Old tubes - Printable Version
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Old tubes - fields 100 - 06-13-2014
I have some low emission globe type tubes.Do you, guys,believe in the rejuvenation of them? I've read some articles on the issue, but the results are mixed to say the least.There is also discussion on the type of a filament most suited for such procedure. The idea is to clean the surface of the filament by running it at the higher voltage for specified period of time. Any ideas?
RE: Old tubes - Eliot Ness - 06-13-2014
It will work for older tubes that have a thoriated tungsten filament, such as the 01A and 99. I have successfully rejuvenated, maybe a dozen, 01A's years ago when I first read about it in an old publication called The Horn Speaker. Later tubes, even if they are globes, probably won't respond as well, if at all. In fact newer 01A's have an oxide coated rather than a thoriated tungsten filament so they will not respond well to the process.
RE: Old tubes - fields 100 - 06-13-2014
Eliot Ness
Thanks,
Where can I find which tube is which? For example, I have 24s, 27s, 47s and 280s? What I mean is how to find out the kind of metal the filaments of the specific tubes are made from?
RE: Old tubes - TA Forbes - 06-13-2014
Here is one method of tube rejuvenation. This is mainly for triodes with a thoriated filament.
Early tubes that do not respond well to this method of rejuvenation: 11, 12, 00, 26, 27, 45, 50, 80, 81, 71A
Another method is to "flash" HV across the tube filament. While I am not exactly sure how this works, I have a Knight tube tester that has been converted into this kind of rejuvenator. With it I have had success bringing back many varieties of tubes, including 75s (triode section), 78s, 80s, 45s, 6A3s, and 6L6s. FYI the 6A3 went from 1300uMho to 4600uMho, as measured on my Weston 981-3.
RE: Old tubes - Randy H. - 06-13-2014
You have 24s, 27s, 47s and 280s... NONE of which will 'take' to rejuvenation. They are coated filament (47, 280) or heater/cathode types, with a coated cathode. (24, 24-A, 27.)
I've never heard ot the system described by TA Forbes, but therre is no reason why it should not work. It SOUNDS like the system used to 'rejuvinate' TV picture tubes. Sometimes they would last for a few months, sometimes only a few days. Might be worth looking into for radio tubes.
RE: Old tubes - fields 100 - 06-13-2014
Randy
Thanks. Then I will make nice display thing out of all tubes.. At least something...
RE: Old tubes - Arran - 06-14-2014
Before you attempt any sort of rejuvenation technique on a globe type tube try re-soldering the tube pins, particularly the cathode and heater pins, and the grid cap. Sometimes a tube with a bad solder joint on the grid cap will test as weak, one with a bad solder joint on the cathode will test weak or test with zero emission. On a 27 or a 26 the grid is connected to one of the small pins, the plate is connected to the other.
Regards
Arran
RE: Old tubes - Randy H. - 06-16-2014
You really should not rely too heavily on what Tube Testers tell you, other than shorted, gassy, or open filament. The ultimate test is, DOES IT WORK satisfactirily in the circuit? I knew a store that made its living selling tubes. People brought in their tubes for testing (Mostly TV) About two out of five tested 'bad'. During a slow week, they could 'thump' a short into most miniature types. But mostly they were not in the circuit with the problem! Low emmission is not NO emission, and a tube that is not carrying a fair amount of power (audio outputs and the like) may have more than enough to operate in your radio.
Try swapping a known good one for the one that seems not
so good in the tester. No difference? Leave the 'bad' one in. Difference? Change it.
Example: the 7F8 in Philco and Zenith AM-FM sets. A brand new one will result in near 'ZERO' emission inside a week. But it will continue to work for a VERY long time.
Hint: In a TV set, keep the original IF tubes as long as possible. (usually 3) They aren't very costly, but replaceing them messes up the alignment of that IF strip. Internal capacities vary from tube to tube, and even more manufacturer to manufacturer. To a lesser degree, the same thing happens to your F.M. I.F..
RE: Old tubes - Phlogiston - 06-16-2014
Here is another tube rejuvenation process for later tubes, those not covered by the one above. It works sometimes and sometimes it makes the tube worse.
The idea is that a good tube will not conduct significantly more at higher filament voltage, where as a cathode that is contaminated will. So you put the tube in at normal filament voltage and push the button. Then you increase the filament voltage by about a third. The reading will go up. (note that this will load down the tester power supply so true filament voltage will be less than indicated so will plate voltage - but ignore that). Watch the needle climb. When it stops climbing take your finger off of the test button and turn the filament back to normal. The additional heating is supposed to clean the cathode.
I would not try this on a good tube or a marginal tube. Note too that gas contamination will often allow a tube to conduct at higher filament voltage. Watch for the blue (purple glow). If you see it, the tube is junk.
RE: Old tubes - Phlogiston - 06-16-2014
Another important tube testing note:
Most tube testers (all I have used) do not use enough plate voltage to properly test eye tubes.
First, the test is rather arbitrary requiring the user to evaluate "brightness". To do this you almost need a NIB tube to compare to.
Generally a tube that generates light bright enough to see in both the open AND closed settings in a well lit room will be quite usable in a radio at higher plate voltages.
This is still a good test though. You will be aware of tubes that have had a lot of use. But since most radios are not "daily drivers" an old tube my be good enough.
Conclusion is to test marginal eye tubes in a radio.
RE: Old tubes - fields 100 - 06-16-2014
OK, here is what I've done.I took 2 of my globe spare 247s and tried them both on my 90B, which is in the good shape. The tubes were marked 400 and 1000 Mhos.Do not know which tester have been used.Long story short, both globes worked good. By good I mean the the audio volume and no distortion in comparison with my other good 47. What else should I expect other than volume level from the audio amplifier tube? Does it mean that I can use them as a good working tubes? BTW, I have a simple Knight 600, and it indicated 72 and 88 on them, chances are it is not calibrated.
RE: Old tubes - Eliot Ness - 06-17-2014
The ultimate test is using a tube in circuit, so it sounds like your 47's and doing fine. If you're not sure who tested them I wouldn't worry about the 400 and 1000 Mhos marked on them. While the Knight is a simple emission type tube tester, it sounds like they tested fairly close and in the good range.
The calibration of the Knight 600 is pretty simple and the manual is located here (600tube.pdf):
http://bama.edebris.com/manuals/knight/600tube/
Replace any caps and out of tolerance resistors and the calibration for your simple tester sounds pretty easy.
RE: Old tubes - codefox1 - 06-18-2014
Agree, if a tube tests a little "weak" and has no other problems and works in your set, use it. Sometimes mismatched output tubes (when the set has no bias adjustment control) can cause some hum and distortion, but they have to be pretty far off to cause any problems. Small signal tubes that test weak will mostl likely either work OK or not.
RE: Old tubes - Phlogiston - 06-18-2014
If the cause for low performance is gas, then the tube can arc. This is especially true for rectifiers and power output tubes. This can cause a output transformer,power transformer, field coil, interstage transformer or choke failure rather quickly.
RE: Old tubes - Tom Hamilton - 07-12-2014
Really good advice here. Thanks for posting all the knowledge.
Tom