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Author Topic: 3-500Z Tube Failure  (Read 853 times)
W0XX
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Posts: 1426




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« on: December 04, 2017, 07:56:33 AM »

What are the typical symptoms if a 3-500Z open or shorts in an amp?

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K8AXW
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Posts: 6360




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« Reply #1 on: December 04, 2017, 09:34:04 AM »

Many answers...from no current draw on keydown to catastrophic destruction of one or more components.

Do you have specific symptoms or just a general question?
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K3VW
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Posts: 22




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« Reply #2 on: December 04, 2017, 09:39:34 AM »

One thing that occurs is if it showing negative grid current on key down, it has a grid-to-filament short, it can be fixed about 50% of the time by putting 120VAC between the grid and filament. Be carefull ! Check the tube with an ohmmeter. If it doesn't light up at all, check below filament pins on the chassis for melted solder.
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W0XX
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« Reply #3 on: December 04, 2017, 10:01:25 AM »

Not keying it yet.  Tube does not light up in the socket.  Another tube swapped in does.  Replacing the suspect tube does not light up.  I did not upon really looking into the tube, a very small piece of what looks like a staple wire sliding around.  Likely a piece of filament, likely shaken loose during transit.  I am guessing the tube is "open".
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K8AXW
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« Reply #4 on: December 04, 2017, 09:31:19 PM »

Looks like you have a bad tube OM..... Open filament.  Check the continuity between the filament pins with an ohmmeter.
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K4JJL
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« Reply #5 on: December 05, 2017, 07:07:00 AM »

I have had plate to grid shorts which blows up all kinds of components.  It's usually cause by someone running the amp laying on its side causing the grid to sag into the plate.
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WA2ISE
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« Reply #6 on: December 05, 2017, 01:11:06 PM »

caused by someone running the amp laying on its side causing the grid to sag into the plate.

Look at any tube datasheet, and there's always a spec on operating position.  Some tubes will work with "any" and others describe allowed positions.  To avoid sagging filaments or grids shorting to something. 

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W0XX
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« Reply #7 on: December 05, 2017, 04:54:32 PM »

I have never been a fan of horizontally mounted tubes...
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KM1H
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Posts: 2593




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« Reply #8 on: December 09, 2017, 05:28:56 PM »

Ive worked on 1-3 tube 3-500 amps for decades and a short time on their side hasnt hurt anything but I wouldnt do a long key down test either.
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AA2UK
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Posts: 363




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« Reply #9 on: Yesterday at 01:31:19 AM »

Not keying it yet.  Tube does not light up in the socket.  Another tube swapped in does.  Replacing the suspect tube does not light up.  I did not upon really looking into the tube, a very small piece of what looks like a staple wire sliding around.  Likely a piece of filament, likely shaken loose during transit.  I am guessing the tube is "open".
Since you have nothing to lose try re-soldering the filament pins on the 3-500Z.
I've brought a few back from the grave doing this.
This assumes the filament voltage is there and the seals are intact in the tube.
Bill, AA2UK
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