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Thread: An experiment for gassy tubes

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    An experiment for gassy tubes

    I'm sure that like me, you may have a couple of old gassy 6V6s or other tubes that you can't use since they'll run away pretty fast. I had two tung-sols (beautiful looking tubes! much cooler than the RCAs) and some RCAs that were all pulls from junk equipment. They glowed bright blue, and redplated after a few minutes.

    I read that CRTs would often have their getter reflashed to rejuvenate the vaccuum. Further, I also read that during factory burn in, intentional arcing was produced in the tube since the arc somehow interacts with the gas in the tube to help either fix it to the getter or break chemical bonds in the gas or something.

    Soo, with nothing to lose (free, already dead tubes) I tried RF heating the getters in the ole microwave. I think I found something on the old RAT lists that inspired me, or maybe it was a HAM site, I can't remember (I am well aware that both of these sources are sometimes crazy ).

    The procedure goes like this:
    1) with nothing in microwave, turn on for 1-5 seconds, until the magnetron can be heard firing...each microwave is different, mine took about 2 seconds after the light & "fan" came on before it would fire.
    2) Place gassy, throw-away tube in microwave, making sure no part of the tube is contacting the metal microwave chassis.
    3) Microwave tube for about 1 second (not too long!) after the magnetron activates
    4) Observe literal flash of the getter
    5) Remove tube and test

    My results were
    Tung Sol #1: Still gassy, not as bad, doesn't redplate anymore
    Tung Sol #2: Still gassy, redplates
    RCA #1: Perfect!
    RCA #2: Less gassy, no redplate.

    This is clearly a longshot, but hey, one out of four is good if you had none to start!
    Any thoughts? Is this insane? Anyone tried it before?

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    I only thought that worked with cats
    go figgure

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    Supporting Member tubeswell's Avatar
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    I'm into a bit of mad science now and again - I must give that a go. Thanks for the handy hint.

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    Building a better world (one tube amp at a time)

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    Senior Member NorCalTuna's Avatar
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    best evar

    this is the greatest posting ever made on this site. Youtube vids plz!

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    The disclaimer is that it whatever you do with your microwave and your tubes is between the three of you .

    Does make a cool flash, no video camera so no youtube yet. I'm interested in hearing if anyone is successful...I ran the Tung-Sols a couple times to see if I could get them to improve. The RCA was night and day.

    I'm trying to remember, there's some other trick for burning cathode material off the grid of old tubes, I read it and promptly forgot.

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    Quote Originally Posted by 6267 View Post
    The disclaimer is that it whatever you do with your microwave and your tubes is between the three of you .

    Does make a cool flash, no video camera so no youtube yet. I'm interested in hearing if anyone is successful...I ran the Tung-Sols a couple times to see if I could get them to improve. The RCA was night and day.

    I'm trying to remember, there's some other trick for burning cathode material off the grid of old tubes, I read it and promptly forgot.
    The old radio men used to 'rejuvenate' tubes from time to time. Sibley has a procedure in "Tube Lore" which you can read up on. I've tried it. It consists of running the tube at 2x filament voltage for about thirty seconds and then letting it cool down.
    Sometimes it works, sometimes it doesn't, but if the tube's beat anyway who cares? I saved some kinda tired Bugle Boys this way one time.

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    I've read about that, it's common with CRTs and it was very common with early audio tubes, apparently. Certainly worth a try! I can't seem to find the source, but there's some trick for burning the cathode material that gets attached to the grids off and reducing grid leakage/current. I think it was a charged capacitor or a slight positive voltage, but I'm not sure and I still can't remember where I read that.

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    Okay,so lets say you can burn the cathode material off the grid,how do you get it back onto the cathode it came off in the first place?Once the cathode is stripped,it is useless,so I dont see how "cleaning the grid" will improve anything.Sounds like a lot of "voodoo" to me.

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    Quote Originally Posted by stokes View Post
    Okay,so lets say you can burn the cathode material off the grid,how do you get it back onto the cathode it came off in the first place?Once the cathode is stripped,it is useless,so I dont see how "cleaning the grid" will improve anything.Sounds like a lot of "voodoo" to me.
    Reverse plate and cathode voltages to reverse the flow?

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    Quote Originally Posted by stokes View Post
    Okay,so lets say you can burn the cathode material off the grid,how do you get it back onto the cathode it came off in the first place?Once the cathode is stripped,it is useless,so I dont see how "cleaning the grid" will improve anything.Sounds like a lot of "voodoo" to me.
    Reverse plate and cathode voltages to reverse the flow?

    - you know how sometimes I claim to have half an idea? This would be a bit less than that.

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    Here's what I found in Ludwell Sibley's book Tube Lore.

    "Rejuvenating tubes was a well known practice in the 1920s but not common later. There are 3 techniques involved. For pure tungsten filaments the goal is to evaporate contaminants by operating the tube at 100 per cent of rated filament voltage for 1/2 hour and then retest. For thoriated tungsten filaments the idea is to restore the trace of thorium on the filament surface by operating at 135 per cent voltage for 30 minutes which makes fresh thorium diffuse to the surface. Test and repeat once if necessary. If that doesn't work, take a deep breath and operate at 350 per cent of rated voltage for 30 seconds, which usually wipes out whatever emission there was. Then let the tube idle at normal voltafe for several hours and the emission may return to normal.....for cathode type filaments, direct action in nothing to lose cases is to go to triple voltage for 30 seconds, let the tube cool, and retest. One or two cycles of this treatment will improve or impair a lot of even modern tubes. Eye tubes cannot be repaired."


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    Well I tried the microwave trick a while back with a NOS 5Y3GT that had all this powdery white stuff on the inside of the tube beside the getter, but alas it didn't bring that tube back to life.

    Good to hear about that other tube rejuvenation stuff.

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    Quote Originally Posted by tubeswell View Post
    Well I tried the microwave trick a while back with a NOS 5Y3GT that had all this powdery white stuff on the inside of the tube beside the getter, but alas it didn't bring that tube back to life.

    Good to hear about that other tube rejuvenation stuff.
    Well, that wouldn't have worked because a white getter means you've lost your vacuum. I did try some of Sibley's tricks on some really dead Bugle Boys and it did make a couple of them usable. I think for a time in the brown amp era Fender was using Bugle Boys-woulda been mid 1960 because my brown Concert had them, and my brown Pro has them.

    Anyway to make a long story short it was worth a try because they were headed for the trash otherwise. It brought the gM up to usable from beat to death.

    I've also used this gambit on 45s that were pretty weak, but then I found out that it's a good idea to monitor filament voltage when testing a tube that has a heavy draw. In this case the 2.5v was being dragged way down under load to 1.8, so setting the filament voltage selector on 3v and monitoring voltage from an adjacent socket got me 2.5v under load and I found out that I was rejecting a lot fewer tubes. Same holds true for rectifier tubes as well.

    Remember that you don't want a global increase in voltages (just filament) so stay away from the line adjust.

    You shoulda seen the filament on the bugle boys at 18 volts....jeez...like a light bulb.

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    Success!

    I just tried this trick on an old Sovtek 5881/6L6WGC. In my 5E7 clone, it tended to "eat" a couple volts of bias, and glow a dull red as it gobbled up extra current. Thankfully, it always stabilized itself at that point, and never went past 25W or so.

    So I saw this thread and had to try it. I upended the tube on top of an upside-down tumbler to position the getter at the center of the microwave. (Those flat-topped Sovtek tubes make this easy!) On my first attempt, I saw the heaters glow at the base, chickened out, and retested the tube in my Seco 107, fearing that I'd destroyed it. Whew, it was fine, so I tried it again.

    This time I managed to wait until there was another white flash at the top of the tube before hitting the Stop button. When I pulled the tube out, I noticed some crazing patterns in the inside of the getter. It tested fine, so I threw it back in my amp.

    I was horrified to see a bright blue haze fill the tube. Thankfully, it abated -- must have been the liberated gases being eaten by the getter. After a few minutes, I was pleased to see that the blue glow was mostly gone, and the plate wasn't glowing at all!

    Time to raise the stakes. I powered down the amp, replaced the 5U4GB with my Sovtek 5AR4, and turned it on again. In my amp, this raises the B+ by about 25 volts, and generally exacerbates the tube's tendency to runaway. After letting the amp "cook" for about half an hour, I'm happy to report that not only is the tube behaving as well as its partner, its current draw is now within 2mA of the other!

    Gotta love those hams for thinking outside the box.

    - Scott

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    Quote Originally Posted by ThermionicScott View Post
    I just tried this trick on an old Sovtek 5881/6L6WGC. In my 5E7 clone, it tended to "eat" a couple volts of bias, and glow a dull red as it gobbled up extra current. Thankfully, it always stabilized itself at that point, and never went past 25W or so.

    So I saw this thread and had to try it. I upended the tube on top of an upside-down tumbler to position the getter at the center of the microwave. (Those flat-topped Sovtek tubes make this easy!) On my first attempt, I saw the heaters glow at the base, chickened out, and retested the tube in my Seco 107, fearing that I'd destroyed it. Whew, it was fine, so I tried it again.

    This time I managed to wait until there was another white flash at the top of the tube before hitting the Stop button. When I pulled the tube out, I noticed some crazing patterns in the inside of the getter. It tested fine, so I threw it back in my amp.

    I was horrified to see a bright blue haze fill the tube. Thankfully, it abated -- must have been the liberated gases being eaten by the getter. After a few minutes, I was pleased to see that the blue glow was mostly gone, and the plate wasn't glowing at all!

    Time to raise the stakes. I powered down the amp, replaced the 5U4GB with my Sovtek 5AR4, and turned it on again. In my amp, this raises the B+ by about 25 volts, and generally exacerbates the tube's tendency to runaway. After letting the amp "cook" for about half an hour, I'm happy to report that not only is the tube behaving as well as its partner, its current draw is now within 2mA of the other!

    Gotta love those hams for thinking outside the box.

    - Scott
    A lot of the really creative ideas came from radio repairmen during WW2 when replacement tubes were made of unobtainium. Later on there was a limited release of certain tube part numbers but they were still hard to get.

    I've been re-reading Tyne's "Saga of the Vacuum Tube" and it makes for interesting reading. Back in the day a number of tubes were made with dual filaments in case of burnouts, and then some people repaired tubes. I think that this was fairly common practice with transmitter tubes.

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    I've zapped a couple more tubes and gotten mixed results.

    One was a tired 5Y3GT from a Muzak PA. The getter flash is dark-brownish, so I figure it wasn't long for this world anyway. It tested at about 50-60% on my Eico 701, and put out roughly 10V less than my other 5Y3s. After zapping it for a few seconds, it pegs the needle of my tester, just like the healthy ones! I guess the microwaving can not only flash the getter, but possibly (re)activate the cathode as well.

    The other tube was a tired GE 6V6GT, from the same PA. It tested around 60%, and would fall off when holding down the lever on the tester. I zapped it long enough to see flashing but I must've gone too long, as I've lost filament continuity.

    Just a couple more data points for you folks...

    - Scott

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by ThermionicScott View Post
    I've zapped a couple more tubes and gotten mixed results.

    One was a tired 5Y3GT from a Muzak PA. The getter flash is dark-brownish, so I figure it wasn't long for this world anyway. It tested at about 50-60% on my Eico 701, and put out roughly 10V less than my other 5Y3s. After zapping it for a few seconds, it pegs the needle of my tester, just like the healthy ones! I guess the microwaving can not only flash the getter, but possibly (re)activate the cathode as well.

    The other tube was a tired GE 6V6GT, from the same PA. It tested around 60%, and would fall off when holding down the lever on the tester. I zapped it long enough to see flashing but I must've gone too long, as I've lost filament continuity.

    Just a couple more data points for you folks...

    - Scott
    OK ... since I have dozens and dozens of these old used rectifier tubes, I tried this with three old American made used 5Y3GT tubes to see what would happen.
    On the Hickok #533's English scale, they all were testing just under what a well worn tube would, but they all still worked, just not in the green "good" part.
    I put them one at a time in microwave oven for a just a couple-few seconds each and then I ran them over to the tube tester...
    One did get better, much to my disbelief, in the green and I was encouraged, but the next one quit working completely and the final one.... well, the damn the glass broke open in the microwave. Spfft.
    I'll try again and report back... but right now, I'm having a hard time buying into it, yet one did get better and I know how to work my test equipment.

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    Quote Originally Posted by tubeswell View Post
    Well I tried the microwave trick a while back with a NOS 5Y3GT that had all this powdery white stuff on the inside of the tube beside the getter, but alas it didn't bring that tube back to life.
    White powdery stuff means the tube has lost vacuum. No microwave's gonna fix that!

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    Ok,
    What seperates a "gassy" tube from just a old wore out tube?

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    Quote Originally Posted by Sweetfinger View Post
    White powdery stuff means the tube has lost vacuum. No microwave's gonna fix that!
    Yep thanks Sweetfinger, Praire Dawg confirmed that (last May - see above)

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    Quote Originally Posted by tubeswell View Post
    Yep thanks Sweetfinger, Praire Dawg confirmed that (last May - see above)
    Y' know, ..I don't seem to have a good excuse for my redundant post. I'll swear I didn't see the earlier info- sometimes I don't see all the posts in a thread and have to click on "more replies below current depth" in the thread window- even if I've clicked "last page". This seems to be a relatively recent development in display modes- 1 or 2 years?
    This quirk got me real twitchy the first time I encountered it. I kept trying to get to MY post in a thread but it wouldn't come up no matter how I switched display modes. Anybody got a fix?

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    Senior Member ThermionicScott's Avatar
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    Quote Originally Posted by Sweetfinger View Post
    This quirk got me real twitchy the first time I encountered it. I kept trying to get to MY post in a thread but it wouldn't come up no matter how I switched display modes. Anybody got a fix?
    Yeah, switch to "linear" mode under the "Display Modes" pull-down menu. Thread modes where you have to click on each post to read it belong in the past.

    For stingray, a gassy tube will have excessive grid current flowing at idle. Positively-charged gas ions are attracted to the grid (the most negatively-charged item in the tube), and each collision makes the grid a little more positive. This causes the tube to draw more current, ionize more gas molecules, and reduce the bias even more, leading to a "runaway" condition in some tubes. Like the ones in 6267's post that would redplate in a short amount of time.

    Excessive gas in a tube wears out the cathode coating, so while a gassy tube may test as weak, a weak tube might not necessarily be gassy.

    - Scott

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Well, I have know killed at least 9 old GE, RCA and Sylv 12AX7s, 12AY7s, 12AT7s and a small handful of really poor 5Y3GTs, 5U4Gs and Bs and a soggy Russian GZ34.... all with less then 2 second bursts from my 1Kw micro wave... I'm afraid this story is really weak and getting weaker.
    Flash to trash.

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