3-500z filament voltage Too high

Discussion in 'Amateur Radio Amplifiers' started by N4FMG, Feb 27, 2012.

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  1. N4FMG

    N4FMG Ham Member QRZ Page

    I've gotten all of my HV protection on my Swan Mark2 and was doing one last check of filament voltage
    I've measured 5.5v AC at the transformer and 5.3 AC at the tube pins.
    I've read about adding resistance to the output side of the transformer due to it being a
    multi output transformer.

    What size wattage resistor am I looking at to drop the .3 to .5 volts with the current on the 2 tubes?
    I'm going to do one more check at the pin base of the tube but I don't think it will be in the 5 to 5.25 A/C volts range.

    Quick edit, I did some calculations with info I pickup here and the data sheet. with 14.6 amps current and to drop .5 volts I'd need
    .0324 ohm at 6.533 watts or better.

    If someone could check that to make sure I'm correct, as I'm not.
    I havent had to figure this stuff for a long time and I'd hate to hurt the amp.
    Last edited: Feb 27, 2012
  2. KM3F

    KM3F Ham Member QRZ Page

    Considering your meter accuracy may be a factor, load down on transmit, and powerline changes over time, I would not be concerned about it.
  3. VK4TUX

    VK4TUX Ham Member QRZ Page

    0.02 ohm of N22 Ni-chrome wire in series will work well. Should result in 4.9v - 5.0v at the filament pins.

    Adrian ... vk4tux
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    What is the accuracy of your voltmeter?
    You are only Fifty Millivolts high and I'll bet your AC voltmeter is plus or minus three percent which is an error of plus and minus one hundred fifty millivolts.

    I wouldn't worry about it.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Me, neither.
  6. KF5LJW

    KF5LJW Ham Member QRZ Page

    I would not worry about either. My bet is your AC primary input voltage is a little on the high side. Going from 120 VAC to 122 VAC on the transformer input will cause the 5 volt output to rise 50 mv. During summer when air conditioning load is high your 5 volt supply will be a little low.

    Last comment is: Have you measured the voltage with the radio keyed up and operating @ Full Power?

    Bet you $100 it will be less than 5 volts. Now what are you going to do about that? Very simple, don't worry about it. Sooner or later you wil learn something about electrical and electronics. We measure it with a micrometer, mark it with chalk, and then cut with an axe.
    Last edited: Feb 27, 2012
  7. N4FMG

    N4FMG Ham Member QRZ Page

    Haven't checked it keyed yet. I've been mainly working on adding some HV protection to the amp.
    Just wasn't sure how critical the 5v filament voltage is,
    Meter accuracy isn't too bad 4 to 400 volts ac is +/- .75%

    I'd still like to know if my calculations are correct. If someone could let me know if
    I did that correctly.
  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    Well I still think you are spinning your wheels for nothing, but to answer your question is simple Ohm's Law using the formulas Voltage = Current x Resistance, Resistance = Voltage / Current. So if you need to drop .5 volts with 14.6 amps flowing you need .5 volts / 14.6 volts = .03 Ohms.

    Kicker is there is no way you can measure that low of a resistance with a DMM. It takes a special meter called a DLRO that uses high calibrated precision current and then read precision milli-volts to determine resistance. At .03 Ohm's you would need a calibrated 1 amp current source, and then look for .03 volts. or use 10 amps and look for .3 volts. They are very expensive meters.

    Other way is guesstimate using a micrometer to measure out a length of wire to desired resistance, mark it with a piece of chalk, then cut it with an axe, and hope you get in the ball park.
  9. VK4TUX

    VK4TUX Ham Member QRZ Page

    50 Millivolts = 0.050v, Is that what you meant Sue rather than 500 Millivolts=0.5v or half a volt etc?

    Adrian ... vk4tux
  10. VK4TUX

    VK4TUX Ham Member QRZ Page

    Nichrome and other type resistive wire comes with a specified resistance value per length unit (metric or imperial)
    So all our friend needs is a ruler or tape measure. If he get's the filament voltage ~5v that's a lot better than 5.3 -5.5v on the lifespan of newly fitted 3-500zg's. I set mine to 4.85v with no issues. Dropping the (too high)filament voltage on already old tubes will be of little benefit to those tubes, but certainly will be beneficial to new replacement tubes life span.

    Adrian ... vk4tux
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