4-400 turning blue.

Discussion in 'Amateur Radio Amplifiers' started by KB4MNG, May 15, 2015.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    I put some solid state rectifiers in my thunderbolt to replace the 866a. When I cut the unit it on, with the plate, all the filaments appear to start lighting as regular. One of my 4-400 turns blue, hums, you will see a long arc on the inside at the base, soon it kicks out the fuse. The other one does not do this.

    Any idea what may be happening?
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    Sounds like your 4-400 has gas, well too much to allow for normal operation anyway.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That tube is history. Very unlikely you can save it.

    Luckily 4-400As are not expensive and still quite available.
  4. KB4MNG

    KB4MNG Ham Member QRZ Page

    No problem on replacing the tube. I just don't want to install another one and destroy it. Does this sound like just a tube failure?

  5. W8JX

    W8JX Ham Member QRZ Page

    Yes what you are seeing is ionizing of gas in tube.
  6. AA9G

    AA9G Premium Subscriber QRZ Page

  7. KB4MNG

    KB4MNG Ham Member QRZ Page

    Ok, Ill get a new tube installed. Just curious, what would cause a failure like this? Has the seals leaked?

    Steven, just want to make sure. When I transmitt into the TB, the realy needs to connect the operating bias and grid, correct? Will the tubes glow red connected in this fashion with no signal applied from the exciter?

  8. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Most likely. It's also possible (but unlikely for that amount) residual gas from manufacture found its way free.

    If you are biased to cutoff, there should be no plate nor grid current, so the only thing glowing should be the filament.
  9. W8JX

    W8JX Ham Member QRZ Page

    Not quite true. This assumes a normal tube with a vacuum. There will still be a very small flow and it takes very little to ionize gas in tube.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Now you have a Tube to make a lamp out of.

    Don't let the smoke out.

    Good Luck on your project.

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