.

also there was the picture tube booster, which went inline with the CRT socket and stepped up the heater voltage to increase the emission and brighten the picture. good to get a few more weeks or months out of the old CRT before spending hundreds on a new one or new TV.
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Old 20th April 2012, 04:36 PM   #7
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For TV I used electrolytic capacitor charged from B+
One shock, and it may work. But may be dead. Does not matter, anyway such picture tubes were almost unusable.

For more life of picture tube 1 turn around horizontal flyback tyransformer primary in addition to 6.3V AC worked well.
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Old 20th April 2012, 07:43 PM   #8
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I recall that I had a few 826s; after wasting a great deal of time wondering what to do with them, I decided to sell them. The buyer was pleased that (at the time) I had an AVO VCM 163 valve tester. I was able to report that all were good except one which was jolly gassy. I said that I would try to revive it.

Well, it was very obstinate - glorious blue glow. Finally I gave it hell; the filament voltage is 7.5 and I ran it at nearly 10 volts whilst dissipating significant anode current. After a good couple of hours the blue gradually disappeared. Restoring it to the correct filament voltage it tested fine. The buyer was delighted. We christened the valve "Lazarus"

Paul
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Old 20th April 2012, 08:43 PM   #9
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if i'm not mistaken those tubes have the zirconium plated molybdenum anodes witch absorb gases once they get near 600C i believe . made to run hot


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Old 20th April 2012, 08:54 PM   #10
7N7 is offline 7N7  United Kingdom
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That's right, in fact they are designed to run with their anodes cherry-red. My tester could not provide that sort of current as I recall so I simply cooked up the filament whilst running a fair few milliamps!

P.
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