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 Post subject: Repairing Open Filament
PostPosted: Dec Fri 04, 2015 3:19 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1487
Location: Orland Park, IL
I know it sounds like an oxymoron, but I've been trying to get continuity out of a 71A tube filament which now measures open. Usually, I would toss the tube, but I can see the filament making continuity from pin 1 through the vacuum seal, and out to pin 4. I took the bakelite base off, so there is nothing but the glass envelope and the copper leads. Visually, there appear to be no breaks in continuity. I thought maybe there is a microscopic break at the vacuum seal, so I tried applying about 20v DC through the filament and tapping the tube with a screwdriver, hoping it would somehow bridge the gap. Still no luck. Has anybody ran into a situation like this? Do you think the filament is fixable?


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 Post subject: Re: Repairing Open Filament
PostPosted: Dec Fri 04, 2015 3:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 13795
Location: S. Dartmouth MA 02748-1225 USA
IMHO, No, it probably saw many hours of service and the filament has become notched, with only the oxide keeping it together.

When buying ST and some "S" bulb style tubes, used, the getter is usually on the side. Look for clarity in the top glass, this area is where some of the filament alloy will accumulate as a grayish haze. The darker the haze the more likely the tube has seen some hours use. Not to be confused with tubes that have an internal graphite coating.

YMMV

Chas

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 Post subject: Re: Repairing Open Filament
PostPosted: Dec Fri 04, 2015 6:13 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1487
Location: Orland Park, IL
I don't have the tube in front of me now, but I can tell you it is not ST. This is a globe 71A. There is not that much getter on it, which is how I was able to look at the filament. I don't recall seeing any haze on the top. It looked pretty clean to me. However, your explanation seems to be as good as any. If the tube is gassy, this could have sped up the filament failure process.


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 Post subject: Re: Repairing Open Filament
PostPosted: Dec Sat 05, 2015 12:09 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11405
Location: Powell River BC Canada
Use a leakage test from an Eico or other bridge tester. If. as you
advance the voltage, the eye closes, there is continuity.

You might try 300 DC volts in series with a 68K 2 watt resistor.

In a dark room, tap the tube. Set up a camera and run a movie
on a cell phone or handy cam pointed at the tube. you might be able
see the break location.

If there are no options left, you might try repeating the above,
with a 0.5 uFd capacitor across the access pins on the tube.

Then when the tube is tapped, about 20 mJ of energy might
arc weld it together. Or blow it open forever.

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 Post subject: Re: Repairing Open Filament
PostPosted: Dec Sat 05, 2015 8:17 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1487
Location: Orland Park, IL
I don't believe it, but it worked!!! I tried the arc weld method, just as you described, monitoring the voltage across the tube while tapping it. Suddenly, the voltage went to zero, indicating continuity. Then, I fired up the filament on a variable DC power supply, and it started to glow! Who knows how long it's going to last though. I'll put the tube back together and see how it goes. Thanks.


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 Post subject: Re: Repairing Open Filament
PostPosted: Dec Sat 05, 2015 8:33 pm 
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Joined: Dec Thu 08, 2011 4:03 am
Posts: 1487
Location: Orland Park, IL
Well, the success was short lived. Turns out, the filament blew apart, and both ends got shorted to the grid, resulting in continuity. At least it was a good attempt at getting some life out of the tube, and there was no expense involved. Thanks for your responses.


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 Post subject: Re: Repairing Open Filament
PostPosted: Dec Sat 05, 2015 10:15 pm 
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Joined: Nov Sat 27, 2010 6:15 pm
Posts: 5449
Yeah, never worked for more than a week or two, and that was on picture tubes. I have heard of otherwise valuable transmitting tubes being evacuated, reworked, and fitted with a new top, but probably not done anymore,? You can get you a 71 easy enough.


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 Post subject: Re: Repairing Open Filament
PostPosted: Dec Sat 05, 2015 10:19 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 37331
Location: Livermore, CA
Sometimes works on tubes with heavier filaments. 45, 280 etc. I use a larger cap, 20 mf,. This gives a hotter spark and usually holds up. I have tubes for testing that were repaired like this years ago.

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