Chinese 811A and 572B tubes possible cause of early failure

Discussion in 'Amateur Radio Amplifiers' started by N8FVJ, Jul 18, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    It is well known Chinese 811A and 572B tubes fail early. The Cetron 572B are known to operate well after 25 years. I do not have information of actual vacuum tube use in hours of operation, but something is amiss with these Chinese 811A & 572B tubes. The Chinese 3-500Z is noted to be going strong after being introduced 10 years ago. No doubt Chinese tube factories can make a high quality tube. Some have been in business since 1950.

    It is not vacuum loss, melted plates due to an undersized plate (dissipation capacity) or internal shorts causing the failures. The 811A and 572B filaments are a thoriated tungsten design. Tubes were also built of pure non-thoriated tungsten. It is easy to see the difference due to the pure tungsten filaments operate bright white like an incandescent bulb. Thoriated tungsten filaments operate at a yellow/white color. The thoria is added so the tungsten filament does not have to operate as hot as pure tungsten filaments and aids in the electron cloud produced in the tube at reduced filament temperatures.

    A thoriated filament will not function long term as the thoria becomes depleted if not controlled. To control thoria loss, the filament must be coated with a carbon film. The carbon film is created with a small shot of a carbon gas such as acetylene inside the tube when under vacuum at -5 to -6 Torr. After the carbon coats the filament part of the vacuum is loss. The tube needs to be re-evacuated after the carbon coating and this takes time. If a tube is not carbon coated at the filament, it takes double the time to finish a tube.

    I have not any hard facts and no one has at least in the USA regarding the early tube failures. But, I am guessing the Chinese 811A & 572B tube filaments are missing the carbon coating. Any other suggestions, I hope to hear from you.
     
  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    TYPO- Should read: If a tube is carbon coated at the filament, it takes double the time to finish a tube.
     
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    I think many failures are due to operator error.

    Users not understanding that RF output is not equal to RF input, And duty cycle.

    And advertisements that confuse users with PEP ratings to make their amps sound better.

    Even cheap tubes can make a lot of power for a short period of time.
     
  4. AI3V

    AI3V Ham Member QRZ Page

    I agree.

    And there are several prolific posters on this website that give the incredibly bad advice to simply tune for maximum grid current, with absolutely no regard to the other currents in the tube, nor the power dissapated in the tube.

    And then there are the guys that advise to "run 'em red hot to getter them"....:rolleyes:

    And a goodly number of xenophobic posters.

    Rege
     
    WQ4G likes this.
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    Some are operators error, but it does not account for emissions failure over a short time. Melting a plate is operators error.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    This tube imported by and made for RF Parts seems to support claims that it's identical to the Cetron in materials and construction. It also ain't cheap: https://www.rfparts.com/tubes/tubes-572b/572b-tay.html

    I must say I've heard nothing but very good reports about this specific model, from users who've bought them.
     
    KD0CAC likes this.
  7. KD0CAC

    KD0CAC Ham Member QRZ Page

    They got the printing on the box backwards ,
    " more watts per dollar "
    More dolars per watt .)
    I guess my stash is worth more .
     
  8. KW4GT

    KW4GT Ham Member QRZ Page


    I have never seen any solid data on these claims one way or the other, only peoples opinions stated as fact (and I too am skeptical of items manufactured in China, I've been involved in laboratory evaluation of communication cable from China and much of it was pure junk). I've also heard from a few users of them that Chinese 572Bs are perfectly fine. I'd love to see some controlled testing to get to the bottom of this.
     
    KD0CAC likes this.
  9. N8FVJ

    N8FVJ Ham Member QRZ Page

    eHam reviews of electron tubes give some info. Also, occasionally read about Chinese tube failures in various posts. RF Parts has NOS Cetron 572B at $200 each. Chinese 572B sell at $50 right now. Build Chinese 572B correctly and charge $100 per tube.
     
  10. KQ0J

    KQ0J XML Subscriber QRZ Page

    The original post is not written very clearly to be able to understand the specifics of the failures referenced. There is also no such thing as ' thoria ' .

    Off the top o my head , I don't recall 811 or 572 filaments being of the thoriated type, maybe its just my bad memory.

    Anyhoo funny rant in any case.
     

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