572B Filament Voltage

Discussion in 'Amateur Radio Amplifiers' started by WQ4G, Feb 9, 2020.

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  1. WQ4G

    WQ4G Ham Member QRZ Page

    Greetings to all:

    According to specs the 572B tube requires 6.3vac Filament Voltage. Knowing that the output of a Filament Transformer varies a bit due to the variation of input voltage from the mains, I am wondering how much variation in Filament Voltage is allowable before the life expectancy of the tube(s) is affected?

    For example: The Filament Transformer in my amp is producing (right now) about 6.8vac under load (Filaments lit). This is .5v (1/2 volt) over the 6.3vac spec for the tube. How detrimental, to filament life, is 6.8vac vs. 6.3vac?

    Just trying to determine if I need to do something to tame the filament voltage down to spec....


    Dan WQ4G
  2. W1NB

    W1NB XML Subscriber QRZ Page

    I’m sure you’ll get a multitude of opinions on this. I think it’s likely dependent on the tube manufacturer, how much you use the amp and how long it’s kept on. Svetlana rated their 572Bs at 6.3 +/- .3V but none of the spec sheets for other brands that I’ve seen indicate a tolerance.

    In my opinion, .5v will not significantly shorten the life of the tube, and inrush current is your greatest foe. If you want to ensure maximum life, add a soft start board/inrush circuit if there is not already one in the amp. Depending on the configuration, it should help protect the HV rectifiers and reduce the current surge through the filaments.
    K0UO likes this.
  3. WQ4G

    WQ4G Ham Member QRZ Page

    Inrush Current Limiters are in place... The tubes are of Chinese Manufacture.

    Dan WQ4G
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Then, your answer may be in a fortune cookie!

    (Kidding...fortune cookies were actually invented in San Francisco.:))

    Eimac tech notes published tons of information regarding filament voltage vs. operating life for lots of their tubes, but they didn't make a 572B so everything they published may not be applicable at all. RCA developed the original 811A from which the 572B was derived years later...possibly their tech notes may be more applicable.
    WQ4G likes this.
  5. VK5QD

    VK5QD Ham Member QRZ Page

    Here is what Tom Rauch (W8JI) posted to this forum some 'couple of hundred'? posts ago in regard to filament voltage. It is a sobering read:

    "How it really works is like this......

    You won't see any statistical difference in life of tubes in amateur service by getting all worried about filament voltage. None. Zip. Zero.
    If you run the 8877 or 3CX800A7 somewhere between Eimac's minimum up to an extra 1/2 volt, you will never see a life history change.
    Tubes like the 8877 or 3CX800A7 are ruined in seconds by excessive grid current, operation before full warm-up, or operation with low filament voltage. The rest of the failures, other than inadequate cooling, are almost entirely from tube manufacturing defects.

    Nearly all failures in thoriated tungsten tubes are caused by thermal cycling of the tube in well-manufactured tubes, although that now has been greatly eclipsed by manufacturing defects in tubes. Operating errors in thoriated tungsten tubes (e.g. 3-500Z) almost exclusively relate to over-dissipation of anodes.

    Some amps have inadequate cooling (e.g. TL922), which leads to seal failures, pin unsoldering, or glass failures. Other than amps with inadequate airflow causing glass or pin seal failures, the majority of failures now are manufacturing quality issues. Second to that is thermal cycling. Filament voltage as a general rule is not even on the statistical RADAR.

    We kept statistics on this stuff for years. The only 8877 ever returned for low emission had a filament mod to reduce voltage to 4.5 volts. The low voltage ate the tube up in about 3 months.

    For Ham use, anything right around the specified range is just perfect. You will never see a life change by getting all anal about filament voltage, but you sure will if you do things like not have a grid trip circuit in a metal oxide cathode tube or run the filament voltage low, or transmit before the cathode is at full temperature.

    Filament voltage is important in commercial BC transmitters. This is because the tubes are not cycled off and on, and they are not heated and cooled during operation while on. They are also operated at a fraction of ratings. This means most BC tube failures are emission life failures. This has never been the case in amateur service, except a few Dentron designs that ran grossly excessive voltage on filaments or amps that people have modified.
    73 Tom
    N2EY likes this.
  6. W2VW

    W2VW Ham Member QRZ Page

    Larger indirectly heated tubes can have shortened life with high heater voltage when it causes secondary emission from the grid after cathode material migrates.

    The 572B is of course directly heated

    The OP might be closer to 6.3 volts with the plate supply delivering current. Was the voltage measured right at the tube pins? If not there's also some drop through the cathode choke.

    Really glad I don't have to defend crummy designs. That must be a gift that just keeps on giving.
    N2EY likes this.
  7. AI3V

    AI3V Ham Member QRZ Page

    Oddly enough, in many magnetron type RADAR sets, you only need filament voltage to get the cathode hot enough the start emitting electrons.

    Once up to power, "back heating" of the cathode will keep it hot enough that you can remove filament voltage entirely and the tube will keep producing power.

    I personally observed this phenomenon with the U.S. navy an/sps-10 RADAR set, I got the tube up to temperature, got it outputting rf, then I removed the filament fuse and the RADAR kept transmitting just fine.

    You probably won't see very much back heating on a 572b...:D

  8. WQ4G

    WQ4G Ham Member QRZ Page

    Actually, I took the measurement at the transformer side of the Bifilar Filament Choke....


    Dan WQ4G
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That will almost always show higher voltage than is actually reaching the filaments due the considerable current power tube filaments draw.

    If you go to this link there's good discussion of filament voltages and how important or not it actually is but if you scroll down towards the bottom of the page it shows you how to properly measure the voltage actually applied to the filaments. https://www.w8ji.com/filament_voltage_life.htm

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