AC or DC for a 3-500z filament voltage?

Discussion in 'General Technical Questions and Answers' started by K5JCJ, May 9, 2019.

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  1. K5JCJ

    K5JCJ Ham Member QRZ Page

    I own a Drake L-75 amplifier where the tube has recently gone dark. When I pull the tube and conduct a continuity test on the filament pins, I get continuity and very low resistance on them, and there are no shorts or other flaws from filament to grid/anode and such. This makes me think the tube might still be good.

    When I test the filament supply voltage with the dark tube in the amp there's a reading of 5.3 VAC. Is this normal? The Eimac 3-500z datasheet doesn't specify AC or DC. If the filament was shorted then I would think fuses would blow or some other indication of trouble, but none of that is happening. Is it possible that the transformer is so stout that it could still supply 5.3VAC even in the face of a shorted filament?

    Thanks in advance for any advice. 73,
    John K5JCJ
  2. G0JUR

    G0JUR Ham Member QRZ Page

    what you need to know is in here.

    "Power line voltage varies seasonally and with local time. Most amateur (and many commercial) products, because of size, weight, or cost, cannot use regulated AC filament supplies. Directly heated cathodes also do not work well on DC supplies. A DC supply biases one end of the filament more negative in relationship to grids, and this can unevenly distribute emission current. This is especially problematic in higher voltage filaments with low bias tubes. "
  3. AG5CK

    AG5CK Ham Member QRZ Page

    Where are you checking filament voltage? If you're checking at the transformer start at the tube and work your way back. You may have a bad connection in the tube socket or an open circuit somewhere.

    Depending on the amplifier if might be easier to install the tube, disconnect the filament supply at the transformer and check the resistance. It should be close to the resistance you measured at the tube pins.
  4. K2XT

    K2XT Premium Subscriber QRZ Page

    Swap the tubes.
    If you have a problem in the socket you will find out.
  5. WQ4G

    WQ4G Ham Member QRZ Page

    Check the tube data sheet...

  6. AA7QQ

    AA7QQ Ham Member QRZ Page

    What is the issue that you are troubleshooting?
    What resistance did you read on the filament?
    If it was shorted, you would be popping fuses.
    AC is fine for the filament.
    There have been many discussions of the benefits of slightly lower filament voltages. I "Think" 5.3VAC is within the suggested range.

  7. KM3F

    KM3F Ham Member QRZ Page

    Can't find a simple open circuit?
    The tube filament draws a lot of current to provide enough temperature to feed over 500 ma of plate current.
    Socket pins lost tension!
    Tube pins lost solder!
    Don't worry so much about absolute filament voltage as long as it is within the +/- specs.
    When you key up and talk voltage drops anyway.
    A look at the plate voltage tells you this.
    A marginal capacity AC feed circuit also contributes.
    Going low on filament voltage can poison the filament emission.
    Going high shortens the life of the filament structure.
    The amplifier works fine until there is a fault.
    Fix the fault. DC supply is not the answer.
    Good luck.
  8. N2EY

    N2EY Premium Subscriber QRZ Page

    Where are you reading the voltage? At the tube pins?

    There is probably an open somewhere. Loose socket contact is a common problem.

    Do you have a spare 3-500Z?

    Note that the filament current is 14.5 amps, so everything in the circuit has to be low resistance.


    DC is not the way to go in this application.

    73 de Jim, N2EY
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    As stated, AC is preferred and what all the amps use unless they're old vacuum tube mobile rigs.

    My first guess was already stated: Bad filament pin socket contact. Look underneath the socket with a strong light and see if one or both filament (cathode) pin(s) are discolored and "darker" in color than the grid pins, or if the socket contact(s) has lost it's "spring" so it doesn't fully close on the tube pin.
  10. K5JCJ

    K5JCJ Ham Member QRZ Page

    I've tested the voltage at both the tube socket and the transformer leads, both readings were 5.3VAC. I just wanted to make sure that was normal, but looking at the schematic I see that it is AC.

    The resistance value I measured was well under 1 Ohm, I believe it was .01 Ohm or less. I don't have a spare tube to swap it with, so it seems I'll have to buy one to get this venerable amp back up and running.

    The socket pins are all nice and clean and seem to have plenty of grab on the pins. I have access to a regulated variable AC supply at work, so I'm going to try to power the filament with it outside the amp and that should determine if it's the tube or the amp.

    Thanks for the suggestions everyone.

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