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.
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.
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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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.
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.
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
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
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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.
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.
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
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
This procedure may be suited to tubes, as author Robert B.Tomer mention it on the book ''Getting the Most of Vacuum Tubes''(1960).
He suggest tubes go sleep after 6 month unused, and gasses may appear with the time.
i'd be weary of doing that whit an bakelite socketed tube .
why don't you try running it for a while as a diode whit a low voltage cc supply
grid tied to ground cathode ofcourse if the tube improves you see voltage going down
and wrap it in alumina foil . run the heaters as hot as you dare and let it cook tubes tend to get Very HOT whit alumina foil as it reflects all the heat back inwards .
v4lve
why don't you try running it for a while as a diode whit a low voltage cc supply
grid tied to ground cathode ofcourse if the tube improves you see voltage going down
and wrap it in alumina foil . run the heaters as hot as you dare and let it cook tubes tend to get Very HOT whit alumina foil as it reflects all the heat back inwards .
v4lve
Cranking up the heater voltage for days is detrimental, risky and likely to extremely shorten the overall tube life on regular smaller receiving tubes.
Its important to realise that the probable cause of poor performance could be either an undesirable coating on the cathode, or else some gas buildup.
Although it may help cook off junk on the cathode, obviously overheating the filament has the negative effect of shortening tube-life. This is not a 'guess', its a fact.
RCA did a study on that, and noted that running a tube either over-voltage or under-voltage more than 5% of its rating for any length of time reduces the life of a tube from about 5,000 hours service to about 3 hours!
Certainly running the tube heater at way over-voltage for more than a couple of hours is a disastrous methodology.
This only worked with the older thoriated tungsten tubes and those huge powertubes with carbon anodes (e.g. 813s!).
Such a strategy would just damage a 'modern' receiving tube.
BETTER HOW-TO
A better way of doing business can be to instead 'cook' the tube by running the heater at regular or only slightly increased voltage (less than 5% higher), and then putting the quiescent (idle) current up to maximum dissapation for the tube.
This will heat up the tube as effectively (and more) as cranking the heater, and will more likely clean up the cathode (the electron emission and current will do the job). Also, if the tube gets hot, the getters may also work to remove excess gas.
Since the screen (G2) does the work in pulling current out of the cathode, it should be charged to near maximum rated voltage but not more. The tube must be properly biased too, to protect it and the screens from too much current and heat-watts dissipation. The best way to do this is self-bias, i.e., put an appropriate high wattage cathode resistor in the circuit.
You cannot do the over-voltage trick with the High Voltage either:
That is, you must keep the HV below the rated maximum voltage to prevent arcing while running the tube. Arcing would probably destroy the tube.
TV repairmen knew the dangers 30 years ago:
The over-voltaging of the filament worked in his final example, only because the heater was improperly manufactured as new!
Its important to realise that the probable cause of poor performance could be either an undesirable coating on the cathode, or else some gas buildup.
Although it may help cook off junk on the cathode, obviously overheating the filament has the negative effect of shortening tube-life. This is not a 'guess', its a fact.
RCA did a study on that, and noted that running a tube either over-voltage or under-voltage more than 5% of its rating for any length of time reduces the life of a tube from about 5,000 hours service to about 3 hours!
Certainly running the tube heater at way over-voltage for more than a couple of hours is a disastrous methodology.
This only worked with the older thoriated tungsten tubes and those huge powertubes with carbon anodes (e.g. 813s!).
Such a strategy would just damage a 'modern' receiving tube.
BETTER HOW-TO
A better way of doing business can be to instead 'cook' the tube by running the heater at regular or only slightly increased voltage (less than 5% higher), and then putting the quiescent (idle) current up to maximum dissapation for the tube.
This will heat up the tube as effectively (and more) as cranking the heater, and will more likely clean up the cathode (the electron emission and current will do the job). Also, if the tube gets hot, the getters may also work to remove excess gas.
Since the screen (G2) does the work in pulling current out of the cathode, it should be charged to near maximum rated voltage but not more. The tube must be properly biased too, to protect it and the screens from too much current and heat-watts dissipation. The best way to do this is self-bias, i.e., put an appropriate high wattage cathode resistor in the circuit.
You cannot do the over-voltage trick with the High Voltage either:
That is, you must keep the HV below the rated maximum voltage to prevent arcing while running the tube. Arcing would probably destroy the tube.
TV repairmen knew the dangers 30 years ago:
Note that this repairman recommends the HV approach FIRST, before trying the drastic method of over-voltaging the Filament!
The over-voltaging of the filament worked in his final example, only because the heater was improperly manufactured as new!
Note in the quote above, the repairman doesn't recommend more than a HALF-HOUR for over-voltaging filaments!
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Search the forum for baking tubes. We discussed it already several times here.
i'd be dammed 5% overvoltage at will decrease life to 3 hours
it quotes 3000-10000 hours of life for a tube operated at 105%
Hell most SQ tubes are guaranteed for 10,000 hours for 95-105% heater voltage that means that the manufacturer still claims those figures as valid for 105% that is whitin spec in this case
hell i even experimented whit running P tubes (i have a few hundred of them) at about 300% heater voltage it light up like a halogen bulb ! yet i had examples giving 30 minutes of flawless service until i got bored of the practice . and decided to go back to on line gaming
they however smell funny . abit like vinegar
v4lve
it quotes 3000-10000 hours of life for a tube operated at 105%
Hell most SQ tubes are guaranteed for 10,000 hours for 95-105% heater voltage that means that the manufacturer still claims those figures as valid for 105% that is whitin spec in this case
hell i even experimented whit running P tubes (i have a few hundred of them) at about 300% heater voltage it light up like a halogen bulb ! yet i had examples giving 30 minutes of flawless service until i got bored of the practice . and decided to go back to on line gaming
they however smell funny . abit like vinegar
v4lve
I suppose that study is for normal working conditions with normal plate voltage/current etc, not just with the filament running?
Read the chart again. its very clear:
the life decreases to 3 hours for UNDER-voltage, not overvoltage.
Overvoltage damage is less severe, but nonetheless drastic enough for RCA to insist on the 5% margins.
At a 10% overvoltage for instance, the average 5000-hr tube will only last 1,500 hrs or less.
Thats over 2/3rds of the tube life unnecessarily lost.
Far too much to be worth the experiment.
Remember also that individual tubes and tube-types may vary, and be far worse.
Part of the problem is that the damage done by such experiments only show up later,
in the form of decreased tube-life statistics.
+- 5% is the borderline .
Experiment with your tubes if you have the money to burn.
RCA had the money to burn, and their findings are based on
the statistics for a large number of tubes tested off the manufacturing line.
Their findings are statistical generalizations, but very rigid nonetheless,
as they made clear in their reports.
They warned designers to stay within +- 5% or void any warrantees.
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The testing details are probably still obtainable somewhere.
I am certain they would have made the tests using a standard,
"max design-center" value for plate current, simulating ideal operating conditions,
and moderate design values.
Most receiving tubes really are this sensitive to voltage variations.
But that is the point I am making.
You can't really run tubes 5% outside of their operating voltages,
or you'll have extreme tube-failure on a statistical basis.
This doesn't mean that individual tubes cannot survive such extreme conditions for short bursts,
but tube-life isn't a gambling exercise, its based on physical wear and tear.
In any case, leaving a tube dangerously over-voltaged for days at a time is reckless advice.
Tube sellers love this kind of "tube lore", as it guarantees that tube sales skyrocket,
and vintage tube stashes for NOS tubes never to be re-made jump in price too.
Right now NOS Mullard 12AX7s sell for $300, because there are very few left.
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