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Eimac 3-500Z Inspection

Discussion in 'Amplifiers' started by 357, Jun 21, 2018.

  1. 357

    357 Walkin' the dog

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    I was wondering what to look for on a 3-500Z that would indicate trouble or quality of the tube.
    Things like the lettering and the plate coating on the edges.

    Mine seem to have initials carved in the plate.

    These are off ebay and I'd like to know they aren't damaged.



    So far there is no foreign materials rolling around inside.

    IMG_3157.JPG IMG_3160.JPG IMG_3161.JPG IMG_3162.JPG IMG_3163.JPG IMG_3164.JPG IMG_3159.JPG r4.JPG
     
    Shadetree Mechanic likes this.

  2. nomadradio

    nomadradio Analog Retentive

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    Look for any "tilt" in the gray anode structure. Eimac had trouble with the spot-welding that holds it to the cone-shaped structure connected to the top cap. Mostly tubes from the 80s had this defect, but I don't know where you might find a range of serial numbers. Wish I had one. A tilted anode is already coming loose and will only get worse.

    A continuity test between one of the two cathode pins and one grid pin. Should be an open circuit, right?

    Now turn the tube on its side, still connected to the continuity test. Tap politely on the sides, top and bottom of the tube. We rotate it around while doing this. What you're looking for is a grid wire that has a broken weld at one end. If it flops around, you'll get a very brief circuit between them. The heat from normal-power operating can make this loose grid wire curl, and touch only while you're keyed. We use a component tester with a CRT display. It catches the very-briefest contact between the cathode and a loose grid wire better than a meter.

    If you have a HV breakdown tester, you should not get any gas leakage from cathode to grid below around 3000 Volts. A really-good vacuum won't show gas current past 4000 Volts. Under 3000 is probably safe enough. And if it breaks down and shows gas current much below 3kV, it might give you trouble.

    Maybe.

    But that's what we do before putting one in the test amplifier to see what power it will show. Better to skip the snap-pow result by testing for breakdowns before that stage.

    73
     
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  3. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Also check the solder in the pins of the filament leads. If it looks dull and like a cold solder joint it is possible the tube was run too hard or without proper cooling. They have been known to literally melt the solder out of those pins.
     
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  4. Hurricane145

    Hurricane145 Active Member

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    I have heard that you can get an idea of how gassy a 3-500Z is by holding up in bright sunlight with a piece of white paper behind it. How grayish it appears inside the tube is supposed to be something of an indicator of how gassy it has become.
    It may be a ham radio wives tale. Anyone ever hear of this?
     
  5. Hurricane145

    Hurricane145 Active Member

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    You would go by the date codes on the glass if they are still there to read. But even then you might not be able to find a very accurate date range.
    I think you are mostly just taking your chances and throwing the dice buying used high power tubes.
     
  6. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Discoloration of the internal elements and especially the glass is an indication of emissions and indicates a well used (or abused) tube.
     
  7. Hurricane145

    Hurricane145 Active Member

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    I don't think it was about discoloration of internal parts or the glass. More about a slightly grayish haze (of gas?) inside the glass that would show that it has gone gassy to at least some degree.
    I'd just as soon buy a new one from RF parts or some place like that than take chances on a used one unless I could get it for a song. I do have an older Eimac for my AL-80B as a spare but I have a new 3-500ZG in it. The older Eimac does okay but doesn't tune up as well as the new one.
    When I bought the amp it was an older amp that had never been out of the box or used. Somewhere along the way the original tube got broken so was given a fairly good older used Eimac for it. I thought it was a good deal at $500 even if I wanted a new tube later.
     
  8. nomadradio

    nomadradio Analog Retentive

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    Molecule by molecule, the metal of the filament will vaporize. Takes a while, though. If you have ever removed an incandescent dashboard lamp from a car with high mileage, it probably looks like a mirror. This is the filament metal, evaporated a few molecules per mile and condensed back to metal on the inside of the glass.

    I you see a shiny patch of this on a 3-500Z, it probably spent too much time on channel 6. Or 26, or 28. This tells you the tube was run W-A-Y too hot. Stay away from that one. It's toast. Normal wear for large glass tubes still puts a very thin layer of this condensed metal vapor, but only gets you a slightly "gray" glass when you hold a sheet of white paper behind it. Takes a lot of hours running at safe power levels to do this.

    The only way to find out how much gas has contaminated the vacuum is with high voltage. Won't be visible in any way. I have seen Eimac 3-500Z tubes that looked brand new. Bright, shiny paint where the logo is printed, but flunked the gas test.

    Everybody has a bad day eventually, and Eimac is not exempt.

    As they say, gas in a tube is odorless and colorless. Until you light it up with a few thousand Volts.

    73
     
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  9. 357

    357 Walkin' the dog

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    The tubes did 1000w on low.
    Without the f-ing box though, they do nothing.
     
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  10. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Never saw or even heard of that one during my 22 years in broadcasting using lots of glass tubes such as 6146B, 2E26, 807, 833A/833C/ 4-400C and a few others. Gas cannot be seen as it is usually simply a partial loss of vacuum and you cannot see air. What you are talking about is a white or grey deposit on the inside of the glass envelope resulting from air entering the tube envelope and causing a chemical reaction as a result of electrons being burned off the metal elements of the tube structure. Imagine breaking a light bulb while it is on. The filament vaporizes instantly and coats the inside whitish gray.
     
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  11. nomadradio

    nomadradio Analog Retentive

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    Here is a 1985 Eimac 3-500Z that had the failed spot welds holding up the anode.

    [​IMG]

    I need to ditch about three dozen various 3-500 and 4-400 tubes that all check completely bad in one or more ways. Thought about putting them on Ebay, six to a carton, and list them as "Guaranteed bad. If you find one that works, send it back for a refund."

    Ought to get pix of the various visible signs of abuse before they go.

    73
     
  12. 357

    357 Walkin' the dog

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    Well the 3-500z's are now freebies.
    Date code 7744

    They look like someone took very good care of them.

    IMG_3198.JPG IMG_3199.JPG IMG_3200.JPG IMG_3201.JPG
     
  13. Hurricane145

    Hurricane145 Active Member

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    Thanks for the explanation. That certainly makes more sense!
     
  14. 357

    357 Walkin' the dog

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